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CHAPTER 11 - GEOLOGIC HAZARDS

A. OVERVIEW OF GEOLOGIC HAZARDS AND THE DEVELOPMENT PLANNING PROCESS
B. EARTHQUAKES
C. VOLCANIC ERUPTIONS
D. TSUNAMIS
CONCLUSIONS
REFERENCES

SUMMARY

The chapter presents planners with (1) a description of the most hazardous geologic phenomena-earthquakes, volcanoes, and tsunamis-and their effects; (2) a discussion of how to use existing information to assess the hazards associated with these phenomena and incorporate mitigation measures early In an Integrated development study; (3) sources of geologic data and maps; and (4) information with which to make key decisions early in the planning process.

The processes that have formed the earth continually act on or beneath its surface. The movement of plates in the earth's crust and local concentrations of heat are a continuing source of hazards to people and their structures. A simplified classification of the major hazard-related geologic phenomena and the hazards they cause is presented in the box below.

This chapter focuses on the use of information about earthquakes and earthquake-induced landslides, volcanic eruptions, and tsunamis (ocean waves caused by earth movement) to improve development planning in Latin America and the Caribbean. For each hazard the chapter presents physical characteristics, information sources, data available for determining the threat posed, and mitigation measures; Chapter 10 provides a more detailed discussion of landslides. Not considered here are certain other geologic phenomena-such as expansive soils, uplift, and subsidence-which are less common, less hazardous, or less amenable to general assessment and mitigation.

The results of the extensive research on geologic hazards that has been conducted to date have been translated into a form accessible to non-scientists, and small-scale maps displaying historic, actual, and potential hazard levels are available. While this chapter does not go into specific geologic hazard assessment techniques, most of which are well beyond the technical, temporal, and budgetary constraints of integrated development planning studies, it presents and discusses existing information which can and should be used during the Preliminary Mission and Phase I stages of a planning study. This information is sufficient to show the planning team whether a hazard constitutes a significant problem in development area and, if so, what detailed studies requiring the services of a specialist are needed.

A. OVERVIEW OF GEOLOGIC HAZARDS AND THE DEVELOPMENT PLANNING PROCESS

Development Planning

Geologic hazards are responsible for great loss of life and destruction of property. In the twentieth century more than a million people worldwide have been killed by earthquakes alone, and the value of the property destroyed by earthquakes, volcanoes, and tsunamis amounts to scores of billions of dollars. Latin America suffers its share of this destructive force: during the period 1985-1987, earthquakes in Ecuador, Mexico, and El Salvador and a volcanic eruption in Colombia killed more than 36,000 people.

The Nazca Plate, sliding slowly eastward on the earth's mantle, slips under the South American Plate along the Peru-Chile Trench. Friction produces stress and temperature increases; the subducted rock melts and expands, causing additional stress and upward movement of the magma. The magma reaches the surface, erupting to form volcanoes, and the crustal rocks respond to the stresses by breaking and moving. Thus the crust above the subduction zone is demarcated by volcanoes and active faults. Movement along the faults causes earthquakes.

This zone of volcanism and earthquakes, involving several plates and trenches and manifested in Latin America by the Andes Mountains and their extension into Central America and Mexico, virtually encircles the Pacific Ocean and is known as the "Ring of Fire." Geologic hazards-earthquakes, the landslides they induce, and volcanic eruptions-are concentrated in this region, and the seismic sea waves called tsunamis most commonly originate from earthquake shocks there as well. Similar geologic conditions extend into the Caribbean, which is considered a part of the Ring of Fire even though not a part of the Pacific Basin.

A SIMPLIFIED CLASSIFICATION OF MAJOR GEOLOGIC HAZARDS

Geologic Event

Hazards They Cause

Earthquake







A. Ground shaking

B. Surface faulting

C. Landslides and liquefaction


1. Rock avalanches


2. Rapid soil flows


3. Rock falls

D. Tsunamis

Volcanic Eruption





A. Tephra falls and ballistic projectiles

B. Pyroclastic phenomena

C. Lahars (mud flows) and floods

D. Lava flows and domes

E. Poisonous gases

With the present state of technology, most geologic events cannot be prevented or even predicted with any precision. Landslides are an exception: they can often be prevented. Areas prone to such events can be identified as earthquake fault zones, active volcanoes, and coastal areas susceptible to tsunamis. However, not all earthquake faults have been identified. Estimates of an occurrence of a given hazardous event are probabilistic, based on consideration of the magnitude of an event and its occurrence in time and space. Other measures-duration, areal extent, speed of onset, geographic dispersion, frequency-can be anticipated with even less precision.

Nevertheless, appropriate mitigation measures can enormously reduce the damage caused by geologic hazards. The City of Los Angeles, California, for example, instituted a system of grading regulations that has resulted in a 90 percent reduction of landslide-related damage to structures that were built after it went into effect (Hays, 1981). High density of population and infrastructure increases the risk, making hazard mitigation even more important.

Geologic events are distinctive for their extremely rapid onset. Unlike a flood or hurricane, whose impact at a site can be forecast hours or days in advance, earthquakes give virtually no warning. Volcanoes often show signs of a general increase in activity but give little or no warning of the actual eruption. (In a few areas where known hazards exist, e.g., Nevado del Ruiz, Mt. St. Helens and the San Andreas Fault, instrumentation has been installed which can give an indication of impending activity.) Tsunamis travel great distances over the open ocean; one triggered off the cost of Peru might hit the coast of Japan 18 hours later, giving reasonable warning time, but the same tsunami would hit the coast of Peru with almost no warning at all.

In addition to speed of onset, geologic hazards also tend to have impacts covering large areas. Earthquakes can cause damage over millions of square kilometers, and tsunamis travel the entire ocean and cause major damage thousands of kilometers from their point of origin. For these reasons, non-structural mitigation measures, such as land-use zoning or the development of monitoring systems, tend to be particularly effective.

Development Planning

The earlier geologic hazard mitigation is incorporated into the development planning process, the more effective it is. Figure 11 -1 summarizes the major issues involved and indicates the most appropriate phase in the process for their consideration in a development planning study.

It must be emphasized that "consideration" means that hard decisions representing real money must be made at each step along the way: Is ground faulting a serious hazard here? Should something be done to avoid it? To mitigate its effects? How much will mitigation works cost? What are the potential costs of not taking action? The planner must provide the information on which to base a decision at each point, but it should be the minimum necessary for a decision of acceptable reliability, since gathering information is expensive. Furthermore, it must be available at the right time, since integrated development planning, if it is to be efficient, operates on a tight schedule.

Figure 11-1 - RELATIONSHIP OF GEOLOGIC HAZARDS ISSUES TO AN INTEGRATED DEVELOPMENT PLANNING STUDY

The ideal type of information for a particular decision may not be available or may be too expensive or too time-consuming to obtain as a part of the development planning study. In that situation it may be possible to substitute other information which, even if not ideal, yields a result with a degree of reliability suitable to the level of the study. This chapter offers a framework for arriving at decisions on the mitigation of geologic hazards at various stages of the development planning process with a minimum expenditure for information gathering. In the successive stages of development planning, the hazard mitigation work becomes more detailed and specialized. Thus, the chapter concentrates on the early phases of development studies, in which the assessment of geologic hazards and the identification of mitigation measures fit comfortably within a planning study.

B. EARTHQUAKES

1. Earthquake Effects and the Hazards They Cause
2. Earthquake Hazard Prediction, Assessment, and Mitigation
3. Types and Sources of Earthquake Information
4. Earthquake Hazards and the Development Planning Process

An earthquake is caused by the sudden release of slowly accumulating strain energy along a fault within the earth's crust. Areas of surface or underground fracturing that can experience earthquakes are known as earthquake fault zones. Some 15 percent of the world's earthquakes occur in Latin America, concentrated in the western cordillera. The Regional Seismologic Center for South America (Centro Regional de Sismología para América del Sur-CERESIS), based in Lima, Peru, has produced a map entitled "Significant Earthquakes, 1900-1979" which shows the significant earthquakes that have occurred in Latin America during this period.

1. Earthquake Effects and the Hazards They Cause

a. Ground Shaking
b. Surface Faulting
c. Earthquake-Induced Ground Failure: Landslides and Liquefaction

Depending on its size and location, an earthquake can cause the physical phenomena of ground shaking, surface fault rupture, and ground failure and, in some coastal areas, tsunamis. Smaller earthquakes, aftershocks, may follow the main shock, sometimes several hours, months, or even several years later.

a. Ground Shaking

Ground shaking or ground motion, a principal cause of the partial or total collapse of structures, is the vibration of the ground caused by seismic waves during an earthquake. Four different types of waves are propagated through and on the surface of the earth at different velocities, arrive at a site at different times, and vibrate a structure in different ways. The first wave to reach the earth's surface the sound wave or P wave and is the first to cause a building to vibrate. The most damaging waves are shear waves, S waves, which travel near the earth's surface and cause the earth to move at right angles to the direction of the wave and structures to vibrate from side to side. Unless a structure is designed and constructed to withstand these vibrations, ground shaking can cause damage. The third and fourth types are slow low-frequency surface waves, usually detected at great distances from the epicenter, which cause buildings to sway and waves to form in bodies of water.

Characteristics (Parameters)

Four principal characteristics which influence the damage that can be caused by an earthquake's ground shaking-size, attenuation, duration, and site response-are discussed here. A fifth parameter, the potential for ground failure (or the propensity of a site to liquefaction or landslides) is dealt with separately later in this section. These factors are also related to the distance of a site from the earthquake's epicenter - the point on the ground above its center.

(1) Earthquake Severity or Size: The severity of an earthquake can be measured two ways: its intensity and its magnitude. Intensity is the apparent effect of the earthquake at a specific location. The magnitude is related to the amount of energy released.

Intensity is measured on various scales. The one most commonly used in the Western Hemisphere is the twelve-level Modified Mercalli Index (MMI), on which the intensity is subjectively evaluated by describing the extent of damage. Figure 11 -2 shows the approximate relationships of magnitude, intensity at the epicenter, and other seismic parameters, comparing energy release with equivalent tons of TNT.

The Richter Scale, which measures magnitude, is the one most often used by the media to convey to the public the size of an earthquake. Magnitude is easier to determine than intensity, since it is registered on seismic instruments, but it does present some difficulties. While an earthquake can have only one magnitude, it can have many intensities which affect different communities in different ways. Thus, two earthquakes with an identical Richter magnitude may have widely different maximum intensities at different locations.

(2) Attenuation: Attenuation is the decrease in the strength of a seismic wave as it travels farther from its source. It is influenced by the type of materials and structures the wave passes through (the transmitting medium) and the magnitude of the earthquake. Figure 11 -3 shows that a given degree of ground shaking can be expected over a much greater area in the eastern United States than in the West, where geologic conditions differ. The figure also shows that structural damage caused by a large earthquake can extend over a million square kilometers. The largest earthquakes have caused damage in areas three to four million square kilometers in extent.

Figure 11-2 - APPROXIMATE RELATION CONNECTING EARTHQUAKE MAGNITUDE, INTENSITY, ACCELERATION, ENERGY RELEASE, AND INCIDENCE

Sources: Adapted from U.S. Atomic Energy Commission, TID-7024. Nuclear Reactors and Earthquakes (August, 1963), pp. 13-14; Gutenberg, B., and Richter, C.F. Seismicity of the Earth and Associated Phenomena (Princeton, New Jersey: Princeton University Press, 1954), p. 18; and U.S. Department of Defense. The Effects of Nuclear Weapons (Washington, D.C.: S. Glasstone ed., Government Printing Office, 1962); pp. 14.

(3) Duration: Duration refers to the length of time in which ground motion at a site exhibits certain characteristics such as violent shaking, or in which it exceeds a specified level of acceleration measured in percent of gravity (g). Larger earthquakes are of greater duration than smaller ones. This characteristic, as well as stronger shaking, accounts for the greater damage caused by larger earthquakes.

(4) Site Response: The site response is the reaction of a specific point on the earth to ground shaking. This also includes the potential for ground failure, which is influenced by the physical properties of the soil and rock underlying a structure and by the structure itself. The depth of the soil layer, its moisture content, and the nature of the underlying geologic formation-unconsolidated material or hard rock-are all relevant factors. Furthermore, if the period of the incoming seismic wave is in resonance with the natural period of structures and/or the subsoil on which they rest, the effect of ground motion may be amplified.

In the 1985 Mexico City earthquake, the period of the seismic wave was close to the natural period of the Mexico City basin, considering the combination of soil type, depth, and shape of the old lake bed. The wave reached bedrock under the city with an acceleration level of about 0.04g. By the time it passed through the clay subsoil and reached the surface, the acceleration level had increased to 0.2g, and the natural vibration period of the buildings with 10 to 20 floors increased the force to 1.2g, 30 times the acceleration in the bedrock. Most buildings would have resisted 0.04g acceleration, and the earthquake-resistant buildings destroyed would have resisted 0.2g, but the waves that were amplified to 1.2g caused all buildings they reached to collapse (Anderson, 1985).

Effects of Ground Shaking

Buildings, other types of structures, and infrastructure are all subject to damage or collapse from ground shaking. Fire is a common indirect effect of a large earthquake since electrical and gas lines may be ruptured. Furthermore, firefighting efforts may be impeded by blocked transportation routes and broken water mains. Damage to reservoirs and dams may result in flash flooding. The damage caused by ground shaking, however, is amenable to mitigation by a number of approaches which will be discussed later in this section. In general, structural measures such as earthquake-resistant design, building codes, and retrofitting are effective. Less costly non-structural measures such as land-use zoning and restrictions can also greatly reduce risk.

An important, if little-appreciated, effect of earthquakes is damage to aquifers. The 1985 Mexico City earthquake undermined major aquifers. It not only broke the encasing impermeable layers, allowing the trapped water to escape, but also permitted the infiltration of contaminants.

b. Surface Faulting

Surface faulting is the offset or tearing of the ground surface by differential movement along a fault during an earthquake. This effect is generally associated with Richter magnitudes of 5.5 or greater and is restricted to particularly earthquake-prone areas. Displacements range from a few millimeters to several meters, and the damage usually increases with increasing displacement. Significant damage is usually restricted to a narrow zone ranging up to 300 meters wide along the fault, although subsidiary ruptures may occur three to four kilometers from the main fault. The length of the surface ruptures can range up to several hundred kilometers.

In addition to buildings, linear structures such as roads, railroads, bridges, tunnels, and pipelines are susceptible to damage from surface faulting. Obviously, the most effective way to limit such damage is to avoid construction in the immediate vicinity of active faults. Where this is not possible, some mitigation measures such as installing pipelines above ground or using flexible connections can be considered. This is discussed in greater detail later in this section.

c. Earthquake-Induced Ground Failure: Landslides and Liquefaction

Landslides occur in a wide variety of forms. The focus of this section is on those induced by earthquakes, but they can also be triggered by other mechanisms. (For a detailed discussion of landslides see Chapter 10.) Not only can earthquakes trigger landslides, they can also cause the soil to liquefy in certain areas. Both of these forms of ground failure are potentially catastrophic.

Earthquake-Induced Landslides

Earthquake-induced landslides occur under a broad range of conditions: in steeply sloping to nearly flat land; in bedrock, unconsolidated sediments, fill, and mine dumps; under dry and very wet conditions. The principal criteria for classifying landslides are types of movement and types of material. The types of landslide movement that can occur are falls, slides, spreads, flows, and combinations of these. Materials are classified as bedrock and engineering soils, with the latter subdivided into debris (mixed particle size) and earth (fine particle size) (Campbell, 1984).

Figure 11-3 - ISOSEISMIC CONTOURS FOR THE 1906 SAN FRANCISCO AND THE 1811 NEW MADRID EARTHQUAKES

Source: Algermissen, S.T. "Integration, Analysis, and Evaluation of Hazard Data" in Proceedings of the Geologic and Hydrologic Hazards Training Program, Open File Report 84-760 (Reston, Virginia: U.S. Geological Survey, 1984), p. 20.

Moisture content can also be considered a criterion for classification: some earthquake-induced landslides can occur only under very wet conditions. Some types of flow failures, grouped as liquefaction phenomena, occur in unconsolidated materials with virtually no clay content. Other slide and flow failures are caused by slipping on a wet layer or by interstitial clay serving as a lubricant. In addition to earthquake shaking, trigger mechanisms can include volcanic eruptions, heavy rainstorms, rapid snowmelt, rising groundwater, undercutting due to erosion or excavation, human-induced vibrations in the earth, overloading due to construction, and certain chemical phenomena in unconsolidated sediments.

Figure 11-4, which is designed for practical use by planners, contains a simplified classification of earthquake-induced landslides indicating the more damaging and/or more common types. The salient characteristics of these landslide types and why each is of concern (degree of damage, frequency of occurrence 1, velocity) are listed in Figure 11-5. This figure, again designed for use by the planner, provides information on the mode of occurrence-under what conditions each type can be expected to occur (geomorphic, topographic, parent materials, moisture content), and their causative factors, including the smallest earthquake that can cause that type of landslide and common trigger mechanisms.

1/ The frequency of occurrence of earthquake-induced landslides is related primarily to the magnitude of the earthquake and aftershock but also to local geologic conditions. The frequency scale used here is based on a survey of landslides associated with 40 historic earthquakes.

Rock avalanches, rock falls, mudflows, and rapid earth flows (liquefaction) account for over 90 percent of the deaths due to earthquake-induced landslides.

(1) Rock Avalanches: Rock avalanches originate on over-steepened slopes in weak rocks. They are uncommon but can be catastrophic when they occur. The Huascarán, Peru, avalanche which originated as a rock and ice fall caused by the 1970 earthquake was responsible for the death of approximately 20,000 people.

(2) Rock Falls: Rock falls occur most commonly in closely jointed or weakly cemented materials on slopes steeper than 40 degrees. While individual rock falls cause relatively few deaths and limited damage, collectively, they rank as a major earthquake-induced hazard because they are so frequent.

Figure 11-4 - CLASSIFICATION OF PRINCIPAL EARTHQUAKE-INDUCED LANDSLIDES INDICATING DEGREE OF DAMAGE PRODUCED a/



Type of Material and Moisture Content

Bedrock

Engineering Soil (Earth and Debris)

Type of Movement

Dry to Wet

Dry to Wet

Very Wet

Falls

ROCK FALL

Earth fall


Slides



Rock slump

EARTH SLUMP


Rock slide

Debris slide



Earth block slide


Lateral spreads


EARTH LATERAL SPREAD


Flows




Dry sand flow

MUD FLOW Debris flow


Loess flow

RAPID EARTH FLOW



Earth flow (mudslide)

Complex

ROCK AVALANCHE, moist to very yet


a/Historical damage caused by earthquake-related landslides.

UNDERLINED CAPITALS; these landslides have caused many casualties and large economic losses.

CAPITALS: these Landslides have caused a moderate number of casualties and moderate economic losses.

Upper and lower case: these landslides have caused few casualties and minor economic Losses.

Sources: Modified from Campbell, R.H., et al "Landslides Classification for Identification of Mud Flow and Other Landslide Hazards" and Keefer, P.K. "Landslides Caused by Earthquakes" in Proceedings of the Geologic and Hydrologic Training Program, Open File Report 84-760 (Reston, Virginia: U.S. Geological Survey, 1984).

(3) Mud Flows: Mud flows are rapidly moving wet earth flows that can be initiated by earthquake shaking or a heavy rainstorm. While the term is used in several ways, in this chapter "mud flow" is used to designate the phenomena associated with earthquake shaking. Underwater landslides, also classified as mud flows, may occur at the margins of large deltas where port facilities are commonly located. Much of the destruction caused by the 1964 Seward, Alaska, earthquake was caused by such a slide. The term "mudflow," in keeping with common practice, is used as a synonym for "lahar," a phenomenon associated with volcanoes.

Liquefaction

Certain types of spreads and flows are designated as liquefaction phenomena. Ground shaking may cause clay-free soil deposits to lose strength temporarily and behave as a viscous liquid rather than as a solid. In the liquefied condition soil deformation may occur with little shear resistance. Deformation large enough to cause damage to constructed works (usually movement of about ten centimeters) is considered ground failure.

The occurrence of liquefaction is restricted to certain geologic and hydrologic environments, primarily in areas with recently deposited sands and silts (usually less than 10,000 years old) with high ground-water levels. It is most common where the water table is at a depth of less than ten meters in Holocene deltas, river channels, areas of floodplain deposits, eolian material, and poorly compacted fills.

Figure 11-5 - CHARACTERISTICS OF PRINCIPAL EARTHQUAKE-INDUCED LANDSLIDES a/

General Characteristics

Mode of Occurrence

Causative Factors

landslide Type and Degree of Damage b/

Frequency Occurrence c/

Velocity

Landform and Geologic Environment

Parent Material

Moisture

Smallest Earthquake (Intensity)

Common Triggers-and Comments

OCCURS ON VERY STEEP 10 GENTLE SLOPES

1. ROCK AVALANCHE

4

Extremely rapid

Slopes steep to very steep

Unsorted colluvium

Moist to very wet

6.0

Earthquake shaking, volcanic eruption, heavy rainstorm

2. Debris slide (debris avalanche)

1

Extremely rapid to rapid

Slopes steep to very steep

Colluvium

Wet to dry

4.0

Earthquake shaking, volcanic eruption, heavy rainstorm

3. Debris flow (debris avalanche)

2

Extremely rapid to rapid

Slopes steep to moderately steep

Colluvium

Very net to wet

NA

Earthquake shaking, volcanic eruption, heavy rainstorm

4. MUD FLOW

NA

Extremely rapid to rapid

Slopes steep to gentle

Colluvium, alluvium

Very wet

NA

Earthquake shaking: Landslides in saturated materials, landslides into or beneath water

5. Rock slump

3

Slow to rapid

Slopes steep to moderately steep

Unconsolidated to poorly consolidated bed-rock

Moist to wet

5.0

Groundwater rise; undercut/surcharge

6. EARTH SUMP

2

Slow to rapid

Slopes steep to moderately steep, sometimes gentle

Unconsolidated deposits

Moist to wet

4.5

Groundwater rise; undercut/surcharge

7. Earth flow (or mudslide)

3

Rapid to slow

Slopes steep to moderately steep, same-tunes gentle

Clay-bearing unconsolidated material

Wet to moist

5.0

Groundwater rise; undercut/surcharge

8. RAPID EARTH FLOW (including wet sane and silt flows)

31

Extremely rapid to rapid

Gentle slopes

Unconsolidated clay, silt, and sand

Very wet to wet

5.0

Earthquake or other dynamic lode, changes in pore water chemistry

OCCURS PARALLEL TO GEOLOGIC DISCONTINUITIES

9. Rock slide

1

Extremely rapid to moderate

Steep to very steeply dipping

Consolidated bed-rock

Wet to dry

4.0

Earthquake shaking; groundwater rise; frost/root wedging

10. Earth block slide

2

Extremely rapid to rapid

Gently, moderately or steeply dipping

Unconsolidated material

Wet to dry

4.5

Groundwater rise; undercut/surcharge

11. Rock block slide

4

Extremely rapid to rapid

Gently, moderately or steeply dipping

Consolidated bed-rock

Wet to dry

5.0

Groundwater rise; undercut/surcharge

12. EARTH LATERAL SPREAD

2

Extremely rapid to rapid

Flat to gently dipping

Unconsolidated material

Wet to moist

5.0

Earthquake shaking; undercut/surcharge

OCCURS IN STEEP LOCAL RELIEF

13. ROCK FALL

1

Extremely rapid to slow

Steep to vertical cliffs

Consolidated bed-rock

Moist to dry

4.0

Earthquake shaking; undercut/surcharge

14. Earth fall

5

Extremely rapid to slow

Bluff faces

Unconsolidated deposits

Moist to dry

4.0

Earthquake shaking; undercut/surcharge

15. Dry sand flow

5

Extremely rapid to rapid

Very steep to steep local relief

Unconsolidated deposits

Dry

NA

Earthquake shaking; undercut/surcharge

16. Loess flow

5

Extremely rapid to rapid

Very steep to steep local relief

Unconsolidated deposits

Dry

NA

Earthquake shaking; undercut/surcharge

a/ Figure does not include landslides for which movement is extremely slow or for which earthquake-induced occurrence is very rare.

b/ Degree of damage from earthquake-induced landslides: Landslide types presented in UNDERLINED CAPITALS - great damage; landslide types presented in CAPITALS - moderate damage; landslide types presented in lower case letters - little damage.

c/ The frequency scale used here is based on a survey of landslides associated with 40 historic earthquakes: (1) very abundant (more than 100,000 landslides associated with the 40 earthquakes); (2) abundant (10,000 landslides); (3) moderately common (1,000 to 10,000 landslides); (4) uncommon (100 to 1,000 landslides); (5) rare (less than 100 landslides); NA - data not available.

d/ All landslides shown can be induced by earthquakes. For some landslides, other trigger mechanisms are more common.

Source: Modified frail Campbell, R.H. et al. "Landslides Classification for Identification of Hid Flow and Other Landslide Hazards"; and Keefer, P.K. "Landslides Caused by Earthquakes" in Proceedings of the Geologic aid Hydrologic Training Program, Open File Report 84-760 (Reston, Virginia: U.S. Geological Survey, 1984).

Ground failures grouped as liquefaction can be subdivided into several types. The two most important are rapid earth flows and earth lateral spreads.

(1) Rapid Earth Flows: Rapid earth flows are the most catastrophic type of liquefaction. Large soil masses can move from tens of meters to several kilometers. These flows usually occur in loose saturated sands or silts on slopes of only a few degrees; yet they can carry boulders weighing hundreds of tons.

(2) Earth Lateral Spreads: The movement of surface blocks due to the liquefaction of subsurface layers usually occurs on gentle slopes (up to 3 degrees). Movement is usually a few meters but can also be tens of meters. These ground failures disrupt foundations, break pipelines, and compress or buckle engineered structures. Damage can be serious with displacements on the order of one or two meters.

In areas susceptible to earthquakes, liquefaction may be one of the most critical effects. Flow failure in loess (wind-blown silt) in the 1960 earthquake in China caused 200,000 deaths. Liquefaction was also a major factor in the earthquakes of 1960 in Chile and 1985 in Mexico and in major earthquakes in California, Alaska, India, and Japan.

In general, liquefaction can be prevented by ground-stabilization techniques or accommodated through appropriate engineering design, but both are expensive methods of mitigation. Avoidance is, of course, the best approach, but it is not always practical or possible in areas already developed or with existing transportation routes, pipelines, etc.

2. Earthquake Hazard Prediction, Assessment, and Mitigation

a. Earthquake Prediction
b. Seismic Risk Assessment
c. Earthquake Mitigation Measures

Minimizing or avoiding the risks from earthquakes involves three subject areas. First is the ability to predict their occurrence. While scientists cannot routinely predict earthquakes, this area is of growing interest and may be a key factor in reducing risks in the future. The second area is seismic risk assessment, which enables planners to identify areas at risk of earthquakes and/or their effects. This information is used to address the third area of earthquake risk reduction-mitigation measures. Following a discussion of prediction, assessment, and mitigation, the types and sources of earthquake information are presented.

a. Earthquake Prediction

A report on an erroneous prediction of an earthquake in Lima, Peru, states:

Earthquake prediction is still in a research and experimental phase. Although a few successful predictions have been made, reliable and accurate predictions having a long lead time, and useful location and magnitude estimates, are many years in the future (Gersony, 1982).

Some progress is being made in regional, long-term prediction and forecasting. "Seismic gaps" along major plate boundaries have been identified: areas with histories of prior large earthquakes (greater than 7 on the Richter scale-Ms7) and great earthquakes (Ms7.75) which have not had such an event for more than 30 years (McCann et al, 1979; Nishenko, 1985; and United Nations, 1978). Recent studies show that major earthquakes do not recur in the same place along faults until sufficient time has elapsed for stress to build up, usually a matter of several decades. In the main seismic regions, these "quiet" zones present the greatest danger of future earthquakes. Confirming the seismic gap theory, several gaps that had been identified near the coasts of Alaska, Mexico, and South America experienced large earthquakes during the past decade. Moreover, the behavior of some faults appears to be surprisingly constant: there are areas where earthquakes occur at the same place, but decades apart, and have nearly identical characteristics. Monitoring these seismic gaps, therefore, is an important component of learning more about earthquakes, predicting them, and preparing for future ones.

On the basis of the seismic gap theory, the U.S. Geological Survey has prepared maps of the coast of Chile and parts of Peru for the U.S. Agency for International Development's Office of Foreign Disaster Assistance (USAID/OFDA), adapted from a study by Stuart Nishenko (Nishenko, 1985). These maps give probability estimates and rank earthquake risk for the time period 1986 to 2006 (see Figure 11-6). USAID/OFDA has commissioned studies to produce similar information for the remainder of the Latin American Pacific coast.

It can be seen, however, that forecasting of this type only delineates relatively large areas in which an earthquake could potentially occur in a general future period of time. There have been successful earthquake predictions, but these are the exception rather than the rule. Earthquake prediction involves monitoring many aspects of the earth, including slight shifts in the ground, changes in water levels, and emission of gases from the earth, among other things. At this stage, it is still a young science.

Figure 11-6 RANKING OF RISK FROM SEISMIC GAPS OF THE CHILEAN SUBDUCTION ZONE

Source: Nishenko, Stuart P. "Seismic Potential for Large and Great Interplate Earthquakes Along the Chilean and Southern Peruvian Margins of South America: A Quantitative Reappraisal" in Journal of Geophysical Research, Vol. 90, No. B5 (April, 1985).

One successful short-term prediction is the often-mentioned case of Haicheng, China, in February 1975, in which people were evacuated six hours before a Ms7.3 earthquake struck. The worst-hit area was around the epicenter, where about 500,000 people lived, and half the buildings were damaged or destroyed. Among the indicators the Chinese had observed were changes in water level In deep wells, increased levels of radon gas, foreshocks, and unusual behavior of animals. Unfortunately, such successful predictions are offset by failures to predict: one year later in Tangshan, China, a great earthquake reportedly killed between 500,000 and 750,000 people.

b. Seismic Risk Assessment

A seismic risk assessment is defined as the evaluation of potential economic losses, loss of function, loss of confidence, fatalities, and injuries from earthquake hazards. Given the current state of knowledge of seismic phenomena, little can be done to modify the hazard by controlling tectonic processes, but there are a variety of ways to control the risk or exposure to seismic hazards. There are four steps involved in conducting a seismic risk assessment: (1) an evaluation of earthquake hazards and prepare hazard zonation maps; (2) an inventory of elements at risk, e.g., structures and population; (3) a vulnerability assessment; and (4) determination of levels of acceptable risk.

Evaluating Earthquake Hazards and Hazard Zonation Maps

In an earthquake-prone area, information will undoubtedly exist on past earthquakes and associated seismic hazards. This can be supplemented with existing geologic and geophysical information and field observation, if necessary. Depending on geologic conditions, some combination of ground shaking, surface faulting, landslides, liquefaction, and flooding w (covered in Chapter 8) may be the most serious potential earthquake-related hazards in an area. Maps should be prepared showing zones of these hazards according to their relative severity. These maps provide the planner with data on such considerations as the spatial application of building codes and the need for local landslide and flood protection. A composite map can be compiled showing the relative severity of all seismic hazards combined (see Chapter 6).

(1) Assessing Ground Shaking Potential: Even though ground shaking may cause the most widespread and destructive earthquake-related damage, it is one of the most difficult seismic hazards to predict and quantify. This is due to the amplification of the shaking effects by the unconsolidated material overlying the bedrock at a site and to the differential resistance of structures. Consequently, the ideal way to express ground shaking is in terms of the likely response of specific types of buildings. These are classified according to whether they are wood frame, single-story masonry, low-rise (3 to 5 stories), moderate-rise (6 to 15 stories), or high-rise (more than 15 stories). Each of these, in turn, can be translated into occupancy factors and generalized into land-use types.

Alternative approaches can be used for planning purposes to anticipate where ground shaking would be most severe:

- The preparation of intensity maps based on damage from past earthquakes rated according to the Modified Mercalli Index.

- The use of a design earthquake to compute intensity.

- In the absence of data for such approaches, the use of information on the causative fault, distance from the fault, and depth of soil overlying bedrock to estimate potential damage.

(2) Assessing Surface Faulting Potential: This is relatively easy to do, since surface faulting is associated with fault zones. Three factors are important in determining suitable mitigation measures: probability and extent of movement during a given time period, the type of movement (normal, reverse, or slip faulting), and the distance from the fault trace in which damage is likely to occur.

QUESTIONS PLANNERS NEED TO ASK TO EVALUATE EARTHQUAKE HAZARDS

- Where have earthquakes occurred in the region?

- How often do earthquakes of a certain magnitude occur?

- What kinds of seismic hazards are associated with the earthquakes?

- How severe have the hazards been in the past? How severe can they be in the future?

- How do the hazards vary spatially and temporally?

In areas of active faulting, fault maps should be prepared at scales appropriate for planning purpose (about 1:50,000 in developing areas and 1:10,000 in urban areas) and kept updated as new geologic and seismic information becomes available. The extent on the areas in jeopardy along the faults should be determined, and maps should be prepared showin the degree of hazard in each of them. Measures such as land-use zonation and building restrictions should be prescribed for areas in jeopardy.

(3) Assessing Ground Failure Potential: The mapping and evaluation of landslide hazards is described in Chapter 10. This method is applicable to earthquake-induced landslides. Liquefaction potential is determined in four steps: (1) a map of recent sediments is prepared, distinguishing areas that are likely to be subject to liquefaction from those that are unlikely; (2) a map showing depth to groundwater is prepared; (3) these two maps are combined to produce a "liquefaction susceptibility" map; and (4) a "liquefaction opportunity" is prepared by combining the susceptibility map with seismic data to show the distribution of probability that liquefaction will occur in a given time period.

Inventory of Elements at Risk

The inventory of elements at risk is a determination of the spatial distribution of structures and population exposed to the seismic hazards. It includes the built environment, e.g., buildings, utility transport lines, hydraulic structures, roads, bridges, dams; natural phenomena of value such as aquifers and natural levees; and population distribution and density. Lifelines, facilities for emergency response, and other critical facilities are suitably noted.

Vulnerability Assessment

Once an inventory is available, a vulnerability assessment can be made. This will measure the susceptibility of a structure or class of structures to damage. It is difficult, if not impossible, to predict the actual damage that will occur, since this will depend on an earthquake's epicenter, size, duration, etc. The best determination can be made by evaluating the damage caused by a past earthquake with known intensity in the area of interest and relating the results to existing structures.

Assessing Risk and Its Acceptability

It is theoretically possible to combine the hazard evaluation with the determination of the vulnerability of elements at risk to arrive at an assessment of specific risk, a measure of the willingness of the public to incur costs to reduce risk. This is a difficult and expensive process, however, applicable to advanced stages of the development planning process. For any particular situation, planners and hazard experts working together may be able to devise suitable alternative procedures that will identify approximate risk and provide technical guidance to the political decisions as to what levels are acceptable and what would be acceptable costs to reduce the risk. Thus, the appropriate mitigation measures can be recommended as part of a development study. Sub-section 4, "Earthquake Hazards and the Development Planning Process," provides a more detailed discussion of where seismic risk assessment fits into the development planning process.

c. Earthquake Mitigation Measures

There is no question that earthquake damage can be reduced. The questions are what techniques or mechanisms are appropriate in a given situation and how can they be applied. The range of mechanisms includes land-use zoning; engineering approaches such as building codes, strengthening of existing structures, stabilizing unstable ground, redevelopment; the establishment of warning systems; and the distribution of losses. In keeping with this chapter's focus on hazard-related aspects of the early phases of development studies, this sub-section discusses only the land use, or nonstructural, mechanisms such as avoiding hazardous areas or restricting types and intensities of land use. See Chapter 3 for a detailed discussion of land-use evaluation and hazards.

Some of these mitigation measures are applicable to new development, some to existing development, and some to both. Consideration must be given to the administrative and political aspects of applying mitigation techniques such as obtaining community support, mobilizing local interests, and incorporating the seismic aspects into a comprehensive zoning ordinance. These issues are discussed amply in a number of publications, including those listed in the box below. The mitigation measures included in this discussion focus on the most important earthquake-related hazards-ground shaking, surface faulting, landslides, and liquefaction. Again, mitigation measures for floods induced by earthquakes are the same as for flooding induced by any other cause. These are discussed in Chapter 8.

Ground Shaking Mitigation Measures

Once the potential severity and effects of ground shaking are established as explained above, several types of seismic zoning measures can be applied. These include:

- Relating general ground shaking potential to allowable density of building occupancy.

- Relating building design and construction standards to the degree of ground shaking risk.

- Adopting ordinances that require geologic and seismic site investigations before development proposals can be approved.

- In areas already developed, adopting a hazardous building abatement ordinance and an ordinance to require removal of dangerous parapets.

ADDITIONAL EARTHQUAKE INFORMATION

The following publications are useful sources of information on earthquakes, other geologic hazards, and their mitigation:

UNDRO. Disaster Prevention and Mitigation: Land Use Aspects (New York: United Nations, 1978).

W. W. Hays (ed.) Facing Geologic and Hydrologic Hazards: Earth Science Considerations. U.S. Geological Survey Professional Paper 1240-B (Reston, Virginia: U.S. Geological Survey, 1981).

U.S. Geological Survey. Proceedings of the Geologic and Hydrologic Hazards Training Program, March 5-30, 1984, Denver, Colorado, Open File Report 84-760 (Reston, Virginia: U.S. Geological Survey, 1984).

W. W. Kockelman. Examples of Use of Geologic and Seismological Information for Earthquake Hazard Reduction in Southern California. U.S. Geological Survey Open File Report 83-82 (Reston, Virginia: U.S. Geological Survey, 1983).

Surface Faulting Mitigation Measures

Since fault zones are relatively easy to delineate, they lend themselves to effective land-use planning. Where assessment of the consequences of surface rupture indicates an unacceptably high possibility of damage, several alternative mitigation measures are available:

- Restricting permissible uses to those compatible with the hazard, i.e., open space and recreation areas, freeways, parking lots, cemeteries, solid-waste disposal sites, etc.

- Establishing an easement that requires a setback distance from active fault traces.

- Prohibiting all uses except utility or transportation facilities in areas of extremely high hazard, and setting tight design and construction standards for utility systems traversing active fault zones.

Ground Failure Mitigation Measures

Land-use measures to reduce potential damage due to landslides or liquefaction are similar to those taken for other geologic hazards: land uses can be restricted, geologic investigations can be required before development is allowed, and grading and foundation design can be regulated. Stability categories can be established and land uses commensurate with these categories can be recommended or ordained. Land-use zoning may not be appropriate in some areas because of the potential for substantial variation within each mapped unit, but even without mandatory use restrictions, stability categories can indicate the precautions appropriate for the use of any parcel of land.

General Land-Use Measures

Where development has already taken place in areas prone to earthquake hazards, measures can be adopted to identify unsafe structures and ordain their removal, starting with those that endanger the greatest number of lives. Tax incentives can be established for the removal of hazardous buildings, and urban renewal policies should restrict reconstruction in hazardous areas after earthquake destruction. The political acceptability of zoning measures can be increased by developing policies which combine earthquake hazards with other land-use considerations.

3. Types and Sources of Earthquake Information

a. Information on Earthquakes (occurrence, size, characteristic effects, relation to geologic features)
b. Information on Seismic Hazards
c. Information on Seismic Risk and Vulnerability
d. Data Substitution

The following categories of information are discussed below: information on the occurrence of earthquakes as hazardous geologic events; information on the hazards or effects of earthquakes; information for assessing seismic risk and undertaking a vulnerability assessment; and substitute data in the absence of other information. This sub-section is intended to identify for the planner the kind of information that might be available, where it might be sought, what it might be used for, and, if it does not exist, what other information can be helpful.

a. Information on Earthquakes (occurrence, size, characteristic effects, relation to geologic features)

Seismic Information

This information covers the occurrence of historic earthquakes and their characteristics.

(1) Earthquake Catalogues: There are several kinds of earthquake catalogues, covering world-wide, regional, national, or more geographically restricted occurrences. Earthquake catalogues normally provide information on location, time, and size for each earthquake recorded and an estimate of the completeness of the seismic record. Of particular relevance is the catalogue published in December 1985 by CERESIS, which includes entries for all earthquakes recorded in South America since 1900 and historical earthquakes dating back 400 years. This catalogue is accompanied by two maps: Seismicity (epicenters) and Large Earthquakes of South America (1520-1981).

(2) Maps Showing Damage Caused by Earthquakes; Maps of Notable or Historic Earthquakes: The U.S. Geological Survey has published such maps for South America and Middle America. The National Oceanic and Atmospheric Administration (NOAA) of the U.S. Department of Commerce has published a world map of significant earthquakes between 1900 and 1979. National maps are also available for some countries.

(3) Epicenter Maps, Data on Hypocenters (Earthquake Foci), Maps and Data on Earthquake Magnitude and Peak Horizontal Ground Acceleration, and Earthquake Recurrence Data: Seismicity information is available from a variety of sources such as CERESIS, national geologic surveys, national agencies of disaster preparedness, USAID/OFDA, and the United Nations Disaster Relief Coordinator (UNDRO).

Seismotectonic Information

This information covers indicators of seismic activity.

(1) Continental and Subregional Seismotectonic Maps: Geologic maps showing seismic indicators such as faults, volcanoes, hot springs and uplifted or down-dropped tectonic blocks are available from hemispheric, regional, and national geologic information sources. In 1985 CERESIS published a Neotectonic Map of South America at a scale of 1:5,000,000, and each of the participating countries produced similar national maps at a scale of 1:2,000,000. For planners, this map is particularly useful for delimiting areas prone to volcanic eruptions. (See Section C of this chapter, "Volcanic Eruptions.") A seismicity map of Middle America is also available.

(2) Seismic Provinces and Source Zones; Macrozonation Maps: Some large countries or those having markedly differentiated geology may be regionalized according to seismic hazard. The principal function of these maps is to distinguish areas with relatively minor hazard from those having a great hazard which requires mitigation. Argentina, for example, is divided into eleven seismotectonic provinces. One advantage of such information is that it permits the setting of priorities for subsequent assessment work. In Argentina the most seismically active provinces were studied to determine the nature and degree of specific seismic hazards in preparation for undertaking mitigation activities. Information on seismic provinces and source zones is available through national disaster mitigation agencies.

(3) Geologic and Geophysical Information: A wide range of geologic information is applicable to the determination of seismic hazard, including surface and subsurface geology (age and rock type), structural geology, stratigraphy, and tectonics. The mapping of quaternary sedimentary geology is useful for determining the liquefaction potential. Fault mapping can be used to approximate seismic parameters in lieu of other data. Geologic information is available from national, state, or urban municipal governments, universities, and private oil, mineral, and engineering firms.

High-resolution seismic reflecting surveys, gravity maps, magnetic maps, and seismic refraction surveys are useful in supplementing or substituting for geologic information in the delineation of seismotectonic features. In addition, information concerning "gaps" in active fault zones provides one of the most widely used means of forecasting earthquakes. National geologic survey agencies are sources of geophysical information, as are mineral and oil exploration companies and repositories of satellite imagery. See Chapter 4 for information on where to obtain satellite imagery.

b. Information on Seismic Hazards

This information covers maps and data on the effects of earthquakes.

Ground Shaking

(1) Intensity and Magnitude Data: The available information includes maps of maximum seismic intensities, intensity observations and maps of intensity distribution, and calculations of the magnitude's upper bound. Mapped data can range in scale from 1:5,000,000 maps suitable for delineating seismic provinces to large scale maps of 1:5,000 suitable for detailed land-use planning for seismic hazards.

In 1985 CERESIS published a Maximum Intensities Map of South America showing the geographic distribution of Modified Mercalli Index units. (See the discussion below on "Earthquake Hazards and the Development Planning Process.") Again, the continental map scale is 1:5,000,000, and the national map scale is 1:2,000,000. Peak ground acceleration can be used as a measure of the severity of ground motion. Maps showing intensity distribution can be constructed by using mathematical models and by plotting historical data.

(2) Seismic Attenuation Data: These include isoseismal maps (maps showing equal-intensity contours) of significant historic earthquakes and strong-motion accelerogram records when available. Since detailed strong-motion data are frequently not available, an interpretation of intensity distribution on isoseismal maps may be used as a substitute.

(3) Site Response Data: There are two general types of site response data: (1) observations of the effects of past earthquakes which correlate the total ground shaking (acceleration in the bedrock plus amplification effects) with the damage caused at a site; and (2) response spectra obtained from accelerogram data or theoretical calculations.

Ground Failure: Landslides and Liquefaction

(1) Continental Map: CERESIS published a map of potential landsliding and liquefaction in South America at a scale of 1:10,000,000 in 1985.

(2) Other Landslide Hazard Information: National maps of the occurrence of landslides and liquefaction at a scale of 1:2,000,000 have been produced by the South American countries under the coordination of CERESIS. Maps or reports on the potential for landslides at the local level are found in some urban areas that have a high potential for the hazard. In the absence of such data, slope maps and geology maps, together with information on soils, topography, vegetation, rainfall regime, groundwater level, and present land use can be used to estimate the landslide hazard. Chapter 10 describes the method to use for this purpose.

(3) Other Liquefaction Information: Liquefaction hazard maps can most commonly be found at the disaster preparedness agency of urban municipal governments. In the absence of such information, the liquefaction hazard can be estimated using maps of Holocene or Quaternary geology or other depictions of recent sediment deposition and maps of groundwater depth.

Other Data

Geologic, seismotectonic, and geophysical data can be used to evaluate the potential for surface faulting. Bathymetric data can be used to estimate tsunami runup in coastal areas and, together with offshore geologic information, provide insights into the potential for subaqueous landslides caused by earthquakes.

c. Information on Seismic Risk and Vulnerability

This information covers maps, records, reports, and data useful for making a seismic risk or hazard assessment and vulnerability analysis.

(1) Seismic Microzonation Maps: These large-scale maps delineate areas according to their seismic risk potential and are useful for estimating the population and property at risk and designating the land uses and earthquake-resistant structural designs appropriate for each land unit. Such maps are only rarely available, usually for metropolitan areas having a history of seismic events. Information for estimating lives and property at risk can be derived from census and land-use data.

(2) Instrumental Site Response Records: These provide information on how the bedrock and soil cover will influence seismic waves at a site. This information is necessary for determining suitable design parameters.

(3) Geotechnic Properties of Materials at Shallow Depth and Earthquake Damage Assessment Reports: In the absence of more definitive information on the seismic hazard of a site, this type of information can be used to make a rough evaluation of the hazard.

(4) Building Codes and Regulations: These legal statutes, which relate seismic hazard mitigation requirements to the degree of exposure to seismic hazards and type of construction, are normally prepared at the city, county, or provincial level, but national guidelines are often available as well.

d. Data Substitution

As was said above, when the necessary information is not available, an approximation can often be made by interpretation from information that is available. Here are some examples: Maximum intensity can be estimated for sites from data on earthquake magnitude in specific areas. Magnitude or intensity can be estimated from the length of the causative fault, and peak ground acceleration can be estimated from the magnitude and distance from the causative fault. Duration of shaking can be correlated with magnitude (Hays, 1980).

4. Earthquake Hazards and the Development Planning Process

a. Preliminary Mission
b. Phase I: Development Diagnosis
c. Phase II: Development Strategy and Project Formulation
d. Project Implementation

a. Preliminary Mission

In the Preliminary Mission of a development planning study, the planner wants to know if earthquakes pose enough of a great threat in all or part of the study area to warrant consideration in development planning. If it can be established that they do not, the planner can move on to other issues. See Figure 11-7 for a diagram of the relationship between development planning and the assessment of earthquake hazards.

PRELIMINARY MISSION (STUDY DESIGN)

QUESTIONS PLANNERS NEED TO ASK:

- Is there a history of significant earthquakes (Modified Mercalli Index of VI or greater or magnitude of 4 or greater) in the study area?

- In what part of the study area did they. or are they likely to, occur? Is there a significant population or infrastructure at risk?

- What is the likelihood of such a quake occurring during the timeframe of the development project (50-100 years)? What hazards are associated with these earthquakes: ground shaking, surface rupture, liquefaction, landslides, other?

- If earthquakes pose a threat in all or part of the study area, is there a national agency or institution that is responsible for data collection, monitoring, hazard mitigation, disaster planning?

KEY DECISIONS TO BE MADE AT THIS STAGE:

- Earthquakes are (or are not) a significant threat in the study area, and therefore consideration of earthquake hazards should (or should not) be included in the development planning process.

- If the information available is insufficient for making a recommendation on the above decision in the Preliminary Mission, Phase I should include a data collection effort designed to permit such a recommendation.

- If earthquake hazards are found to constitute a significant threat in the study area, mitigation efforts should be built into the study design.

Two kinds of information are needed to evaluate an earthquake threat: (1) the potential severity of an earthquake; and (2) the likelihood of a damaging earthquake during the timeframe of a project or planning application. While both are necessary for a full evaluation, one or the other may not exist for a given location, in which case a partial evaluation can be made with the information available.

Potential severity is usually defined historically; that is, the largest earthquake determined to have occurred in an area is taken as the maximum that is likely to occur there again. The severity of an earthquake can be measured in terms of intensity by the Modified Mercalli Index (MMI) or in terms of magnitude by the Richter Scale (see Figure 11-2 and the discussion of earthquake severity in sub-section B.1 above). In the figures below, MMI is used for South America and Middle America (Mexico to Panama) and Ms (surface-wave magnitude) for the Caribbean. MMI of VI or greater and Ms of 4 or greater are taken to indicate significant threat.

The likelihood of occurrence is measured in terms of conditional probability or seismic potential. Conditional probability is an estimate, expressed as a percentage, of how likely it is that a large or great earthquake will occur within a specified time period. In the following tables and maps, data generated by Stuart Nishenko of the U.S. Geological Survey are used for South and Middle America. William McCann of the University of Puerto Rico defines seismic potential as the likelihood of occurrence of a large or great earthquake in the near future and provides values for the Caribbean on a scale of 1 (high potential) to 6 (low potential).

The information presented below will usually be sufficient to guide a planner in answering the questions and arriving at decisions for the preliminary mission indicated in the box above.

South America

The best source of information for determining the seismic threat of an area is a seismic zone map, available for several South American countries such as Argentina, Peru, and Venezuela. The planner can locate the study area on such a map and use the seismic hazard ratings indicated. When a seismic zone map is unavailable, the CERESIS Map of Maximum Intensities covering all of South America can be used. More detail can be obtained from the national maps of maximum intensities available for individual South American countries. Figure 11-8 shows the geographic distribution of the historical occurrence of maximum intensities of VI or greater by political subdivision in South America. The figure also indicates the occurrence of soil liquefaction and significant landslides, although there may be other areas prone to landslides or liquefaction not shown on the table. Figure 11-9 shows by province and department the conditional probabilities of an earthquake occurring in the next 5, 10, or 20 years and the maximum likely intensity of such a shock.

Figure 11-7 RELATIONSHIP OF DEVELOPMENT PLANNING STAGES TO EARTHQUAKE-RELATED HAZARD ASSESSMENT (Showing Direct and Indirect Sources of Information)

PRELIMINARY MISSION:
PROCEDURE FOR INITIAL SEISMIC HAZARD ASSESSMENT IN LATIN AMERICA AND THE CARIBBEAN

INITIAL REVIEW

- For French Guiana, Paraguay, and Suriname: seismic hazards are not a significant threat.

- For Brazil and Uruguay: GO TO NOTE 1.

- For Barbados, St. Vincent and the Grenadines, and Trinidad and Tobago: GO TO NOTE 2.

- For Argentina, Bolivia, Chile, Colombia, Ecuador, Guyana, Peru, Venezuela; and

- Belize, Costa Rica, El Salvador, Guatemala, Honduras, Mexico. Nicaragua, Panama; and

- Antigua and Barbuda, Cuba, Dominica, Dominican Republic, Grenada, Guadeloupe, Haiti, Jamaica, Martinique, Montserrat, Puerto Rico, St. Bans, Saint Eustatius, Saint Lucia, the Virgin Islands; GO TO STEP 1.

NOTE 1 - In Uruguay, seismic hazards constitute a minor threat in the Department of Artigas. In Brazil, they constitute a minor threat in the states of Bahia, Ceará, Minas Gerais, Rio Grande do Norte, Santa Catarina. and Sao Paulo (see Figure 11-8). Elsewhere In these countries they are not considered a significant threat. They do not require study in the Preliminary Mission or in Phase I, but consideration of seismic hazards should be incorporated into the Project Formulation and Implementation phases of specific projects.

NOTE 2 - For these countries there is no record of large shocks (they are considered to have a relatively low potential for a large or great earthquake). There is no information : on estimated maximum magnitude (see Figure 11-17). Earthquakes probably do not | constitute a major threat.

STEP 1 - Determine if the study area is included in areas of MMI of VI or greater or of magnitude of 4 or greater on the maps and tables presented below. If not, earthquakes do not pose a significant hazard in the study area. If yes, go to Step 2.

STEP 2 - Incorporate seismic hazards in phases I and II of the planning study. Include suitable mitigation measures commensurate with historic maximum intensity. Conduct studies of specific seismic hazards, initiate earthquake zoning, modify building codes to incorporate seismic considerations, and recommend emergency preparedness actions. (Before undertaking these actions, go to Step 3.)

STEP 3 - Determine probability of a large or great earthquake occurring in the near future (see Figures 11-9, 11-11,11-14 and 11-17). If the area has a high probability of a large earthquake in the near future, the actions described in Step 2 should be undertaken with greater urgency.

Figure 11-8 - GEOGRAPHIC DISTRIBUTION OF MAXIMUM EARTHQUAKE INTENSITIES, SOIL LIQUEFACTION, AND SIGNIFICANT LANDSLIDES IN SOUTH AMERICA

Location

Maximum Earthquake Intensity

A - Soil Liquefaction

VI

VII

VIII

IX

X

XI

B - Signification Landslide

ARGENTINA


Province









Catamarca

x

x







Chaco

x








Chubut

x








Córdoba

x

x


L





Corrientes

x








Entre Ríos

x








Jujuy

x

x

x




A, B


La Rioja

x

x

x

L



B


Mendoza

x

x

x

x



A, B


Neuquen

x

x







Río Negro

x








Salta

x

x

x

x



A, B


San Juan


x

x

x



A, B


San Luis

x

x

x






Santa Cruz

x








Santiago del Estero

x








Tierra de Fuego

x








Tucumán

x


L





BOLIVIA


Department









Cochabamba

x

x





B


Chuquisaca

x

x

x




A, B


La Paz

x

x

x




B


Oruro

x

x







Pando









Potosi

x

x

x




B


Santa Cruz

x

x





A


Tarija

x

x

x

x




BRASIL


State









Bahia

L

L







Ceará

x

L







Minas Gerais

L

L







Rio Grande de Norte

L








Santa Catarina

x








São Paulo

L







CHILE


Province









Aisén

x








Aconcagua


x

x

x

x




Antofagasta

x

x

x






Arauco



x




A, B


Atacama

x

x

x

x

L




Bio Bio

x

x

x






Cautin

x

x

x




A, B


Chiloé

x

x

x






Colchagua

x

x

x






Concepción



x

x





Coquimbo


x

x

x





Curicó

x

x

x






Linares

x

x

x

x





Llanquihue

x

x

x



L

A, B


Magallanes

x

x







Malleco

x

x

x


L




Maule



x

x

L




Nuble

x

x

x

x

L




O'Higgins

x

x

x






Osorno

x

x

x




B


Santiago


x

x

x

L


B


Talca

x

x

x

x





Tarapacá


x

x

x



B


Valdivia

x

x

x


L


A, B


Valparaiso



x

x

x

L


COLOMBIA


Department









Antioquia

x

x

x

L



A, B


Arauca

x

x

x






Atlántico

x

x







Bolivar

x

x

x






Boyacá

x

x

x






Caldas



x

x



B


Caquetá

x

x

x




B


Cauca

x

x

x

x

x


B


Chocó


x

x

x



A, B


Córdoba

x

x

x






Cundinamarca


x

x

x



B


Guajira

x

x

x






Huila


x

x

x

x


A, B


Magdalena

x

x

x




B


Meta

x

x

x

x



B


Nariño


x

x

x

x


B


Norte de Santander


x

x

x

x


B


Putumayo

x

x





B


Santander

x

x

x

x



B


Tolima


x

x

x





Valle del Cauca


x

x

x





Vaupes

x








Vichada

x







ECUADOR


Province









Azuay


x

x






Bolivar



x

x



B


Cañar


x







Carchi


x

x

x





Chimborazo



x

x

x

L

A, B


Cotopaxi


x

x

x

x

L



El Oro



x




B


Esmeraldas


x

x

x

x




Guayas


x

x

x



A, B


Imbabura


x

x

x





Los Rios


x

x






Loja

x

x

x






Manabí



x

x





Morona Santiago

x

x

x






Napo

x

x

x




B


Pastaza

x

x







Pichincha


x

x

x

L




Tungurahua


x

x

x

x

L

B


Zamora- Chinchipe


x

x





GUYANA


x

x





PERU


Department









Amazonas

x

x

x

x



B


Ancash

x

x

x

x


L

A, B


Apurimac


x

x






Arequipa

x

x

x

x

x

L

B-B


Ayacucho

x

x

x

x





Cajamarca

x

x


L



B


Cuzco

x

x

x

x



A


Huancavelica

x

x

x




B


Huánuco

x

x

x






Ica


x

x

x

L


A


Junin

x

x

x

x

L

L



La Libertad

x

x

x

x





Lambayeque

x

x

x






Lima

x

x

x

x

L

L

A


Loreto

x

x

x

x

x




Madre de Dios

x

x







Moquegua


x

x

x


L



Pasco

x

x

x

L





Piura


x

x

x



A


Puno

x

x

x






San Martin

x

x

x

x

x


A


Tacna


x

x

x





Tumbes



x

x



A, B

URUGUAY


Department









Artigas

x







VENEZUELA


State









Delta Amacuro

x

x

x

x



A


Amazonas

x








Apure

x

x

x






Aragua


x

x

x



A


Anzoátegui

x

x

x

x



B


Barinas


x

x

x





Bolívar

x

x

x






Carabobo


x

x




A


Cojedes


x

x




B


Distrito Federal




x

L


B


Falcón


x

x




B


Guarico

x

x

x




B


Lara




x

x




Mérida




x

L


A. B


Miranda



x

x

x


A, B


Monagas



x

x



A


Portuegesa


x

x

x



B


Sucre




x

L


A, B


Táchira



x

x

x


B


Trujillo



x

x



A, B


Yaracuv



x

x





Zulia


x

x

x



A, B

Legend:

x = Contour value covering all or part of the area.
L = Observed localized Intensity greater than the contour values.

Source: Adapted from Regional Seismological Center for South America (CERESIS). Maximum Intensity Map of South Ana-tea (Santiago, Chile: CERESIS, 1985); and Regional Seismological Center for South America. Map of Soil Liquifaction and landslides Associated with Earthquakes in South America (CERESIS, 1985).

Figure 11-9 - MAXIMUM SEISMIC INTENSITY AND CONDITIONAL PROBABILITY OF OCCURRENCE OF A LARGE OR GREAT EARTHQUAKE FOR COASTAL LOCATIONS IN SOUTH AMERICA

Location

Maximum Likely Seismic Intensity

Conditional Probability a/

1989-1994 (%)

1989-1999 (%)

1989-2009 (%)

COLOMBIA


Department






Cauca

X

£1

£1

£1


Chocó

IX

?

?

?


Nariño





 

North

X

£1

£1

£1

 

South

X

8

19

6


Valle

IX

£1

£1

£1

CHILE


Province






Aconcagua

X

£1

£1

£1


Aisén





 

North

VI

£1

£1

£1

 

South

VI

?

?

?


Antofagasta





 

North

VIII

(10)

(20)

(39)

 

South

VIII

£1

£1

15


Arauco

VIII

1

3

12


Atacama





 

North

IX

£1

£1

15

 

South

IX

2

4

10


Cautín

VIII

1

3

12


Chiloé

VIII





Colchagua





 

North

VIII

£1

£1

£1

 

South

VIII

17

33

59


Concepción

IX

1

3

12


Coquimbo





 

North

IX

2

4

10

 

South

IX

11

24

49


Curicó

VIII

£17

£33

£59


Llanquihue

VIII

£1

£1

£1


Magallanes





 

North

VII

?

?

?

 

South

VII

4

11

29


Maule

IX

1

3

12


Ñuble

IX

1

3

12


Osorno

VIII

£1

£1

£1


Santiago

IX

£1

£1

£1


Talca

IX

1

3

12


Tarapacá

IX

10

20

39


Valdivia





 

North

VIII

1

3

2

 

South

VIII

£1

£1

£1


Valparaíso

X

£1

£1

£1

ECUADOR


Province






El Oro

VIII

?

?

?


Esmeraldas

X

(41)

(66)

(90)


Guayas

IX

?

?

?


Manabí





 

North

IX

(41)

(66)

(90)

 

South

IX

?

?

?

PERU


Department






Ancash





 

North

IX

?

?

?

 

South

IX

£1-3

£1-8

1-24


Arequipa





 

North

X

(£1)

(£1)

(£1)

 

Central

X

6

13

29

 

South

X

(£1-12)

(£1-23)

(£1-43)


Ica





 

North

IX

(14)

(27)

(47)

 

South

IX

(£1)

(£1)

(£1)


La Libertad

IX

?

?

?


Lambayeque

VIII

?

?

?


Lima





 

North

IX

£1-3

£1-8

1-24

 

South

IX

£1

£1

£1


Moquegua

IX

(£1-12)

(£1-23)

(£1-43)


Piura

VIII

?

?

?


Tacna





 

North

IX

(£1-12)

(£1-23)

(£1-43)

 

South

IX

4

11

29


Tumbes

IX

?

?

?

a/ Conditional probability refers to earthquakes caused by inter-plate movement.
? No information available.
() All values in parenthesis represent less reliable estimates.

Source: Adapted from Regional Seismological Center for South America (CERESIS). Maximum Intensity Map of South America (Santiago, Chile: CERESIS, 1985), and Nishenko, S.P. Summary of Circum-Pacific Probability Estimates (unpublished table) (Golden, Colorado: U.S. Geological Survey, 1989).

Middle America

Information for seismic hazard evaluation of Mexico includes published data on the Pacific Coast subduction zone and mapped data on maximum acceleration and maximum velocity of earthquake waves expected in 50-year, 100-year, and 500-year periods, available from the Institute de Ingeniería of the Universidad Nacional Autónoma de México. For unpublished information planners should contact the Instituto de Geofísica and the Institute de Ingenieria, Universidad Nacional Autónoma de México; the U.S. Geological Survey; the Institute for Geophysics at the University of California, Santa Cruz; or the Lamont Dougherty Geological Observatory in Palisades, New York.

Figure 11-10 presents the conditional probability of a large or great earthquake between 1986 and 1996 along the 13 segments of the Mexican Subduction Zone (Nishenko and Singh, 1987). The data on these segments are summarized in Figure 11-11.

New maps and reports by Randall White and Stuart Nishenko, both of the U.S. Geological Survey, on seismic hazards in Central America became available in 1988 and 1989. Figure 11-12 is a map of the region showing MMI values recorded historically and carefully analyzed by White. Figure 11-13 shows the occurrence by province and department of MMI of VI or greater for Belize, Costa Rica, El Salvador, Guatemala, Honduras, and Nicaragua (information on Panama is still limited). Figure 11-14 shows conditional probability and MMI by province and department.

Caribbean

Information for estimating seismic hazards of the Caribbean Basin is available from William McCann from the Department of Geology of the University of Puerto Rico. Figure 11-15, a map of seismic potential in the Caribbean region, rates potential in terms of the length of time since the last large earthquake (McCann, 1987). The map is a composite of two earlier ones: the 1982 Seismic Potential Map, which covers the whole basin, and the 1983 Seismic Potential Map, covering the area from Dominica to the eastern portion of the Dominican Republic.

Long-term seismic activity expressed in terms of magnitude is shown in Figure 11-16 from a 1984 map which covers the same area as the 1983 map. This map is close to the "model sources" map needed by engineers to estimate ground shaking (McCann, 1985). Combining data from two maps, Figure 11-17 categorizes the Caribbean countries in terms of their seismic potential (likelihood of experiencing a large earthquake) and long-term seismic activity (likely maximum size of an earthquake).

McCann, who has written extensively on Eastern Caribbean seismology, emphasizes:

The eastern margin of the Caribbean from Trinidad to Puerto Rico has the potential for large earthquakes. The northern Lesser Antilles is in the category of highest seismic potential. This region appears ripe for a major earthquake. While this estimate may have an error of 25 years or more, it does indicate the strong likelihood of a great earthquake before the turn of the century in the northern Lesser Antilles (McCann, unpublished paper).

This is obviously a consideration that planners should bear in mind.

b. Phase I: Development Diagnosis

This phase of a planning study requires the diagnosis of a region, including spatial and natural resource stet. Further evaluation of earthquake-prone areas and areas where the data are insufficient for evaluating the seismic threat depends on the type of development programmed for the area and resource allocation considerations for subsequent studies. Parallel to the hazard studies, the planning study team may subdivide the study area into present and near-future use-intensity zones, and hazard studies should proceed in selected zones according to the judgment of the planner. Recommended mapping scales for this work are 1:250,000 to 1:50,000.

The evaluation and zoning of specific seismic hazards is conducted as an element of the natural resource evaluation in the selected areas. The hazards can be grouped as follows: ground shaking/fault rupture, landslides, and liquefaction. Since it has already been established that the selected areas are earthquake-prone, seismic data such as intensity and magnitude, strong motion, attenuation, site response, fault movement, and damage reports of past earthquakes are likely to be available, as are data on landslides and/or liquefaction. At this point a substantial effort in the collection of existing data is justified.

If the existing data on seismic hazards are inadequate, the use of some substitute data can provide adequate information for this stage of evaluations. See the discussion on data substitution in the preceding sub-section, and Chapter 10 for a discussion of the factors associated with landslide activity. An indication of the susceptibility to liquefaction can be estimated from data on Holocene sedimentary geology and depth to groundwater.

Figure 11-10 - CONDITIONAL PROBABILITY ESTIMATES ALONG THE MEXICAN SUBDUCTION ZONE

Map view of the conditional probability of the occurrence of a large or great earthquake in 13 segments of the Mexican Subduction Zone for the time interval 1986-1996. Probabilities are based on estimates of the expected recurrence time using either historically observed data or recurrence behavior extrapolated from other segments. Segmentation into separate source zones is based on the lateral extent of the last large or great earthquake in each segment.

Source: Adapted from Nishenko, S.P., and Singh, S.K. "Conditional Probabilities for the Recurrence of Large and Great Interplate Earthquakes Along the Mexican Subduction Zone," submitted to the Bulletin of the Seismological Society of America (1987).

Figure 11-11 - CONDITIONAL PROBABILITY OF A LARGE OR GREAT EARTHQUAKE ALONG THE MEXICAN SUBDUCTION ZONE a/

Figure 11-12 MAXIMUM SEISMIC INTENSITIES IN CENTRAL AMERICA

Source: Based on White, Randall A. Maximum Earthquake Intensities in Central America (draft map) (Menlo Park, California- US Geological Survey, 1988).

Figure 11-13 GEOGRAPHIC DISTRIBUTION OF MAXIMUM EARTHQUAKE INTENSITIES IN CENTRAL AMERICA

Location

Maximum Earthquake Intensity

VI

VII

VIII

IX

X

BELIZE


District







Stann Creek

x






Toledo

x

x




COSTA RICA


Province







Alajuela

x

x

x




Cartago


x

x




Guanacaste


x

x




Heredia

x

x

x




Limón

x

x

x




Puntarenas

x

x

x




San José

x

x

x



EL SALVADOR


Department







Ahuachapán


x

x




Cabañas

x

x





Chalatenango

x

x





Cuscatlán

x

x





La Libertad


x

x




La Paz


x

x




La Unión

x

x

x




Morazán


x

x




San Miguel

x

x

x




San Salvador

x

x

x




San Vicente


x

x




Santa Ana

x

x

x




Sonsonate


x

x




Usulatán


x

x



GUATEMALA


Department







Alta Verapaz

x

x

x




Baja Verapaz


x

x




Chimaltenango


x

x




Chiquimula


x

x




El Petén

x






El Progreso

x

x

x




El Quiché

x

x

x




Escuintla


x

x




Guatemala


x

x

x



Huehuetenango


x

x

x



Izabal


x

x




Jalapa

x

x





Jutiapa


x

x




Quezaltenango


x

x

x



Retalhuleu


x

x




Sacatepéquez



x




San Marcos



x

x



Santa Rosa


x

x

x



Sololá


x

x




Suchitepéquez


x

x




Totonicapán


x

x




Zacapa


x

x



HONDURAS


Department







Atlántida

x

x

x




Choluteca

x

x





Colón

x

x





Comayagua

x

x

x




Copán

x

x

x




Cortes

x

x

x




Distrito Central

x






El Paraíso

x






Fco. Morazán

x






Gracias a Dios

x






Intibuca

x

x

x




La Paz

x






Lempira

x

x

x




Ocotepeque

x

x

x




Olancho

x






Santa Barbara

x

x

x




Valle

x

x





Yoro

x





NICARAGUA


Department







Boaco

x






Carazo


x





Chinandega

x

x

x




Chontales

x






Granada


x

x




León

x

x

x




Managua

x

x

x




Masaya


x

x




Matagalpa

x






Rio San Juan

x






Rivas


x




Source: Adapted from White, R.A. Maximum Earthquake Intensities in Central America (unpublished map). (Menlo Park, California: U.S. Geological Survey, 1988).

Figure 11-14 MAXIMUM SEISMIC INTENSITY AND CONDITIONAL PROBABILITY OF OCCURRENCE OF A LARGE OR GREAT EARTHQUAKE FOR SELECTED LOCATIONS IN CENTRAL AMERICA

Location

Maximum Likely Seismic Intensity

Conditional Probability a/

1989-1994 (%)

1989-1999 (%)

1989-2009 (%)

COSTA RICA


Province






Alajuela





 

West

VIII

9

43

93

 

Central and East

VIII

£1-3

£1-8

4-25


Guanacaste





 

West

VIII

16

31

55

 

East

VIII

9

43

93


Heredia (West)

VIII

£1

£1

£4


Puntarenas





 

North

VIII

3-9

8-43

25-93

 

Central

VIII

£1

£1

£4


San José (West)

VIII

£1

£1

£4

EL SALVADOR


Department






Ahuachapán

VIII

29

51

79


Cabañas

VII

£1

£1

£1


Cuscatlán

VII

29

51

79


La Libertad

VIII

29

51

79


La Paz





 

West

VIII

29

51

79

 

East

VIII

£1

£1

£1


San Miguel (West)

VIII

£1

£1

£1


San Salvador

VIII

29

51

79


San Vicente

VIII

£1

£1

£1


Santa Ana

VIII

29

51

79


Sonsonate

VIII

29

51

79


Usulatán

VIII

£1

£1

£1

GUATEMALA


Department






Alta Verapaz

VIII

(4)

(8)

(15)


Baja Verapaz

VIII

(4)

(8)

(15)


Chimaltenango

VIII

10

23

50


Chiquimula

VIII

29

51

79


El Progreso

VIII

29

51

79


Escuintla

VIII

10

23

50


Guatemala

X

10-29

23-51

50-79


Huehuetenango





 

East

X

(4)

(8)

(15)

 

West

X

5

13

34


Izabal





 

East

VIII

£1

£1

£1

 

West

VIII

(4)

(8)

(15)


Jalapa

VII

29

51

79


Jutiapa

VIII

29

51

79


Quezaltenango

IX

5

13

34


Quiché

VIII

(4)

(8)

(15)


Retalhuleu

VIII

5

13

34


Sacatepéquez

VIII

10

23

50


San Marcos

IX

5

13

34


Santa Rosa

IX

10-29

23-51

50-79


Sololá

VIII

10

23

50


Suchitepéquez

VIII

10

23

50


Totonicapán

VIII

10

23

50


Zacapa

VIII

(4)

(8)

(15)

HONDURAS


Department






Comayagua

VIII

?

?

?


Copán





 

East

VII

£1

£1

£1

 

West

VIII

(4)

(8)

(15)


Intibuca

VIII

?

?

?


Lempira

VIII

?

?

?


Ocotepeque





 

East

VII

£1

£1

£1

 

West

VIII

(4)

(8)

(15)


Santa Barbara (West)

VIII

£1

£1

£1

NICARAGUA

VIII

?

?

?

a/Conditional probability refers largely to earthquakes caused by inter-plate movement.
? No information available.
() All values in parentheses represent less reliable estimates.

Source: Adapted from White, R.A. Maximum Earthquake Intensities in Central America (unpublished map) (Menlo Park, California: U.S. Geological Survey, 1988), and Nishenko, S.P. Summary of Circum-Pacific Probability Estimates (unpublished table) (Golden, Colorado: U.S. Geological Survey, 1989).

Figure 11-15 - SEISMIC POTENTIAL IN THE CARIBBEAN REGION: POTENTIAL FOR OCCURRENCE OF A LARGE EARTHQUAKE

Figure 11-16 - LONG-TERM SEISMIC ACTIVITY IN THE CARIBBEAN REGION: ESTIMATED MAXIMUM MAGNITUDE

Source: McCann, William R. "On the Earthquake Hazards of Puerto Rico and the Virgin Islands" in Bulletin of the Seismological Society of America. Vol. 75, No. 1. (February 1985): pp. 251-262.

Figure 11-17 - SEISMIC HAZARD IN THE CARIBBEAN REGION

Country or Area

Seismic Potential a/ (Likelihood of occurrence of a large earthquake in the near future)

Estimated Maximum Magnitude (Ms)

Anguilla

5

7-7.5

Antigua

3

>8

Barbados

5

NA

Barbuda

3

>8

Cuba:




Extreme south

3

NA


Remainder

NA

NA

Dominica

3

>8

Dominican Republic

1,3,6

>8

Grenada

4

NA

Guadeloupe b/

3,5

>8, ±4

Haiti

1,3,4,5

NA

Jamaica

4

NA

Martinique

3,5

NA

Montserrat c/

?

±4

Puerto Rico and Virgin Islands d/

2

8 - 8.25

St. Barts

3

>8

St. Eustatius e/

?

±4

St. Kitts and Nevis

?

±4

Saint Lucia

3

NA

St. Martin

5

7-7.5

St. Vincent and the Grenadines

5

NA

Trinidad and Tobago

5

NA

a/Potential for Large or Great Earthquake

1 High potential; large earthquake more than 200 years ago
2 Moderately high potential; large earthquake 150-200 years ago
3 Moderate potential; large earthquake 100-150 years ago
4 Moderately low potential; large earthquake 50-100 years ago
5 No record of large shocks
6 Low potential; large earthquake less than 50 years ago

b/ Volcanic source in part of area: seismic potential unpredictable; maximum magnitude low but intensity high

c/ Volcanic source: seismic potential unpredictable; maximum magnitude low but intensity high

d/ Numerous seismic sources possible

? Indeterminate

NA Not available

Source: McCann, W.R. "On the Earthquake Hazards of Puerto Rico and the Virgin Islands" in Bulletin of the Seismological Society of America, vol. 75, no. 1 (February 1985), pp. 251-262.

The result of the work up to this point will enable a determination to be made of which hazards constitute a significant threat. For those that do, a rough zonation map (see Chapter 6) should be prepared; an appropriate scale is 1:50,000. This hazard zonation map is an important part of the overall regional diagnosis and constitutes a valuable element for strategy formulation and the identification of specific action proposals. It gives an idea of where intensive development is appropriate, which areas should be left relatively undeveloped, what precautions will be necessary where the development of hazardous areas is deemed necessary or unavoidable, and where mitigation is necessary in already developed areas.

c. Phase II: Development Strategy and Project Formulation

The analysis of the hazard information in conjunction with other elements of the regional development study at this point results in project actions and priorities which in turn create new demands for hazard information. This can include undertaking vulnerability studies which, together with the hazard zoning, can be used to produce earthquake risk maps for each hazard individually or the combination of hazards.

These maps can then be combined with maps of other hazards, e.g., flooding, to produce multiple hazard maps, and the hazard maps can be combined with maps of present and potential land use to produce land-use zoning maps. They serve to guide future development and provide the spatial units to which the elements of a building code can be addressed. At this stage, detailed data are required, including characteristics of bedrock and overlying soil, site-specific intensity, acceleration data, site response data, damage reports of previous earthquakes affecting the area. Appropriate scales are 1:50,000 to 1:10,000. See Chapter 6 for a further detailed discussion.

d. Project Implementation

Finally, projects are studied at the final design stage and are implemented. Parallel hazard activities such as the preparation of building and grading regulations, the retrofitting of existing structures to make them more earthquake-resistant, and the redevelopment of damaged areas are beyond the scope of this chapter.

C. VOLCANIC ERUPTIONS

1. Volcanic Hazards
2. Classification, Assessment, Mapping, and Mitigation of Volcanic Hazards
3. Volcanic Hazards and the Development Planning Process

Even though ash from very large volcanic eruptions such as Krakatoa, in what is now Indonesia, can encircle the earth in a matter of a few days and may affect sunsets for years afterwards, serious damage is restricted to small areas compared with the extent of damage from large floods or great earthquakes. Yet volcanic eruptions can take a high toll in human life and property. There are reasons for this seeming contradiction.

The decomposition of most volcanic materials yields rich agricultural soils-particularly significant in tropical areas where other soils tend to be low in nutrient content-and to use them farmers are willing to risk the hazard of a new eruption. Furthermore, the densest rural population in Latin America and some of the largest cities are located in the Andean range and its extension into Mesoamerica along the zone of contemporary volcanism. Finally, many of the volcanoes in the small islands of the Caribbean are still very active. Three of the world's most catastrophic eruptions took place on Guadeloupe, Martinique, and St. Vincent, where there is not much room to hide. The seriousness of volcanic hazards in Latin America and the Caribbean is documented in Figures 11-18 and 11-19. Nearly 60,000 lives have been lost and 250,000 severely affected by eruptions during this century. In the past 10,000 years, 250 volcanoes in Latin America and the Caribbean have erupted nearly 1,300 times.

Since it is impossible to prevent the use of areas subject to volcanism, it becomes imperative to determine which of them are susceptible to particular hazards, to plan their development appropriately, and to establish systems of monitoring, warning and evacuation.

The restricted geographic distribution of volcanic eruptions makes it easier to mitigate their detrimental effects. Throughout Latin America and the Caribbean, only areas that have experienced eruptions since the Pliocene Epoch are subject to significant danger. These areas are indicated in Figure 11-20 by the location of active volcanoes in Latin America and the Caribbean.

Volcanic eruptions range from gentle outpourings of lava to violent explosions. The difference is determined largely by the viscosity of the magma, or molten rock, and its content of dissolved gas. Fluid magmas, rich in iron and magnesium, tend to allow the volcanic gases to escape and often reach the surface in the form of quiet lava flows. More viscous magmas, rich in silica, tend to trap the volcanic gases, resulting in a build-up of pressure, and thus have a greater propensity to violent eruptions. The ejecta of volcanic explosions include blobs of molten lava, which solidify quickly to form glass, and solid fragments ranging from fine ash to house-size boulders. The nature of volcanic hazards is determined by the material ejected by an eruption and the force with which it is ejected.

Figure 11-18 - SUMMARY OF IMPACTS OF RECENT VOLCANIC ERUPTIONS IN LATIN AMERICA AND THE CARIBBEAN

Date

Volcano, Country

Description

1985

Ruiz, Colombia

23,000 death toll caused by lahar (mudflow) through town of Armero.

1985

El Chichón, Mexico

Most of the 153 deaths resulted from roof collapse and fires ignited by incandescent tephra.

1979

Soufriere, St. Vincent

Evacuation of 20,000 people for a month.

1963-1965

Irazu, Costa Rica

Tephra fall forced the 230,000 inhabitants of San Jose to wear goggles, bandannas or even gas masks almost every day for months. Lahar (mudflow) 12m thick in places.

1961

Calbuco, Chile

Explosive eruption, phreatic explosion, lava flows, and mudflow resulted in destruction of extensive agricultural land.

1902

Soufriere, St. Vincent

Pyroclastic flow killed 1680 (77 km2 impacted by pyroclastics).

1902

Mt. Pelee, Martinique

28,000 deaths caused by pyroclastic gases and mudflows; 58 km2 destroyed.

1902

Santa Maria, Guatemala

About 40% of the more than 5,000 deaths resulted from collapsing house roofs under the weight of tephra. The town of Quezaltenango, 15 km from volcano, was destroyed. Eruption lasted 18 hours.

Source: Modified from Krumpe, P.F. Briefing Document on Volcanic Hazard Mitigation (Washington, D.C.: USAID/Office of Foreign Disaster Assistance, March 11, 1986).

1. Volcanic Hazards

a. Tephra Falls and Ballistic Projectiles
b. Pyroclastic Phenomena
c. Lahars and Floods
d. Lava Flows and Domes
e. Other Hazards

Volcanic hazards include tephra falls and ballistic projectiles, pyroclastic phenomena (flows, surges, and laterally directed blasts), lahars (or mudflows), lava flows, hazards associated with lava domes, phreatic explosions, and emissions of poisonous or corrosive gases. Summary information on the characteristics, warning periods, and effects of these hazards can be found in Figures 11-21 and 11-22.

a. Tephra Falls and Ballistic Projectiles

Tephra includes all sizes of rock fragments and lava blobs ejected into the atmosphere by the force of an eruption which accumulate to form deposits as the airborne materials fall back to earth. Eruptions associated with major tephra falls can have main eruptive phases lasting from an hour to two or three days. The eruptions may occur as single events separated by long quiet intervals or as multiple events closely spaced in time over a period of months or years. Tephra deposits consist of variable proportions of low-density material (pumice and scoria) and high-density rock fragments with particle sizes ranging from ash (2mm) to blocks and bombs (up to several meters in diameter). These larger fragments, hurled with great force from the volcano, are considered ballistic projectiles.

Figure 11-19 - NUMBER OF VOLCANOES, ERUPTIONS, AND INCIDENTS OF VOLCANIC ERUPTIONS CAUSING SIGNIFICANT DAMAGE IN LATIN AMERICA AND THE CARIBBEAN DURING THE LAST 10,000 YEARS

Country

Number of Volcanoes

Number of Eruptions

Number of Eruption Causing

Agricultural Land

Fatalities

Destruction

Mudslides

Tsunamis

Mexico

30

88


4

1


Guatemala

24

120

5

10

3


El Salvador

19

119

3

17



Nicaragua

22

94


9

1


Costa Rica

11

66

2

5

2


Honduras

2

2





Panama

1

3


2



Colombia

13

62

4

4

3


Ecuador

10

82

5

14

29



Galapagos

13

61





Peru

10

38

1

4

1


Bolivia

15

15





Chile

62

271

5

12

13

1

Argentina

3

3





West Indies:








Saba

1

1






St.Eustatius

1

1






St.Kitts and Nevis

2

4






Montserrat

1

1






Guadaloupe

1

10



2



Dominique

4

4






Martinique

1

24

1

2

3



Saint Lucia

1

1






St. Vincent

1

210

1

3

5



Grenada

2

2






250

1282





Source: Compiled from Simkin, T. et al. Volcanoes of the World (Stroudsburg, Hutchinson Ross Publishing Company, 1981): pp. 89-103.

Figure 11-20 LOCATION OF ACTIVE VOLCANOES IN LATIN AMERICA AND THE CARIBBEAN

Source: Krumpe, P.F. Briefing Document on Volcanic Hazards Mitigation (Washington, D.C.: USAID/Office of Foreign Disaster Assistance 1988).

Figure 11-21 SUMMARY ESTIMATES OF THE PHYSICAL PROPERTIES OF SELECTED VOLCANIC HAZARDS



Distances at Which Effects Experienced

Area Affected

Velocity

Temperature

Average

Maximum

Average

Maximum

Average

Maximum

Hazard

(km2)

(km2)

(km2)

(km2)

(ms-1)

(ms-1)

(oC)

Tephra falls

20-30

800+

100

100.000+

15

30

Usually ambient air

Ballistic projectiles

2

15

10

80

50-10

100

1000

Pyroclastic flows and debris avalanches

10

100

5-20

10,000

20-30

100

600-7000

Lahars

10

300

5-20

200-300

3-10

30+

100

Lava flows

3-4

100+

2

1.000+

5

30

750-1150

Acid rains and gases

20-30

2.000+

100

20,000

15

30

ambient air

Airshock waves

10-15

800+

1.000

100,000+

300

500

ambient air

Lightning

10

100+

300

3,000

12x105

12x105

Above ignition point

Source: Modified from Blong, R.H. Volcanic Hazards (Sydney, Australia: Macquarie University Academic Press, 1984).

Figure 11-22 WARNING PERIODS AND LIKELY EFFECTS OF SELECTED VOLCANIC HAZARDS

Hazard

Warning Period

Capacity to Cause

Likelihood of Severe

Severe Damage

Injury or Death

Tephra falls

Minutes to hours

Minor-moderate

Low-moderate

Ballistic projectiles

Seconds

Extreme

Very high

Pyroclastic flows and debris avalanches

Seconds

Extreme

Extreme

Lahars

Minutes to hours

Very high

Very high

Lava flows

Usually hours or days

Extreme

Very high

Acid rains and gases

Minutes to hours

Very low

Usually very low

Airshock waves

Seconds to minutes

Minor

Very low

Lightning

None

Moderate

Very high

Source: Modified from Blong, R.H. Volcanic Hazards (Sydney, Australia: Macquarie University Academic Press, 1984).

Tephra can cause casualties or property damage by the impact of falling fragments, by forming a layer covering the ground, by producing a suspension of fine-grained particles in the air, and by heat close to the volcano. The greater the thickness and coarseness of the deposit, the more detrimental the effects.

Large bombs can fly as far as 15km from the vent. Small bombs and lapilli (rock fragments ranging in size up to 64mm) can be carried as far as 80km from the eruption. Ash may be deposited to a depth of 10cm as far as 30km from the eruption and to much greater depths closer by. Accumulation of tephra can cause buildings to collapse, break power lines, and kill vegetation. Deposits only a few centimeters thick can disrupt vehicular traffic. The addition of moisture worsens these effects. Suspended tephra in the air can cause serious respiratory problems, damage machinery, especially internal combustion engines, short-circuit electrical transmission equipment, and disrupt air. rail, and highway transportation. Fragments falling as far as 10km from the vent may still be hot enough to start fires.

b. Pyroclastic Phenomena

Pyroclastic Flows

Pyroclastic flows are masses of hot, dry pyroclastic material and hot gases that move rapidly along the ground surface. The term includes a range of volcanic phenomena known as pumice flow, ash flow, block-and-ash flow, nuee ardente. and glowing avalanches. Pyroclastic flows consist of two parts: a basal flow, which is the pyroclastic flow proper, and an overriding turbulent ash cloud that includes both hot pyroclastic surges and towering columns of ash. Basal flows are dense mixtures of ash, gas, and volcanic rock whose movement is controlled by gravity. They therefore tend to move along topographic depressions. Maximum temperatures of pyroclastic flows soon after deposition range from about 350°C to 700°C. Pyroclastic flows are common throughout the world, ranging in area from less than 1 km to more than 10,000km2. The dangers associated with pyroclastic flows include asphyxiation, burial, incineration, and impact injury and damage.

Pyroclastic Surges

Pyroclastic surges are turbulent, low-density clouds of gases and rock debris that move above the ground surface at high speed. They are generally associated with pyroclastic flows, but because of their greater mobility they affect broader areas. Pyroclastic surges pose all the dangers of pyroclastic flows plus those of noxious gases and high-velocity clouds. Their great mobility makes escape impossible once they have formed. Zones of potential pyroclastic surge inundation should be evacuated at the start of an eruption that may be accompanied by such an event.

Laterally Directed Blasts

Laterally directed blasts are among the most destructive of volcanic hazards. They occur within a period of a few minutes, without warning, and can affect hundreds of square kilometers. Within areas affected by such blasts, virtually all life can be expected to be extinguished and all structures destroyed. Volcanic explosions can project material upward or at any other angle. Laterally directed blasts have an important low-angle component which contributes to their destructiveness.

Rock fragments may be ejected out of a blast site on ballistic trajectories, as pyroclastic flows or surges, or in some combination. Whatever the transport mechanism, the debris is carried at speeds that greatly exceed those expected from simple gravitational acceleration. At Mt. St. Helens, the blast cloud had an initial velocity of about 600km/hr, slowing to about 100km/hr at 25km from the volcano. The deposited material may be cold or may be hot enough to start fires.

c. Lahars and Floods

A lahar (or mudflow) is a flowing slurry of volcanic debris and water that originates on a volcano. The eruption of a snow-covered volcano can melt enough snow to cause a lahar. Similarly, an eruption in a crater lake can cause a flood which becomes a lahar as it entrains rock and eroded earth from the slopes of the volcano. Lahars in which at least 50 percent of the particulate matter is the size of sand or smaller are called mud flows, while those with a lower content of fine particles are called debris flows (see Figures 11-4 and 11-5).

Lahars can be produced in several ways: a sudden draining of a crater lake by an explosive eruption or the collapse of a crater wall, the melting of snow or ice by the deposition of hot rock debris or lava, the mixing of a pyroclastic flow with water, the avalanching of water-saturated rock debris from a volcano, the fall of torrential rain on unconsolidated fragmental deposits, or the collapse of dams formed by lava flows (Crandall, 1984). A lahar was the principal cause of death in the 1985 eruption of the Nevado del Ruiz in Colombia.

The distance reached by a lahar depends on its volume, water content, and gradient, and may be as much as 300km. By incorporating additional sources of water, such as a reservoir in the case of Colombia, it can greatly augment its speed and length. The Ruiz lahar averaged 30km/hr for 90km. The shape and gradient of a valley will also affect lahar length: a steep narrow valley will permit a lahar of a given volume to move a greater distance.

Lahars sometimes reach astonishing speeds. One from a Japanese volcano had a velocity of 180km/hr. A lahar initiated by the eruption of Cotopaxi Volcano in Ecuador had an average speed of 27km/hr over a distance of 300km.

With their high-bulk density and velocity, lahars can destroy structures in their path such as bridges, bury towns and crops, and fill stream channels, thus decreasing their capacity to carry water. This can result in flooding as the water overflows the smaller channel or in the formations of dams by volcanic debris that back up the water and increase the potential for flash flooding.

d. Lava Flows and Domes

Fluid lava forms long thin flows on slopes and flat-topped lava lakes in flat areas and topographic depressions, while viscous lava forms short stubby flows on slopes and steep-sided domes around their vents. In either case lava flows seldom threaten human life because they move slowly and their path can be predicted. The distances they reach are determined by their volume and viscosity and by the local topography. Basalt flows may reach distances of a few hundred kilometers from their sources, but more viscous lava such as andesite rarely extends more than 20km. Lava flows can cause extensive damage by burning, crushing, or burying everything in their path.

A volcanic dome is formed when lava extruded from a vent is too viscous to flow more than a few tens or hundreds of meters so that movement is principally upwards towards the center of the dome. The sides become unstable and avalanches may form, which can be triggered by volcanic explosions or by the growth of the dome itself. Explosions may result in pyroclastic flows, which are the main source of damage associated with the development of domes.

e. Other Hazards

Phreatic explosions occur when magma heats groundwater to the point that it forms steam and blasts through the overlying rock or sediment. Volcanic gases may carry toxic elements that can kill humans and animals and acids that can harm vegetation and corrode metal. Nearly 3,000 deaths were attributed to the release of poisonous gas or carbon dioxide in the August 1986 eruption in the Cameroons. Indirect hazards include volcanic earthquakes, tsunamis, ground deformation, structural collapse due to the withdrawal of magma, airshock waves, and lightning.

2. Classification, Assessment, Mapping, and Mitigation of Volcanic Hazards

a. Classification of Volcanic Hazards
b. Volcanic Hazards and Risk Assessment
c. Volcanic Hazard Zonation Map
d. Mitigation of Volcanic Hazards

Understanding the nature of volcanoes and the hazards they present can lead to development-related mitigation. This sub-section discusses in general terms the classification of volcanoes by frequency of eruption, the assessment of volcanic hazards, the preparation of a hazard zonation map, and the focus of mitigation. Their relationship to the development planning process follows.

a. Classification of Volcanic Hazards

At the outset it is necessary to consider the periodicity of eruptions. The Sourcebook for Volcanic Hazards Zonation published by UNESCO distinguishes between short-term and long-term hazards. A short-term hazard is defined as a volcano that erupts more than once a century-people can expect to experience an eruption in their lifetime. Long-term hazards have a periodicity of more than 100 years (Crandall, 1984).

In this primer, the definitions are modified as follows: a short-term hazard is defined as having a periodicity of 100 years or less and/or as having erupted since 1800; a long-term hazard has a periodicity of more than 100 years and has not erupted since 1800. An additional category is also proposed: imminent hazard signifies volcanoes for which reliable geologic evidence indicates an eruption can be expected within a year or two.

b. Volcanic Hazards and Risk Assessment

An evaluation of the likelihood that a given volcano will erupt in a particular timeframe and an estimation of the severity of such an eruption is based on historic and prehistoric information and on the behavior of similar volcanoes elsewhere in the world. If data from the historic and prehistoric records are adequate, then the frequency of past eruptions can be determined and the possible frequency of future eruptions can be estimated. This assumes that the future behavior of a volcano will reflect its history over the past few thousand years. The behavior of similar volcanoes elsewhere can provide an indication of events of low probability but great magnitude that could take place.

The volcanic hazard assessment involves establishing a stratigraphic record of the products of past eruptions and determining the aerial extent of deposits, their origin in the stratigraphic sequence, and the date of the eruptions. To accomplish this, information that exists in the historic record usually must be supplemented with field analysis.

Implicit in establishing the stratigraphic record is classifying the volcano type in terms of morphology and eruption characteristics 2/ and determining the rock types of the volcanic deposits, both of which are indicators of the propensity for violent explosions. Once the stratigraphic sequence is determined, the deposits are classified as to the type of hazard (tephra, pyroclastic flow, lava flow, etc.), dated (a number of techniques are available that can be used to supplement the historic record), and mapped. The resulting products are maps and reports which depict the volcanic hazards of an area. Finally, the volcanic hazard can be graded in terms of severity on a volcanic hazard zonation map.

2/ For classifications of volcano types see Steinbrugge, 1982, and Simkin, et al, 1981.

c. Volcanic Hazard Zonation Map

The Sourcebook for Volcanic Hazards Zonation provides an excellent discussion of preparing hazard zonation maps:

Volcanic hazards zonation maps have two primary purposes, namely, for long-range planning for uses of land around volcanoes that are thought to be compatible with the hazard from future eruptions, and for determining which areas should be evacuated and avoided during eruptions. Maps prepared for these two purposes have similarities as well as differences. A hazards-zonation map and an accompanying report designed to guide land-use planning could include estimates of the frequency of events anticipated in the future. Such reports could also include quantitative or other estimates of relative degrees of hazard. In contrast, a zonation map prepared chiefly for the purpose of evacuation could subdivide kinds of hazard, so that people could be removed selectively from different areas according to whether the eruption was expected to produce lava flows, airfall deposits, pyroclastic flows, lahars or combinations of these. Maps such as these could also be divided into zones based on the anticipated scales of future eruptions, or into sectors determined by which flank of a volcano, or which valley system, might be affected most often by eruptions. The expectable high cost and social disruption caused by evacuation might be reduced by the use of such maps. Both kinds of uses of hazards-zonation maps should be considered during their preparation; both kinds of maps can be prepared from the same basic data, and in some cases one map could be prepared that would serve both purposes (Crandall, 1984).

Examples of volcanic hazard zonation maps used for development planning purposes are shown in Figures 11-23 and 11-24. Suggestions for preparing hazard zonation maps for specific volcanic hazards together with numerous examples are available from the UNESCO Sourcebook.

d. Mitigation of Volcanic Hazards

Development-related aspects of volcanic hazard mitigation-reducing the potential loss of life and property damage that can be caused by a volcanic eruption-primarily involve hazard assessments and land-use planning. Other mitigation procedures such as the establishment of monitoring and warning systems, emergency evacuation measures, protective measures, insurance programs, and relief and rehabilitation measures are not treated in this chapter. Many of these activities are associated with preparedness, which is another phase of hazard management (see Chapter 1).

Volcanoes which present a short-term hazard and which clearly threaten life or property should be kept under surveillance, and restrictions should be placed on permanent habitation in the areas of greatest hazard. For volcanoes that have long-term periodicity and therefore may or may not pose a hazard during the lifetime of a project, land-use restrictions may not be warranted on purely economic grounds, but development should be planned with a knowledge of the potential consequences of future eruptions. Obviously, an imminent eruption requires constant monitoring and vigilance and the taking of suitable measures to cope with the impending event.

3. Volcanic Hazards and the Development Planning Process

a. Preliminary Mission
b. Phase I - Development Diagnosis
c. Phase II - Development strategy and project formulation

Compared with earthquakes, volcanic hazards are simpler to cope with in development planning because of their point source, the limited extent of the area in which active volcanoes occur, and the limited distance from the source for which volcanic activity poses a serious hazard. The process involves addressing key questions listed in the box above.

Figure 11-23 VOLCANIC HAZARD ZONES OF FUEGO VOLCANO, GUATEMALA

Source: Rose, W.I., et al. Volcanic Hazards of Fuego Volcano, Preliminary Report (Houghton, Michigan: Michigan Technological University, 1981).

Figure 11-24 - VOLCANIC HAZARD ZONES OF MT. ST. HELENS VOLCANO, U.S.A.

Source: Adapted from Crandall, D.R. et al. Sourcebook for Volcanic-Hazards Zonation, Natural Hazard 4 (Paris, France: UNESCO, 1984)

KEY QUESTIONS PLANNERS NEED TO ASK SEQUENTIALLY ABOUT VOLCANOES AS PART OF A DEVELOPMENT STUDY

- Are volcanic eruptions a concern in the Study area?
- How imminent is an eruption?
- What specific hazards present a danger and where?

a. Preliminary Mission

During the preliminary mission of an integrated development planning study, an initial review is made of the information available. At this time the first two questions shown in the box above can be answered with an acceptable degree of reliability by undertaking an initial volcanic hazard assessment. The procedure, outlined in the box on page 59, uses information from the Preliminary Neotectonic Map of South America and Figure 11 -25, which is a listing of volcanoes active in the Holocene period, their periodicity, and other summary information on each. When necessary, local information can supplement this. No specialized expert is needed for this task.

b. Phase I - Development Diagnosis

Phase I of a development study requires a diagnosis of a region's development potential. The results of the initial volcanic hazard assessment will lead to different information needs if a volcano in the study area is identified as an imminent, short-term, or long-term threat.

If an eruption is determined by geologic evidence to be imminent, mitigation actions must take precedence over all other activities. The statement appears too self-evident to merit mention, yet surprisingly the principle is not always heeded. For example, the Nevado del Ruiz gave clear signs of the approach of a major eruption in November 1985, one year before the eruption that killed 23,000 people (Tomblin, 1986). The moment an eruption appears imminent, full-scale monitoring of the volcano must be initiated if it is not already being done. Warning and evacuation systems must be established. Large reservoirs in the path of potential lahars should be either drained or lowered sufficiently to serve as a trap rather than a lubricant to moving mud and water. People occupying the flanks of the volcano should be relocated. Planners can play a role in seeking suitable relocation sites and helping to define relocation mechanisms. Areas adjacent to the volcano that are vulnerable to any specific hazard, particularly lahars and pyroclastic phenomena, should be delineated-at first simply by topographic considerations-and suitable precautions taken. In short, if an eruption is found to be imminent in an area, the planner's focus abruptly changes from the future to the immediate present. When a short-term volcanic hazard is identified, additional information is needed.

Additional information on individual volcanoes can be found in the resources listed in the box on page 60. These sources may be supplemented by more detailed local data such as maps and studies of specific volcanic hazards or historical event and damage assessment studies. Additional information can be inferred from geologic, tectonic, and seismic maps, particularly maps of Holocene or Quaternary geology. Data on wind (prevalent direction and speed) are relevant to evaluating tephra hazard. Topography and interpretative soil studies are important for the evaluation of tephra, lava flow, pyroclastic flow, and lahar hazards. The location of reservoirs and other major sources of water that can cause flooding or contribute to the movement of lahars is particularly important data for volcanic hazard mitigation.

Information on elements at risk is the same as for earthquake hazards. In some areas with severe volcanic hazards, maps of volcanic hazards, risks, and volcanic hazard land-use zonation are available. Sources of information include national geologic agencies, national and international volcanic and hazard data centers, national disaster mitigation agencies, universities, and research centers.

Volcanoes posing a short-term hazard can be plotted on 1:10,000- to 1:100,000-scale topographic maps. Local information commonly exists for volcanoes in this category, and some hazard mitigation program may already have been instituted. In this case, the task of the planner is to promote land uses and protective measures commensurate with the degree of risk of any area.

p376.gif Figure 11-25 - ACTIVE VOLCANOES IN LATIN AMERICA AND THE CARIBBEAN, ASSOCIATED VOLCANIC HAZARDS, AND PERIODICITY OF ERUPTIONS DURING THE LAST 10,000 YEARS

p377.gif Figure 11-25 - ACTIVE VOLCANOES IN LATIN AMERICA AND THE CARIBBEAN, ASSOCIATED VOLCANIC HAZARDS, AND PERIODICITY OF ERUPTIONS DURING THE LAST 10,000 YEARS (continued)

p378.gif Figure 11-25 - ACTIVE VOLCANOES IN LATIN AMERICA AND THE CARIBBEAN, ASSOCIATED VOLCANIC HAZARDS, AND PERIODICITY OF ERUPTIONS DURING THE LAST 10,000 YEARS (continued)

p379.gif Figure 11-25 - ACTIVE VOLCANOES IN LATIN AMERICA AND THE CARIBBEAN, ASSOCIATED VOLCANIC HAZARDS, AND PERIODICITY OF ERUPTIONS DURING THE LAST 10,000 YEARS (continued)

p380.gif Figure 11-25 - ACTIVE VOLCANOES IN LATIN AMERICA AND THE CARIBBEAN, ASSOCIATED VOLCANIC HAZARDS, AND PERIODICITY OF ERUPTIONS DURING THE LAST 10,000 YEARS (continued)

p381.gif Figure 11-25 - ACTIVE VOLCANOES IN LATIN AMERICA AND THE CARIBBEAN, ASSOCIATED VOLCANIC HAZARDS, AND PERIODICITY OF ERUPTIONS DURING THE LAST 10,000 YEARS (continued)

Notes:

1. Sources of information for name of volcano, location, periodicity, location, date of last eruption, effects, and volcanic hazards: Simkin, T. et at. Volcanoes of the World. (Stroudsburg, Pennsylvania: Hutchinson Ross Publishing Company, 1981), and Smithsonian Institution. Global Volcanism Network. (Washington D.C.: Smithsonian Institution, 1989-90)..

Volcanoes with short-term periodicity are presented in capital letters. A volcano having short-term periodicity is defined for this table as one with an eruption periodicity of 100 years or less and/or one that has erupted since 1800.

2. Date of last eruption is simplified from Volcanoes of the World using three categories: (1) "Historic" - the actual eruption date is given, sometimes qualified by "?" when data is questionable. (2) "Holocene" - including the following subcategories: (a) eruptions dated by Carbon 14, hydrophone data, dendrochronology, varve count, anthropologic evidence, lichenometry, magnetism, tephrochronology, hydration rind or fission track analysis; (b) volcanoes now displaying fumarolic or solfataric activity and giving obvious evidence of recent, although undated, eruption; (c) volcanoes virtually certain to have erupted in postglacial time even though neither dated products nor thermal features are present. (3) "Uncertain" - signifying possible Holocene activity but questionable documentation.

3. Fatalities caused by one or more eruptions.

4. Destruction of agricultural land or other property damage caused by one or more eruptions.

5. One or more eruptions were explosive.

6. Pyroclastic flows or surges and/or laterally directed blasts were associated with one or more eruptions.

7. Phreatic explosion was associated with one or more eruptions.

8. Lava flow, lava domes, or spines were associated with one or more eruptions.

9. Destructive mudflows were associated with one or more eruptions.

10. VEI = Volcanic Explosivity Index: the size or "bigness" of a historic eruption. The VEI combines total volume of products, eruptive cloud height, duration of eruption, tropospheric injection, stratospheric injection, and some descriptive terms to yield a 0-8 index of increasing explosivity as follows: 0 nonexplosive, 1 small, 2 moderate, 3 moderately large, 4 large, 5 very large, 6-8 cataclysmic.

11. Volcano number as per reference found on: Regional Seismological Center for South America (CERESIS). Preliminary Neotectonic Map of South America. (Santiago, Chile: CERESIS, 1985).

PRELIMINARY MISSION: PROCEDURE FOR INITIAL VOLCANIC HAZARD ASSESSMENT IN LATIN AMERICA AND THE CARIBBEAN

INITIAL REVIEW

- For Colombia, Ecuador, Peru, Bolivia, Chile, Argentina: GO TO STEP 1.

- For Mexico, Guatemala, El Salvador, Nicaragua, Costa Rica, Honduras, Panama; GO TO STEP 2.

- For Saba, St. Eustatius, St. Kitts and Nevis, Montserrat, Guadeloupe, Dominica, Martinique. Saint Lucia. St. Vincent, Grenada: GO TO STEP 3.

For all other countries in Latin America and the Caribbean, volcanic hazards are not a significant concern.

STEP 1: Determine if any part of the planning area lies within the area designated "Pliocene-Holocene Volcanic Cover" on the Preliminary Neotectonic Map of South America and/or within 30km of an active volcano indicated on the map. If "no," volcanic hazards are not a significant concern in the study area. If "yes," go to Step 2.

STEP 2: Determine if any part of the study area includes or lies within 30km of any of the volcanoes listed in Figure 11-25. K "no," volcanic hazards are not a significant concern in the study area. If "yes," go to step 3.

STEP 3: Using Figure 11-25, classify the eruption periodicity of each volcano in the study area as short-term or long-term. Volcanoes with short-term periodicity are shown in capitals. If the volcano is classified as short-term, go to Step 4.

STEP 4: For short-term volcanoes, determine from local authorities if there is any geologic evidence that an eruption is imminent, and if hazard zonation maps have been prepared.

NOTE: The 30km distance is arbitrary, based on the distance from a volcano that lahars, ash, pyroclastic flows, etc., may be dangerous. The radius may be shorter or longer depending on such factors as the difference in elevation between the volcano and threatened areas, slope, channel morphology, and prevailing winds.

If a hazard zonation map does not exit, one should be prepared as part of the development planning study and should become an integral part of the integrated natural resource inventory. In this case it will be necessary to obtain the services of a volcanic hazard expert. Having completed the preliminary hazard work during the preliminary mission, the planner will be prepared to draft precise terms of reference for the specialist. With the results of additional studies, the planner can identify potential mitigation measures, comparing costs and potential benefits with all the other elements involved in the development of the study area.

If long-term volcanic hazards are determined to occur in the study area, incorporating hazard considerations into a development study can offer additional benefits. Long-term hazards are often ignored in spite of the fact that surprising eruptions from volcanoes considered dormant or inactive are responsible for tremendous damage. If no local information exists, a difficult decision will be required as to whether the preparation of a volcanic hazard zonation map is justified. A volcanic hazard expert can advise on the degree of risk and, commensurately, the effort that should be devoted to additional studies and mitigation measures.

c. Phase II - Development strategy and project formulation

In developing areas with short-term volcanic hazards, mitigation measures should be selected if they are not already part of the project identification information. Land-use restrictions should be instituted for areas having a potential threat of pyroclastic phenomena. In areas where volcanic ash can constitute a hazard, building codes should stipulate appropriate roof construction. In many cases only lahars would merit mitigation measures. Valley areas in the path of potential lahars could be delineated and land-use restrictions or protective measures in keeping with economic rationality could be instituted. The mitigation measures that can be justified economically for these short-term hazards are limited, since "short-term" is still a lengthy period of time. Awareness of the potential danger may permit a more reasoned development plan.

ADDITIONAL VOLCANO INFORMATION

One of these two sources may provide details of a short-term volcano's history:

Simkin, T., et al Volcanoes of the World, Smithsonian Institution (Stroudsburg, Pennsylvania: Hutchinson Ross Publishing Co., 1981).

International Association of Volcanology (ed.). Catalog of Active Volcanoes of the World Including Solfatara Fields (Rome: Istituto di Geologia Applicata, Facultà di Ingegneria).

D. TSUNAMIS

1. Tsunami Hazards and Their Assessment and Mitigation
2. Tsunamis and the Development Planning Process

Tsunamis are water waves or seismic sea waves caused by large-scale sudden movement of the sea floor, due usually to earthquakes and on rare occasions to landslides, volcanic eruptions, or man-made explosions.

1. Tsunami Hazards and Their Assessment and Mitigation

a. Tsunami Hazards
b. Tsunami Hazard Assessment
c. Mitigating the Effects of Tsunamis

a. Tsunami Hazards

Life-threatening tsunamis have not been known to occur in the Atlantic Ocean since 1918, but are a serious problem in the Pacific. Although the Caribbean Basin's tectonic configuration indicates that the area is susceptible to seismic activity, these earthquakes are rarely tsunamigenic. Since 1690 only two significant occurrences have been registered. The 1867 tsunami swept away villages in Grenada, possibly killed 11 or 12 people in St. Thomas, and killed an additional five persons in St. Croix. The 1918 occurrence created abnormally large waves for two to three hours in different parts of the Dominican Republic, and killed 32 people in Puerto Rico (NOAA, 1989). In view of the rarity of these events, it would be difficult to establish an economic justification for mitigation measures. On the Pacific coasts of Mexico, Guatemala, El Salvador, Costa Rica, Panama, Colombia, Ecuador, Peru, and Chile, on the other hand, between 1900 and 1983, there were 20 tsunamis that caused casualties and significant damage. A devastating tsunami occurred in Africa, then in Peru, in 1868. Ships were carried five kilometers inland by a wave that exceeded 21m in height. This and subsequent waves 12m high swept over the city, killing hundreds of people. The earliest recorded tsunami in Latin America occurred in 1562, inundating 1,500km of the Chilean coastline.

Tsunamis differ from other earthquake hazards in that they can cause serious damage thousands of kilometers from the causative faults. Once they are generated, they are nearly imperceptible in mid-ocean, where their surface height is less than a meter. They travel at incredible speeds, as much as 900km/hr, and the distance between wave crests can be as much as 500km. As the waves approach shallow water, a tsunami's speed decreases and the energy is transformed into wave height, sometimes reaching as high as 25m, but the interval of time between successive waves remains unchanged, usually between 20 and 40 minutes. When tsunamis near the coastline, the sea recedes, often to levels much lower than low tide, and then rises as a giant wave.

The effects of tsunamis can be greatly amplified by the configuration of the local shoreline and the sea bottom. Since a precise methodology does not exist to define these effects, it is important to examine the historic record to determine if a particular section of coastline has been subjected to tsunamis and what elevation they reached. An attempt should also be made to determine the possible amplifying effects of the coastal configuration, even with the crude methodologies available (Nichols and Buchanan-Banks, 1974). It should be noted, as shown by the diagram in Figure 11 -26, that because of the force of the wave, the runup can reach an elevation substantially higher than the crest of the wave at the shoreline.

Seiches are phenomena similar to tsunamis but occur in inland bodies of water, generally in elongated lakes. Seiche waves are lower (less than three meters high) than those of tsunamis and are oscillatory in nature. They can cause structural failure and flooding in low-lying areas.

b. Tsunami Hazard Assessment

Estimates of the risk of future tsunamis are based primarily on two types of information: the past history of tsunamis and the prediction of tsunamigenic earthquakes. This information must, of course, be qualified by local conditions such as near-shore marine and terrestrial topography. The most readily available sources of information on historic tsunamis, including current activities in tsunami research, are listed in the box above.

TSUNAMI INFORMATION SOURCES

World Data Center A for Solid Earth Geophysics, National Geophysical Data Center. Tsunamis in the Pacific Basin 1900-1983 (map) (Boulder, Colorado: July 1986) and Tsunamis in Peru-Chile (Boulder, Colorado: NOAA, 1985).

The Pacific Tsunami Warning Center, National Oceanic and Atmospheric Administration. Communication Plan for the Tsunami Warning System, Tenth Edition (Ewa Beach, Hawaii: NOAA, February, 1984).

The International Tsunami Information Center and Intergovernmental Oceanographic Commission, Tsunami Newsletter. (P.O. Box 58027, Honolulu, Hawaii 96850-4993).

The Pacific Marine Environmental Laboratory, National Oceanographic and Atmospheric Administration. THRUST, Third Annual Report (Seattle, Washington: NOAA, 1986).

TSUNAMI MITIGATION MEASURES

- Avoid tsunami runup areas in new development except marine installations and others requiring proximity to water. Prohibit setting of high-occupancy and critical structures.

- Place areas of potential inundation under floodplain zoning, prohibiting all new construction and designating existing occupancies as non-conforming.

- Where economically feasible, establish constraints to minimize potential inundation or to reduce the force of the waves. These measures include:

* constructing sea walls along low-lying stretches of coast and breakwaters at the entrances of bays and harbors

* planting belts of trees between the shoreline and the areas requiring protection

- Where development exists, establish adequate warning and evacuation systems.

- Set standards of construction for structures within harbors and known runup areas.

(Nichols and Buchanan-Banks, 1974; Blair, 1979)

Figure 11-26 TSUNAMI RUNUP HEIGHT

Source: Adapted from Steinbrugge, K.V. Earthquakes, Volcanoes and Tsunamis: An Anatomy of Hazards (Skancia, New York: 1982).

The prediction of tsunamigenic earthquakes is principally based on the seismic gap theory discussed earlier in this chapter.

c. Mitigating the Effects of Tsunamis

While tsunamis cannot be prevented, the Pacific Tsunami Warning Center is constantly monitoring the oceans and in many cases can warn a local population of an impending tsunami with sufficient lead time to make evacuation possible. Such warnings, however, cannot prevent the destruction of boats, buildings, ports, marine terminals, and anything else within the runup area. The areas at risk can be identified, and stringent controls such as those proposed in the box above should be applied.

It should be emphasized, however, that such measures are generally more applicable in areas of high population concentration and that, since significant protection against a large tsunami is virtually impossible economically, avoidance and warning systems are the best mitigation measures for many areas.

To protect against seiches, land-use controls should be applied to low-lying areas of earthquake-prone regions on the borders of large lakes and to areas of potential inundation downstream from large water-retaining structures.

While it is beyond the scope of this chapter to deal with site-specific evaluation of tsunami hazard and design of mitigation measures, techniques for these purposes have been developed for planners. The box below identifies two sources of information.

2. Tsunamis and the Development Planning Process

a. Mexico-Ecuador
b. Colombia-Chile

Tsunamis can be neither prevented nor predicted. The low probability of a large tsunami striking a particular site, together with the potential for great damage if one does hit, makes incorporating tsunami considerations into development planning a tricky proposition. The problem is reduced somewhat in Latin America because of differential trans-Pacific transmission: while a large earthquake in Chile or Peru can generate a tsunami capable of causing damage in Alaska, Hawaii, and Japan, there is little likelihood that an earthquake in the western or northern Pacific will cause damage in Latin America. Of the 405 tsunamis recorded in the Pacific Basin from 1900 to 1983, 61 were recorded on the west coast of Latin America. The source region of all but five of these was the west coast of Latin America. Those five had low to moderate runup and caused negligible to little damage (Tsunamis in the Pacific Basin, 1986 map; and Hebenstreit, 1981). A large tsunami generated in Chile or Peru, however, can cause serious damage thousands of kilometers away on the same coast.

SOURCES FOR TSUNAMI EVALUATION AND THE DESIGN OF MITIGATION MEASURES

National Science Foundation. Comprehensive Planning for Tsunami Hazard Areas. Prepared by Urban Regional Research (1988).

National Science Foundation. Land Management in Tsunami Hazard Areas. Prepared by Urban Regional Research (1982).

KEY PROPOSITIONS FOR CONSIDERING TSUNAMIS IN A DEVELOPMENT PLANNING STUDY

- Any low-lying coastal population center within a zone subject to tsunamis is at risk.

- Mitigation measures other than land-use regulations are generally economically unfeasible except for major metropolitan areas.

- The Pacific Tsunami Warning System covers eight Latin American countries and is designed to alert eastern Pacific countries to tsunamis generated by earthquakes in the western Pacific and vice versa. The system is not designed to warn population centers on the west coast of Latin America of tsunamis generated on the same coast where there may only be about 30 minutes between an earthquake and the subsequent tsunami.

- A new warning system, THRUSH, designed to warn eastern Pacific localities of tsunamis generated on the same coast is in the experimental stage. When it becomes operational, warning lag time should be reduced to about ten minutes.

Studies are being conducted that should greatly enhance the capability for tsunami risk assessment. Until these studies are completed, however, the information in this chapter will serve as an interim guide for planners. Given the propositions listed in the box above, it is important for a planner to know whether or not the study area lies within a zone prone to tsunami damage. If it is, the planner needs to ensure that an adequate warning system is in place and can propose land-use regulations to the extent that they are economically reasonable.

Evaluation of the tsunami hazard is discussed below for two overlapping subregions: Mexico-Ecuador and Colombia-Chile.

a. Mexico-Ecuador

The best data available for estimating the likelihood of a damaging tsunami striking a given site within a given time period in this part of Latin America are found in the record of past tsunamis from the Tsunamis in Latin America Data File (National Geophysical Data Center, 1986). The data for Mexico to Ecuador, indicating 52 tsunamis between 1732 and 1985, are summarized in Figure 11-27.

Areas not included in this figure can be considered to have little threat of damaging tsunamis. While the data are insufficient for statistical prediction, they do provide a general indication of probability based on past events.

b. Colombia-Chile

The historical seismicity patterns and the seismic gap theory have been used in one study to estimate the tsunami hazard in the near future (about 50 years) on the Pacific coast of South America (Hebenstreit and Whitaker, 1981).

Figure 11-27 TSUNAMIS ON THE PACIFIC COAST OF LATIN AMERICA: MEXICO TO ECUADOR

Source: Lockridge, Patricia A. and Smith, Ronald H. Tsunamis in the Pacific Basin - 1900-1983. (Map).(Boulder, Colorado: National Geophysical Data Center and World Data Center A for Solid Earth Geophysics, 1984); and Tsunamis in Latin America Data File (computer print-outs. Tables 1, 2 and 3) (Boulder, Colorado: National Geophysical Data Center, 1986).

Figure 11-28A - MAXIMUM WATER LEVEL ANOMALIES CALCULATED FOR TSUNAMIS GENERATED BY UNIFORM UPLIFT EARTHQUAKES IN PRINCIPAL SEISMIC GAP AREAS ON THE PACIFIC COAST OF SOUTH AMERICA

Figure 11-28B

Source: Adapted from Hebenstreit, G.T., and Whitaker, R.E. Assessment of Tsunami Hazard Presented by Possible Seismic Events- Near Source Effects (McLean, Virginia: Science Applications, 1981).

Figure 11-29 - COASTLINE INDEX POINTS AND MAIN POPULATION CENTERS FOR THE AREA COVERED BY FIGURES 28A AND 28B

Source: Hebenstreit, G.T., and Whitaker, R.E. Assessments of Tsunami Hazards Presented by Possible Seismic Events: Near Source Effects (McLean, Virginia: Science Applications, 1981)

Figure 11-30 TSUNAMI HAZARDS FOR POPULATION CENTERS IN SOUTH AMERICA

COUNTRY, Department or Province

Location of Calculated and/or Reported Wave Height

2-3m

3-5m

Above 5m

COLOMBIA

Cauca

Guapi (h)



Nariño

San José (c)

Pizarro (h)


Majagual (c)

La Chorrera (h)


San Juan (c)

Chagui (h)



Trapiche (h)



Tumaco (h)



Papayal (h)


ECUADOR

Esmeraldas

Muisne (c)

Esmeraldas (h)


Manabí

Pedernales (c)

Isla Salango (c)



Bahía de Caraquez (c)



Manta (c)


Guayas

Guayaquil (h)

Isla Puna (c)


El Oro

Machala (c)



PERU

Tumbes

Pto. Pizarro (c)



Piura

Paita (c)

San Pedro (c)


Bayóvar (c)

Balneario Leguía (c)



Sechura (c)


Lambayeque


San José (c)

Pimentel (b)



Santa Rosa (c)



Puerto de Etén (b)

La Libertad


Trujillo (h)

Pacasmayo (c)


Tambo (h)

Puerto Chicama (c)



Santiago de Cao (c)



Huanchaco (c)



Víctor Larco Herrera (c)



Salaverry (c)

Ancash

Chimbote (h)

Santa (h)

Santa (c)


Samancos (h)

Chimbote (c)


Casma (h)

Samancos (c)


Caleta Tortuga (h)

Caleta Tortuga (c)



Casma (c)



Culebras (c)



Huarmey (c)

Lima



Pativilca (c)

Ancón (c)



Barranca (c)

Callao (a)



Supe (b)

Lima (c)



Huaura (c)

Lurín (c)



Huacho (c)

Pucusana



Hualmay (c)

Chilca (c)



Salinas (b)

Mala (c)



Chancay (c)

San Vicén

Ica


Pisco (h)

Tambo de Mora (c)



Pisco (c)



San Andrés (c)



Paracas (c)



Pto. Caballos (c)



San Juan (c)

Arequipa

Lomas (h)

Mollendo (h)

Lomas (c)

Quilca (c)



Yauca (c)

Matarani (c)



Chala (b)

Islay (b)



Atico (c)

Mollendo (c)



Camaná (c)

Mejía (c)

Moquegua



Ilo (b)

Tacna



Los Baños (c)



La Yarada (c)



Pascana del Hueso (c)

CHILE

Tarapacá



Arica (b)



Pisagua (b)



Iquique (b)



Chanabaya (h)



Caleta Pabellón de Pica (h)



Punta Lobos (b)



Guanillo del Norte (h)

Antofagasta



Tocopilla (b)



Cobija (h)



Mejillones (b)



Antofagasta (b)



Taltal (c)

Atacama


Huasco (h)

Chanaral (b)



Caldera (b)



Carrizal Bajo (c)



Huasco (c)

Coquimbo

Tongoy (c)

La Serena (c)

Coquimbo (h)


Coquimbo (c)



Los Vilos (c)


Aconcagua


Papudo (c)



Zapallar (c)


Valparaíso


Quintero (c)

Juan Fernandez Is. (h)


Valparaíso (h)

Concón (c)



Viña del Mar (c)



Valparaíso (c)



Laguna Verde (c)



Algarrobo (c)



El Quisco (c)

Santiago



El Tabo (c)



Las Cruces (c)



Cartagena (c)



San Antonio (c)



Llolleo (c)

Colchagua



Pichilemu (c)

Curicó



Iloca (c)

Maule


Chanco (c)

Constitución (b)



Curanipe (c)

Ñuble



Buchupureo (c)



Coloquecura (c)

Concepción

Laraquete (c)

Dichato (c)

Coelemu (h)


Tome (b)

Cerro Verde (c)


Coronel (h)

Penco (c)



Talcahuano (b)



Concepción (b)



Coronel (c)



Schwager (c)



Lota (c)

Arauco

Arauco (c)

Lebu (b)



Pto. Tirna (h)


Cautín


Pto. Saavedra (c)

Isla Mocha (h)


Nahuentue (c)

Mehuín (b)



Toltén (c)



Pto. Saavedra (h)

Valdivia

Mancera Is.(h)

Niebla (c)

Corral (h)


Corral (c)

Valdivia (h)

Osorno



Mansa River (h)

Chiloé

Pindo Is. (h)


Ancud (h)



Chiloé Is. (h)



Guafo (h)

Aisén

Puerto Aisén (h)



Legend:

c: Calculated wave height
h: Historically recorded wave height
b: Both c and h

Source:

Based on Hebenstreit, G.T., and Whitaker, R.E. Assessment of Tsunami Hazard Presented by Possible Seismic Events: Near Source Effects (McLean, Virginia: Science Applications Inc., 1981); and Lockridge, P.A. Report Report SE-39 - Tsunamis in Peru-Chile (Boulder, Colorado: World Data Center A for Solid Earth Geophysics, 1985).

A mathematical model combines hypothetical earthquakes with ocean-bottom topography to estimate the height of tsunamis that could be generated by different mechanisms in six gap areas, giving both near-source and far-field heights along the coast from central Colombia to southern Chile. While the study does not attempt to predict actual earthquakes and resulting tsunamis, the results are probably representative of those that could occur in a given area. The water-level anomaly, or wave height above mean sea level, calculated for tsunamis generated by uniform-uplift earthquakes in the principal seismic gap areas of the Pacific coast of South America is shown in Figures 11 -28A and 11 -28B. Figure 11 -28A covers Chile from the Department of Valdivia to the northern border. Figure 11-28B covers the area from the southern border of Peru to the Department of Chocó in Colombia. The approximate location of population centers is shown in Figure 11-29; Figure 11-30 summarizes the results in tabular form. Certain areas seem to be threatened by all or most tsunamis, regardless of the location of the earthquake that generated them. Such locations include the stretch from Guayaquil, Ecuador, to Chimbote, Peru; Callao and Pisco, Peru; and Arica, Iquique, Taltal, Caldera, and Coquimbo-to-Valdivia, Chile.

Of course, as the author of the study, Gerald Hebenstreit, points out, the threat is not uniform, but "making a distinction between a seven meter wave and a twelve meter wave seems pointless. Both are going to be highly destructive in most cases" (Hebenstreit, 1981).

CONCLUSIONS

A great deal of information on geologic hazards and their mitigation now exists for Latin America and the Caribbean. There is a gap, however, between the existence of this information and its use by development planners: planners may find it difficult to obtain it or incorporate it into the development process.

This chapter has provided guidelines on the use of geologic hazard information for development planning, and catalogues information at a general level. The next obvious step is to proceed to the national level. For each member state, a compilation should be made of existing information and information being prepared on hazards associated with ground shaking, landslides, liquefaction, volcanic eruptions, and tsunamis and on mitigation, monitoring, and warning measures now in place. Such a catalogue could also include a brief guide on how to use the information in a development planning study. These guides could be prepared cheaply and quickly. Yet they could greatly increase the value of expenditures already made for scientific and engineering studies of geologic hazards.

REFERENCES

Key to symbols found at the beginning of selected citations:

H = General Hazards
G = General Geologic Hazards
E = General Earthquake Hazards
EG = Ground Shaking and Fault Rupture
V = Volcanic Hazards
EL = Landslides and Liquefaction
T = Tsunami Hazards
* = Important reference (any category)

E* Algermissen, S.T. "Integration, Analysis, and Evaluation of Hazard Data" in Proceedings of the Geologic and Hydrologic Hazards Training Program, Denver, Colorado, March 5-30,1984, Open File Report 84-760 (Reston, Virginia: U.S. Geological Survey, 1984).

E* Algermissen, S.T., and Perkins, D.M. "A Technique for Seismic Zoning: General Considerations and Parameters" in Proceedings of the International Conference on Microzonation, vol. 2 (Seattle, Washington, 1972), pp. 865-878.

EG Anderson, I. "The Harmony That Caused Disaster" in New Scientist (October, 1985).

EG Bernard, R.R., et al. "On Mitigating Rapid Onset Natural Disasters: Project THRUST (Tsunami Hazards Reduction Utilizing Systems Technology)" in EOS, Transactions, vol. 69 (American Geophysical Union, 1988).

EG; L* Blair, M.L., et al. Seismic Safety and Land Use Planning: Selected Examples California, U.S. Geological Survey Professional Paper 941-B (Reston, Virginia: U.S. Geological Survey, 1979).

V* Blong, R.J. Volcanic Hazards (Sydney, Australia: Macquarie University Academic Press, 1984).

V* Bolt, B.A., et al. Geological Hazards (New York: Springer-Verlag, 1975).

Booth, B. "Assessing Volcanic Risk" in Geological Society of London Journal, vol. 136 (1979), pp. 331-340.

E;H G;L* Brown, Robert D., and Kockelman, W.J. Geological Principles for Prudent Land A Decision-Maker's Guide for the San Francisco Bay Region, U.S. Geological Survey Professional Paper 946 (Reston, Virginia: U.S. Geological Survey, 1983).

V Bullard, Fred M. Volcanoes of the Earth (Austin, Texas: University of Texas Press, 1962).

EL* Campbell, R.H., et al. "Landslide Classification for Identification of Mud Flow and Other Landslide Hazards" in Proceedings of the Geologic and Hydrologic Hazards Training Program, Denver, Colorado, March 5-30, 1984, Open File Report 84-760 (Reston, Virginia: U.S. Geological Survey, 1984).

E* Castano, J.C. "Earthquake Hazard Studies in South America" in Proceedings of the Geologic and Hydrologic Hazards Training Program, Denver, Colorado, March 5-30, 1984, Open File Report 84-760 (Reston, Virginia: U.S. Geological Survey, 1984).

E* Centre Regional de Sismología para América del Sur. Mapa de Intensidades Máximas de América del Sur. Programa para la Mitigación de los Efectos de los Terremotos en la Región Andina (Proyecto SISRA), vol. 12 (Lima, 1985).

G* - Mapa Neotectónico Preliminar para América del Sur. Programa para la Mitigación de los Efectos de los Terremotos en la Región Andina (Proyecto SISRA), vol. 11 (Lima, 1985).

G* - Mapa de Grandes Terremotos en América del Sur, 1520-1981. Programa para la Mitigación de los Efectos de los Terremotos en la Región Andina (Proyecto SISRA) (Lima, 1985).

G* - Mapa de Sismicidad de América del Sur, 1520-1981. Programa para la Mitigación de los Efectos de los Terremotos en la Región Andina (Proyecto SISRA) (Lima, 1985).

G* - Catálogo de Terremotos para América del Sur. Programa para la Mitigación de los Terremotos en la Región Andina (Proyecto SISRA), vols. 1-9 (Lima, 1985).

V* Crandall, D.R., et al. Sourcebook for Volcanic-Hazards Zonation, Natural Hazards 4 (Paris: UNESCO, 1984).

Esteva, L. Regionalización Sísmicade México para Fines de Ingeniería, Patrocinado por Institute de Investigaciones de la Industria Eléctrica, Comisión Federal de Electricidad (México: Universidad Nacional Autónoma de Mexico, Abril 1970).

Fleming, R.W., and Taylor, F.A. Estimating the Costs of Landslide Damage in the United States, U.S. Geological Survey Circular 832 (Reston, Virginia: U.S. Geological Survey, 1980).

Fleming, R.W., Varnes, D.J., and Schuster, R.L. "Landslide Hazards and Their Reduction" in Journal of the American Planning Association, vol. 45 (1979), pp. 428-439.

E Fournier, E.M. "Problems of Earthquake Risk Assessment" in Proceedings of the Geologic and Hydrologic Hazards Training Program, Denver, Colorado, March 5-30,1984, Open File Report 84-760 (Reston, Virginia: U.S. Geological Survey, 1984).

E* Ganse, R.A., and Nelson, J.B. Catalog of Significant Earthquakes, 2000 B.C.-1979 (Boulder, Colorado: World Data Center A for Solid Earth Geophysics, July 1981).

E Gersony, R. Lima Disaster Preparedness Report, vol. XV (Washington, D.C.: U.S. Agency for International Development, Office of Disaster Preparedness, 1982).

Gutenberg, B., and Richter, C.F. "Earthquake Magnitude, Intensity, Energy, and Acceleration" in Bulletin of the Seismological Society of America, vol. 46 (1956), pp. 105-145.

H* Hays, W.W. (ed.). Facing Geologic and Hydrologic Hazards: Earth Science Considerations, U.S. Geological Survey Professional Paper 1240-B (Reston, Virginia: U.S. Geological Survey, 1981).

EG* Hays, W.W. Procedures for Estimating Earthquake Ground Motions, U.S. Geological Survey Professional Paper 1114 (Reston, Virginia: U.S. Geological Survey, 1980).

T Hebenstreit, G.T. Assessment of Tsunami Hazard Presented by Possible Seismic Events: Far-Field Effects, Prepared for U.S. Agency for International Development (McLean, Virginia: Science Applications, October 1981).

T* Hebenstreit, G.T., and Whitaker, R.E. Assessment of Tsunami Hazard Presented by Possible Seismic Events: Near Source Effects, Prepared for U.S. Agency for International Development (McLean, Virginia: Science Applications, November 1981).

EG Institute de Ingeniería, Universidad Nacional Autónoma de México. Aceleraciones Maximas y Velocidades Máximas del Terreno con Períodos de Recurrencia de 50 años, 100 anos y 500 años. Unpublished.

V* International Association of Volcanology (ed.). Catalog of Active Volcanoes of the World Including Soliatara Fields (Rome: Istituto di Geologia Applicata, Facultà di Ingegneria).

T International Tsunami Information Center and International Oceanic Commission. Tsunami Newsletter, vol. XIX, no. 2 (Honolulu, Hawaii: August, 1986).

EL* Keefer, D.K.O. "Landslides Caused by Earthquakes" in Proceedings of the Geologic and Hydrologic Hazards Training Program, Denver, Colorado, March 5-30, 1984, Open File Report 84-760 (Reston, Virginia: U.S. Geological Survey, 1984).

EL Kockelman, W.J. 'Techniques for Reducing Landslide Hazards" in Proceedings of the Geologic and Hydrologic Hazards Training Program, Denver, Colorado, March 5-30, 1984, Open File Report 84-760 (Reston, Virginia: U.S. Geological Survey, 1984).

E* - Examples of Use of Geologic and Seismologic Information for Earthquake Hazard Reduction in Southern California, Open File Report 83-82 (Reston, Virginia: U.S. Geological Survey, 1983).

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E Kuroiwa. H. "Microzonificación Sísmica Aplicada al Planeamiento Urbano para la Prevención de Desastres" in Tecnia, vol. 2, no. 2 (Lima: November 1983).

T* Lockridge, P.A. Tsunamis in Peru-Chile, Report SE-39 (Boulder, Colorado: World Data Center A for Solid Earth Geophysics, U.S. Department of Commerce, NOAA, July, 1985).

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E;T McCann, W.R., et al. "Seismic Gaps and Plate Tectonics: Seismic Potential for Major Boundaries" in Paleop, vol. 117 (Basel, Switzerland: Birkhouser Verlag, 1979).

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T* National Geophysical Data Center. Tsunamis in Latin America Data File (computer printout) (Boulder, Colorado: National Geophysical Data Center, U.S. Department of Commerce, NOAA, July, 1986).

T National Oceanic and Atmospheric Administration, National Environmental Satellite, Data, and Information Service. United States Tsunamis 1690-1988 (Boulder, Colorado: NOAA, 1989).

T National Oceanic and Atmospheric Administration, Pacific Marine Environmental Laboratory. THRUSH (Tsunami Hazard Reduction Using System Technology). Third Annual Report (Seattle, Washington: NOAA, 1986).

T National Oceanic and Atmospheric Administration, Pacific Tsunami Warning Center. Communications Plan for the Tsunami Warning System, 10th ed. (Ewa Beach, Hawaii: NOAA, February, 1984).

National Science Foundation. Comprehensive Planning for Tsunami Hazard Areas (Washington, D.C.: National Science Foundation, 1988).

- Land Management Guidelines in Tsunami Hazard Zones (Washington, D.C.: National Science Foundation, 1982).

E;L Nichols, D.R., and Buchanan-Banks, J.M. Seismic Hazards and Land Use Planning, Circular 690 (Washington, D.C.: U.S. Geological Survey, 1974).

E;T Nishenko, S.P. "Seismic Potential for Large and Great Interplate Earthquakes Along the Chilean and Southern Peruvian Margins of South America: A Quantitative Reappraisal" in Journal of Geophysical Research, vol. 90, no. B5 (April 10,1985), pp. 3589-3615.

EG* - Circum-Pacific Seismic Potential 1989-1999 (Golden, Colorado: U.S. Geological Survey, 1989).

- Summary of Circum-Pacific Probability Estimates (unpublished table) (Golden, Colorado: U.S. Geological Survey, 1989).

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