Facing Geologic and Hydrologic Hazards

Earth-Science Considerations

Geological Survey Professional Paper 1240-B
United States Department of the Interior

MANUEL LUJAN, Jr., Secretary 

U.S. Geological Survey 

DALLAS L. PECK, Director

First printing 1981 

Second printing 1992

WE EXPRESS our appreciation to the several authors identified with the individual 

sections of this report, to P.P. Clarke, G.P. Eaton, F.N. Houser, D.A. Rickert, C.F. 

Shearer, and R.L. Wesson for numerous discussions that led to the content and organization of the report, to Ms. E.A. Buffa who prepared the manuscript, and to many 

other members of the U.S. Geological Survey staff who reviewed the report and 

provided technical assistance in its final preparation. Without all of these 

contributions, the expeditious preparation of this report would not have been possible.

Library of Congress Catalog Card No. 81-600123

COVER: Towering clouds of scorching volcanic gases, ash, and rock from the 

May 18, 1980, eruption of Mount St. Helens.
Facing Geologic and Hydrologic Hazards

Earth-Science Considerations

Edited by W. W. Hays

Geological Survey Professional Paper 1240-B

This is the second in a series of Earth-Science Special Reports being prepared by the U .S. Geological 

Survey to provide the public with timely and useful 

information on earth-science considerations relative to critical land, mineral, and water resource 

issues that currently face the Nation.

U.S. Government Printing Office, Washington, D.C., 1981
m '*>*&?&'<: 

-.A i . B .- -.JVS. «. -\^<( V
SAN

ANDREAS 

FAULT 

ZONE

Foreword

Geologic and hydrologic hazards have affected the 

lives of many Americans. The recent eruption of Mount 

St. Helens has demonstrated that we cannot ignore any 

natural hazard, even one that happens as infrequently 

as a volcanic eruption.

Earthquakes, floods, ground failures, and volcaniceruptions will continue to occur in the future. The Nation 

must devise and implement a variety of actions to reduce 

losses when these hazards happen. Formulation and implementation of effective loss-reduction actions will require participation and cooperation of all levels of government and private industry. Many questions must be 

answered and many problems must be solved to achieve 

this national goal.

Information has been assembled in this report to 

answer some of the broader questions concerning earthquakes, floods, ground failures, and volcanic eruptions 

and their impact on our Nation. The report suggests 

actions based on earth-science considerations which may 

be taken to reduce losses. Although the report provides 

only a brief overview of these geologic and hydrologic 

hazards, 1 believe it will be useful to those making decisions in the future about reducing losses from geologic 

and hydrologic hazards.

Doyle G. Frederick 

Acting Director

A high potential for loss of property and lives due to 

earthquake-induced ground shaking and movement of 

surface faults exists in the San Francisco metropolitan 

area. These illustrations (from left to right) are an aerial 

photograph of part of Daly City, a topographic map of 

the same area, and an aerial photograph of part of the 

San Francisco Peninsula. Outlined on each is the area 

of the San Andreas Fault Zone where the San Andreas 

fault (black line) enters the ocean from the land. Movement of this fault in the San Francisco earthquake of 

April 18, 1906, resulted in the loss of about 700 lives and 

millions of dollar's worth of damage. An earthquake of 

similar magnitude (8.3 on the Richter scale) today would 

result in about $24 billion worth of property damage and 

an estimated 5,000 deaths and 700,000 personal injuries 

from ground shaking alone! (See discussion on page 12.)

 Daly City aerial photograph flown May 17,1980. Color infrared, scale 

about 1:24,000, Frame 7121, U.S. Army Corps of Engineers.

 Topographic map, 1973. Scale 1:24,000. San Francisco Smith Quadrangle, U.S. Geological Survey.

 San Francisco Peninsula aerial photograph flown February 14, 1977. 

Color infrared, scale about 1:131,000, Frame ID No. 5770024660937, 

NASA U-2 photography.

FACING HAZARDS BV
Facing Geologic and 

Hydrologic Hazards

1. Introduction

By Walter W. Hays and Clem F. 

Shearer

1 Purpose and scope of the report

2 Why do we study geologic and

hydrologic hazards? 

Selected references

2. Hazards From Earthquakes

Ground Shaking 

By Walter W. Hays

6 What causes earthquakes? 

8 What causes ground shaking? 

10 What is the size of the area affected

by ground shaking? 

12 What is the economic impact of

ground shaking? 

14 What can be done to reduce losses

from ground shaking? 

14 Can earthquakes be predicted?

Surface Faulting

By Manuel G. Bonilla

16 What is surface faulting?

17 What are the effects of surface

faulting? 

20 Can the location, size, and type of

future faulting be predicted? 

23 What can be done to reduce losses

from surface faulting?

Earthquake-Induced Ground Failures

By T. Leslie Youd and David K. Keefer

23 What is the economic impact of 

earthquake-induced ground 

failures?

24 What types of ground failures are

caused by liquefaction? 

28 What can happen to quick clays

during ground shaking? 

30 What types of landslides are induced

by earthquakes? 

30 What can be done to reduce losses

from earthquake-induced ground

failures?

Tsunamis

By Walter W. Hays

32 What causes a tsunami?

34 Where have tsunamis occurred 

historically?

35 What is the economic impact of 

tsunamis?

36 What can be done to reduce losses 

from tsunamis?

3. Hazards From Floods

Floods

By George W. Edelen, Jr.

39 What is the cause of flooding?

41 What is man's role in aggravating 

flooding?

42 What is the impact of flooding on 

the economy?

Flash Flooding

46 What causes flash floods?

46 What are the physical characteristics

of flash floods? 

46 Where do flash floods occur?

Riverine Floods

48 What causes riverine floods?

48 What are the physical characteristics 

of riverine floods?

49 Where do riverine floods occur?

Tidal Floods

50 What causes tidal floods?

50 What are the physical characteristics

of tidal floods? 

50 Where do tidal floods occur?

What can be done to reduce losses from 

floods?

Selected references

Selected references
4. Hazards From Ground Failures

Landslides

By Robert L. Schuster, David J. 

Varnes, and Robert W. Fleming

55 How significant are landslides?

56 What is the economic impact of

landslides? 

60 What are the types and processes of

slope movement? 

62 What is the distribution of

landslides?

62 What causes landslides? 

64 What can be done to reduce losses

from landslides?

Expansive Soils

By Robert L. Schuster

66 What are the parent materials of

expansive soils? 

66 What is the economic impact of

expansive soils? 

68 What is the distribution of expansive

soils? 

70 What is the physical mechanism

causing expansive soils? 

70 What geologic factors cause swelling

and shrinking? 

70 Can expansive soils be recognized?

72 What can be done to reduce losses 

from expansive soils?

Subsidence

By Susan Lee and Donald R. Nichols

73 What is the economic impact of 

subsidence?

74 What are the locations and causes of

subsidence? 

83 What can be done to reduce losses

from subsidence?

Selected references

5. Hazards From Volcanic Eruptions 6.

Volcanic Eruptions

By Donal R. Mullineaux

87 What are the hazards from volcanic

eruptions? 

87 What is the economic impact of

volcanic eruptions?

89 What areas are likely to be affected 

by future volcanism?

90 What are the kinds of volcanic

eruptions and their effects? 

98 What is the frequency of occurrence

and volume of volcanic eruptions?

98 Can future volcanism be forecast?

99 What can be done to reduce losses 

from volcanism?

Selected references

Suggestions for Improving 

Decisionmaking To Face 

Geologic and Hydrologic 

Hazards

By Walter W. Hays and Clem F. 

Shearer

103 What is the cost of not attempting to 

reduce losses from geologic and 

hydrologic hazards?

104 What is the difference between good 

and bad decisions?

105 What can communities do?

106 What is the benefit-cost ratio of

reducing losses from geologic and 

hydrologic hazards?

107 What are the local-State-Federal 

roles in reducing losses from 

geologic and hydrologic hazards?

Selected references

FACING HAZARDS BVII
Example of a community where people are taking actions 

to reduce losses from geologic and hydrologic hazards. 

Some of the suburbs in this view, looking southeast from 

above Daly City, California, lie across strands of the 

San Andreas fault (red lines). The 1906 San Francisco 

earthquake caused as much as 8 feet of horizontal displacement along fault strands like these. Planners and 

decisionmakers in local communities such as Santa Clara 

County, identified approximately by the yellow lines at 

the top of the illustration, are using earth-science data 

to devise loss-reduction actions. State laws enacted in 

1972 require geologic investigations prior to construction 

of a public structure in the "special studies zone" (area 

in orange) and prohibit building of public structures 

across the red lines. (From Robinson and Spieker, 1978. 

Photograph by R. E. Wallace.)

INTRODUCTION
1. Introduction

Purpose and Scope of the

Report

The purpose of this report is to suggest actions 

involving earth-science considerations that planners and decisionmakers can take to reduce losses 

from geologic and hydrologic hazards. These naturally occurring phenomena take place because of 

continuing natural processes. Throughout history, 

these hazards have impacted man and his activities, 

causing considerable damage, injury, and loss of 

life. Increasing losses are expected in the future 

unless man modifies his activities in light of experience and knowledge of the earth sciences.

This report provides basic information on the 

hazards from earthquakes, floods, ground failures, 

and volcanic eruptions. It describes their physical 

characteristics, identifies the locations in the United 

States where they tend to happen; specifies their 

impact on the Nation's population, buildings, structures, and economy; and discusses actions that can 

reduce losses. Information in the report is presented from a national and regional perspective;

therefore, it generally will not be adequate for sitespecific applications. Other natural hazards, although important, are not discussed. The role of 

man in triggering some of the geologic and hydrologic hazards is discussed; this role was also 

treated in Synthetic Fuels Development published 

in 1979 by the U.S. Geological Survey. The social 

issues concerning geologic and hydrologic hazards 

are beyond the scope of this report and will not be 

discussed except in a general way.

The report is organized in a manner that will 

make it easy for a reader to learn about a specific 

hazard of interest by reading the chapter describing 

the hazard and Chapter 6, Suggestions for Improving Decisionmaking To Face Geologic and 

Hydrologic Hazards. Selected references are provided for the reader who wishes more detail. Additional data can be obtained from the geologic 

survey in each State and from the U.S. Geological 

Survev.

FACING HAZARDS Bl
Why Do We Study 

Geologic and Hydrologic

Hazards?

Geologic and hvdrologic hazards are studied to 

understand more completely the physical processes 

which cause them. Understanding these processes 

is a key step in devising methodologies for reducing 

losses to the Nation when these hazards occur in 

the future. The potential losses are high. According 

to J. H. Wiggins (1973), natural hazards in 1973 

collectively accounted for direct costs each year 

exceeding 1 percent of the Nation's gross national 

product. Average annual losses and the potential 

for sudden loss from occurrences of earthquakes, 

floods, ground failures, and volcanic eruptions are 

greater now and increasing fairly rapidly as a consequence of factors such as:

 Increasing numbers of the Nation's population 

are living in flood-prone areas, areas of high 

seismic risk, exposed coastal locations, and 

landslide-prone areas and near potentially active volcanoes.

 Urban centers are growing annually through 

construction of homes, schools, hospitals, 

high-rise buildings, factories, utility systems, 

dams, oil refineries, airports, and other facilities. This growth causes greater valued property to be exposed to geologic and hvdrologic 

hazards every year.

Planning and decisionmaking with respect to the 

hazards from earthquakes, floods, ground failures, 

and volcanic eruptions take place at all levels in 

our Nation. At each level, earth-science information 

is needed so that wise choices can be made from 

the possible responses. These choices are difficult 

to make for two reasons. First, future geologic and 

hvdrologic hazards occur at uncertain times and 

places with great variation in magnitude and probability of occurrence. Second, reducing losses requires integration of earth-science information 

within the planning and decisionmaking process.

Decisionmakers sometimes wrongly conclude 

that actions to reduce losses, based on earth-science 

information, are incompatible with other important 

social or economic needs such as housing or industrial development. Recently, however, geologists, planners, and decisionmakers have been able 

to work together in some areas to achieve a balance 

between the conflicting goals of public safety and 

adequate housing or economic development.

The variety of actions for reducing losses from 

geologic and hvdrologic hazards includes:

 Avoidance. Avoid the hazard by selecting other 

appropriate areas in which to live and build 

where the probability of occurrence of the 

hazard is lowest.

 Land-uae zoning. Reduce losses to certain 

types of structures susceptible to a particular 

hazard either by reducing their density or by 

prohibiting them within parts of the area characterized by a relatively high severity or probability of occurrence of the hazard.

 Engineering design. Allow all types of structures within a potentially hazardous area, but 

require site-specific engineering design and 

construction to increase the capability of the 

site or the structure to withstand the hazard.

 Distribution of losses. Use insurance and other 

financial methods to distribute the potential 

losses in a potentially hazardous area. 

By implementing one or more of these loss-reduction actions, a community can greatly increase 

public safety and achieve a higher standard of living.

The following chapters discuss the hazards from 

earthquakes, floods, ground failures, and volcanic 

eruptions and describe specific loss-reduction actions in more detail. Chapter 6 discusses the earthscience information needed by those devising and 

implementing loss-reduction actions.

B2 INTRODUCTION
Selected References Bolt, B. A., Horn, W. L., Macdonald, G. A., and others, 

1975, Geological hazards; earthquakes, tsunamis, volcanoes, avalanches, landslides and floods: New York, 

Springer-Verlag Publishing Co., 328 p.

Rickert, D. A., Ulman, W. J., and Hampton, E. R., 

eds., 1979, Synthetic fuels development Earth-science considerations: U.S. Geological Survey, Washington, D.C., 45 p.

Robinson, G. D., and Spicker, A. M., eds., 1978, Nature 

to be commanded: U.S. Geological Survey Professional Paper 950, 95 p.

White, G. F., and Haas, J. E., 1975, Assessment of 

research on natural hazards: Cambridge, Mass., The 

MIT Press, 487 p.

Wiggins, J. H., 1973., The risk in-balance in current 

public policies: Proceedings of Symposium on Risk 

Acceptance and Public Policy, Denver, Colo., International System Safety Society, p. 1-35.

Water level in chimney, approximately 6:00 p.m., 

May 9, 1981. Winter Park, Florida, sinkhole; view to 

the east. (Photograph by A. S. Navoy.)

FACING HAZARDS B3
2. Hazards From 

Earthquakes

r damage 

(omitted in California)

(From National Oceanic and Atmospheric Administration, 1970)

Symbols locate epicenters only; 

areas affected were much larger.

Location of damaging historic earthquakes through 1970 in the United States (from Robinson and Spieker, 1978). 

Earthquakes happen most frequently in Alaska and less frequently in California and are relatively infrequent in 

the Central and Eastern United States. Earthquakes cause loss from ground shaking, surface faulting, ground 

failures, and tsunamis. The economic loss increases as the magnitude increases and is most extensive for the 

infrequent great earthquakes of magnitude 8 and above. The 1906 San Francisco, California, and 1964 Prince 

William Sound, Alaska, earthquakes were the last great earthquakes. Communities throughout the Nation, however, 

face the greatest threat of potential loss from moderate and large earthquakes which happen more frequently than 

great ones.

B4 EARTHQUAKES
Hazards associated with earthquakes include the 

phenomena of ground shaking, surface faulting, 

earthquake-induced ground failures, and tsunamis. 

Although earthquakes have caused much less economic loss annually in the United States than 

ground failures and floods, they have the potential 

for causing great sudden loss. Within 1 to 2 minutes, an earthquake can impact part or all of a city 

through ground shaking, surface fault rupture, and 

earthquake-induced ground failures. A tsunami can 

also be generated in some earthquakes and impact 

local and distant coastal communities. Depending 

on its location and magnitude (an indication of the 

energy released), an earthquake can damage buildings and homes valued collectively in billions of 

dollars, can cause loss of life and injury to tens of 

thousands, and can disrupt social and economic 

functions of communities.

120° 110° 100° 90°

_ _ _ _ __| Mo.. 1895 .<~- _____ '. -

Communities throughout the Nation face the 

possibility of loss from the several thousand earthquakes that happen each year. The greatest threat 

is from moderate (magnitudes of 6-7) and large 

(magnitudes of 7-8) earthquakes because they happen more frequently than a great one (magnitudes 

of 8 and above). For example, one moderate earthquake takes place on the average about once every 

3 years in California, but a great one happens only 

about once every 100 to 150 years. Earthquakes 

happen most frequently in Alaska and least frequently in the Eastern United States. A large earthquake, such as the 1811-12 New Madrid, Missouri, 

earthquakes, happens about once every 600 to 700 

years. Locations of moderate and large earthquakes 

in the east include the St. Lawrence River region 

from 1650 to 1928, in the vicinity of Boston in 1755, 

in the central Mississippi Valley in 1811-12, and 

near Charleston, South Carolina, in 1886.

In the past 20 years, the two most destructive 

earthquakes were the Prince William Sound, Alaska, 

earthquake of March 27, 1964 (a great earthquake), 

and the San Fernando, California, earthquake of 

February 9, 1971 (a moderate earthquake,. Losses 

in comparable dollars of about $500 million and 

scores of deaths and injuries resulted from each 

earthquake.

Location of notable historic earthquakes in the United 

States that have caused damage (from Hays, 1980) Although many earthquakes take place every year, most 

are small and do not cause damage. All or parts of 39 

States lie in regions classed as having major and moderate seismic risk. Within these 39 States, more than 70 

million people are exposed to earthquake hazards of 

ground shaking, surface faulting, earthquake-induced 

ground failures, and tsunamis.

FACING HAZARDS B5
Ground Shaking

What CaUSeS Earthquakes? An earthquake the sudden motion or trembling

in the Earth caused by an abrupt release of slowly 

accumulating strain is one of the severest natural 

hazards. Each year, several million earthquakes 

happen throughout the world, varying in size from 

minor tremors that are perceptible only to sensitive 

instruments to a few great earthtjuakes that cause 

considerable damage, injuries, and loss of life. The 

theory of plate tectonics can explain earthquakes. 

In this theory, which was introduced in 1967, the 

"solid" Earth is broken into several major plates. 

These 50- to 60-mile-thick rigid plates or segments 

of the Earth's crust and upper mantle move slowly 

and continuously over the interior of the Earth, 

meeting in some areas and separating in others. 

Velocities of relative motion between adjacent 

plates range from less than a fraction of an inch to 

about 5 inches per year. Although these velocities 

are slow by human standards, they are rapid by 

geologic ones; a motion of 2 inches per year adds 

up to 30 miles in only 1 million years. As these 

plates move, strain accumulates. Eventually, faults 

along or near plate boundaries slip abruptly and an 

earthquake occurs. Scientists monitoring borehole instruments.

W GROUND SHAKING
INDIA

AUSTRAUAN 

PUTE

60°

18(f 180°

Map showing the major tectonic plates of the world. Earthquake activity marks plate boundaries. The double line 

indicates a zone of spreading from which plates are moving, apart. Line with barbs indicates a zone where one plate 

is sliding beneath another. A single line indicates a strike-slip fault along which plates are sliding past one another. 

(Compiled and adapted from many sources; much simplified in complex areas )

FACING HAZARDS B7
What Causes Ground Shaking?

B8 GROUND SHAKING

Ground shaking is a term used to describe the 

vibration of the ground during an earthquake. 

Ground shaking is caused by body and surface seismic waves. As a generalization, the severity of 

ground shaking increases as magnitude increases 

and decreases as distance from the causative fault 

increases. Although the physics of seismic waves 

is complex, ground shaking can be explained in 

terms of body waves, compressional, or P, and 

shear, or S, and surface waves, Rayleigh and Love. 

P waves propagate through the Earth with a speed 

of about 15,000 miles per hour and are the first 

waves to cause vibration of a building. S waves 

arrive next and cause a structure to vibrate from 

side to side. They are the most damaging waves, 

because buildings are more easily damaged from 

horizontal motion than from vertical motion. The 

P and S waves mainly cause high-frequency vibrations; whereas, Rayleigh and Love waves, which 

arrive last, mainly cause low-frequency vibrations. 

Body and surface waves cause the ground, and consequently a building, to vibrate in a complex manner. The objective of earthquake-resistant design 

is to construct a building so that it can withstand 

the ground shaking caused by body and surface 

waves.

One of the partially collapsed reinforced-concrete columns of the heavily damaged six-story Imperial County 

Services Building, El Centra, California. Damage occurred during the October 15, 1979, Imperial Valley 

earthquake.

In land-use zoning and earthquake-resistant design, knowledge of the amplitude, frequency composition, and the time duration of ground shaking 

is needed. These quantities can be determined 

from empirical data correlating them with the magnitude and the distribution of Modified Mercalli 

intensity of the earthquake, distance of the building 

from the causative fault, and the physical properties 

of the soil and rock underlying the building. The 

subjective numerical value of the Modified Mercalli Intensity Scale indicates the effects of ground 

shaking on man, buildings, and the surface of the 

Earth.
Schematic illustration of the directions of vibration 

caused by body and surface seismic waves generated 

during an earthquake. When a fault ruptures, seismic 

waves are propagated in all directions, causing the 

ground to vibrate at frequencies ranging from about 0.1 

to 30 Hertz. Buildings vibrate as a consequence of the 

ground shaking; damage takes place if the building cannot withstand these vibrations. Compressional and shear 

waves mainly cause high-frequency (greater than 1 

Hertz] vibrations which are more efficient than low-frequency waves in causing low buildings to vibrate. Rayleigh and Love waves mainly cause low-frequency vibrations which are more efficient than high-frequency 

waves in causing tall buildings to vibrate. Because amplitudes of low-frequency vibrations decay less rapidly 

than high-frequency vibrations as distance from the fault 

increases, tall buildings located at relatively great distances (60 miles) from a fault are sometimes damaged.

LOVE WAVE

AYLEIGH WAVE

\ \

COMPRESSIONAL

WAVE FRONTS

FACING HAZARDS B9
What Is the Size of the Area

Affected by Ground

Shaking?

The size of the geographic area affected by 

ground shaking depends on the magnitude of the 

earthquake and the rate at which the amplitudes 

of body and surface seismic waves decrease as distance from the causative fault increases. Comparison of the areas affected by the same Modified 

Mercalli intensity of ground shaking in the 1906 

San Francisco, California, the 1971 San Fernando, 

California, the 1811-12 New Madrid, Missouri, 

and the 1886 Charleston, South Carolina, earthquakes shows that a given intensity of ground shaking extends over a much larger area in the Eastern 

United States. Ground shaking affects a larger area 

because amplitudes of seismic waves decrease 

more slowly in the east than in the west as distance 

from the causative fault increases.

A seismologist at the National Earthquake Information 

Service in Golden, Colorado, checks seismograph records from stations throughout the United States.

810
95°

19O6 SAN FRANCISCO

1811 NEW MADRID

1971 SAN FERNANDO.

3f

Comparison of isoseismal contours for a great earthquake, the 1906 San Francisco; 

a moderate earthquake, the 1971 San Fernando; and two large earthquakes, the 

1811-12 New Madrid and 1886 Charleston. The contour lines connect sites having the 

same value of Modified Mercalli intensity, a numerical index of the effects of an 

earthquake on man, the Earth's surface, and on buildings. Each area shown in red

corresponds to an intensity of VII or greater and denotes the zone of most severe 

ground shaking and damage. The area shaded blue corresponds to an intensity of VI; 

in this area,the ground shaking is felt by all, many are frightened and run outdoors, 

but damage is slight. The effects of ground shaking extend over a much larger area 

in the Eastern United States.

FACING HAZARDS Bll
What Is the Economic Impact 

of Ground Shaking?

Most of the spectacular damage that takes place 

during earthquakes is caused by partial or total 

collapse of buildings as a result of ground shaking. 

In addition, ground shaking can induce destructive 

ground failures. In the 1971 San Fernando, California, earthquake, for example, a large part of the 

$500 million loss was caused directly by structural 

damage from ground shaking. A repeat today of the 

San Francisco, California, earthquake, which in 

1906 destroyed buildings with costs translated into 

1978 dollars of almost $170 million and took 700 

lives, probably would cause $24 billion in building 

damage and (depending on the time of day of the 

occurrence) about 5,000 deaths and 700,000 injuries from ground shaking alone (Wiggins, 1979?). 

Much of this hundredfold increase in losses, relative to 1906, is due to the greater number and value 

of buildings and the growth in population. A similar 

estimate for a 1990 repeat of the 1906 San Francisco

earthquake suggests losses of $30 billion from 

ground shaking, excluding losses from fire and 

damage to infrastructure (Wiggins, 1979?). Loss 

estimates for the Los Angeles area are slightly 

higher than for San Francisco. If an earthquake 

should occur today on the Newport-Inglewood fault 

near Los Angeles, it probably would cause losses 

of about $45 billion to buildings, and as many as 

23,000 deaths, depending on whether the earthquake occurred at noontime or during the rush 

hours (the "worst" hazardous times) or in the early 

morning while most people are in bed (the "best" 

hazardous time) (U.S. Federal Emergency Management Agency, 1981).

A repeat of the 1811-12 New Madrid, Missouri, 

earthquakes is estimated to cause losses comparable with the "worst-case" estimates for San Francisco or Los Angeles.

118°22'30"

Zone of surface rupture; 

San Fernando earthquake

B12 GROUND SHAKING
o 124° 123° 122° 121° 120?

CALIFORNIA

N'

a \

o 35T

<^ s_

"P 0

-^

0 100 I 1 I 1 I

-^

O^s'S^ V

* ^

200 MILES^ 

|

.Sar

Index map of California showing 

location of the San Fernando Valley

Sketch map showing San Fernando valley where the most 

severe ground shaking took place in the 1971 San Fernando, California, earthquake. Photograph shows damage from ground shaking at Los Angeles Olive View 

Medical Center. The 850-bed center, costing approximately $24 million, was heavily damaged. Three stair 

towers toppled and broke through the roof of the ground 

story, an ambulance port collapsed, and the first story 

of the psychiatric unit collapsed. Fortunately, only three 

persons were killed. At Olive View, the ground shaking 

was rated XI on the Modified Mercalli Intensity Scale; 

above VII throughout the valley. (Photograph by R. E. 

Wallace.)

FACING HAZARDS B13
What Can Be Done To Reduce

Losses From Ground

Shaking?

Old existing buildings that fail to meet present 

standards for earthquake resistance face the greatest threat from ground shaking. The number of 

such buildings in the United States is very large. 

Applying loss-reduction measures to these substandard buildings is a major unresolved problem 

because of economic, social, and political factors. 

The primary choices for reducing losses from substandard buildings include (1) engineering redesign 

and retrofitting to strengthen the structure, (2) reduction in intensity of use, and (3) removal.

To reduce losses in new buildings, the primary 

choices include (1) avoiding the areas of most severe 

ground shaking, (2) land-use zoning either to prohibit certain types of structures susceptible to damage or to reduce the density of certain uses in areas 

having a high probability of severe ground shaking,

(3) incorporating the earthquake-resistant design 

provisions of the Uniform Building Code during 

construction, and (4) earthquake insurance.

Ground-shaking hazard maps can be used in selecting a loss-reduction action for new buildings. 

The map shown was used in the definition of seismic risk zones in the Applied Technology Council's 

model code of 1978. The ground-shaking hazard is 

presented in terms of contoured values of the peak 

horizontal ground acceleration expected in a 50- 

year period at the 90-percent probability level for 

sites underlain by rock. These values of peak acceleration must be modified to account for possible 

amplification of ground shaking by the soil column 

at the specific site before they are used in earthquake-resistant design.

Can Earthquakes 

Be Predicted?

An effort to understand the physical conditions 

that precede an earthquake was initiated following 

the destructive Prince William Sound, Alaska, 

earthquake of March 27, 1964. This effort had the 

objective of predicting the size, time, and location 

of an impending shock. It was increased after the 

damaging San Fernando, California, earthquake of 

February 9, 1971. In 1977, the Earthquake Hazards Reduction Act was passed; it contained earthquake prediction as a major element of a national 

program to reduce losses from future earthquakes 

in the United States.

Earthquake prediction is a rapidly emerging scientific field offering great promise for loss reduction. Although accurate predictions of the size 

(magnitude), time, and location of future earthquakes in the United States may still be years away, 

scientific information needed for making reliable

predictions within the next decade are emerging 

from studies by earth scientists from many different 

institutions in the United States and in several 

other countries, including the Soviet Union, Japan, 

and China.

As with most new technological developments, 

earthquake prediction must be approached carefully. Earthquake prediction will save lives; this 

has already been demonstrated by the successful 

prediction of the destructive earthquake that struck 

Haicheng, China, on February 4, 1975. However, 

in the United States, a prediction can cause serious 

socioeconomic problems if it is not properly implemented. Scientists, planners, and decisionmakers must be aware of and resolve complexly interrelated technical-socioeconomic-political factors to 

benefit from earthquake prediction (Hays, 1980).

 B14 GROUND SHAKING
Map of peak horizontal ground acceleration expected at 

sites in the conterminous United States underlain by rock 

in a 50-year period (from Algermissen and Perkins, 

1976). This map represents the ground-shaking hazard 

in terms of the peak amplitude of horizontal acceleration, 

one characteristic of the strength of the seismic shaking. 

Locations having the same value of peak acceleration are 

connected with a contour line. Values shown on each 

contour and on the map are percentages of the acceleration of gravity. There is a 10-percent probability that 

these values will he exceeded in a 50-year period. This 

map takes into account the relative differences in rate 

of seismic activity in the Eastern and Western United 

States. Areas where peak acceleration exceeds 10 percent 

of the acceleration of gravity are shaded. The largest values shown, along the California coast, are 80 percent of 

gravity.

FACING HAZARDS B15
Surface Faulting 

What is Surface Faulting? Surface faulting the differential movement of 

the two sides of a fracture at the Earth's surface 

is of three general types: strike-slip, normal, and 

reverse. Combinations of the strike-slip type and 

the other two types of faulting can be found. Although displacements of these kinds can result from 

landslides and other shallow processes, surface 

faulting, as the term is used here, applies to differential movements caused by deep-seated forces 

in the Earth, the slow movement of sedimentary 

deposits toward the Gulf of Mexico, and faulting 

associated with salt domes.

STRIKE-SLIP FAULT 

Horizontal displacement

NORMAL FAULT

Principally vertical displacement with the side above the 

inclined fault moving downward

BID SURFACE FAULTING

it i t » i / 

t f

 * t

REVERSE FAULT

Principally vertical displacement with the side above the 

inclined fault moving upward

Types of displacements of faults. Actual ruptures are 

more complex than shown on these diagrams. (Modified 

from Borcherdt, 1975.)
What Are the Effects of 

Surface Faulting?

SAN

118°22'30" 

GABRIEL MOUNTAINS

Zone of surface rupturer 

San Fernando earthquake

SAN FERNANDO \VALLEY 

SANTA MONICA MOUNTAINS

Sketch map of active fault zone crossing an urban area 

and an example of damage to a house caused by an abrupt 

fault rupture at the surface during the 1971 San Fernando, California, earthquake.

Death and injuries from surface faulting are very 

unlikely, but casualties can occur indirectly through 

fault damage to structures. Surface faulting, in the 

case of a strike-slip fault, generally affects a long 

narrow zone whose total area is small compared 

with the total area affected by ground shaking. 

Nevertheless, the damage to structures located in 

the fault zone can be very high, especially where 

the land use is intensive. A variety of structures 

have been damaged by surface faulting, including 

houses, apartments, commercial buildings, nursing 

homes, railroads, highways, tunnels, bridges, canals, storm drains, water wells, and water, gas, and 

sewer lines. Damage to these types of structures 

has ranged from minor to very severe. An example 

of severe damage occurred in 1952 when three railroad tunnels were so badly damaged by faulting

that traffic on a major rail linking northern and 

southern California was stopped for 25 days despite 

an around-the-clock repair schedule.

The displacements, lengths, and widths of surface fault ruptures show a wide range. Fault displacements in the United States have ranged from 

a fraction of an inch to more than 20 feet of differential movement. As expected, the severity of potential damage increases as the size of the displacement increases. The lengths of the surface fault 

ruptures on land have ranged from less than 1 mile 

to more than 200 miles. Most fault displacement 

is confined to a narrow zone ranging from 6 to 1,000 

feet in width, but separate subsidiary fault ruptures 

may occur 2 to 3 miles from the main fault. The 

area subject to disruption by surface faulting varies 

with the length and width of the rupture zone.
118°22'30"

SAN GABRIEL MOUNTAINS

PER NAN DO \ VALLEY 

SANTA MONICA MOUNTAINS

An example of damage to street pavement and 

sidewalks caused by surface fault rupture in the 1971 

San Fernando, California, earthquake. Several commercial and industrial buildings nearby were damaged 

severely. The location of the photograph is shown on 

the sketch map.

An example of horizontal and vertical movement of 

pavement, curb, and sidewalk in the 1971 San Fernando, California, earthquake. The nursing home in the 

background was damaged severely and had to be 

removed. The location of the photograph is shown on 

the sketch map. (Photograph by T. L. Youd.)

RJ8 SURFACE FAULTING

 ^sz
Displacement of crop rows hy strike-slip movement, Imperial Valley, California. This type of displacement damaged underground drain tile, canals, and highways over 

more than 15 miles.

Surface faulting (shown hy white hand) that occurred 

in Nevada in 1915. The faulting extended discontinuously 

for 38 miles and had vertical displacements as much as 

22 feet. (Photograph hy R. E. Wallace.)

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Can The Location, Size, and

Type of Future Faulting

Be Predicted?

Almost all historic surface faulting has taken 

place on faults that exhibit geologically young displacements. Therefore, future faulting is expected 

to take place on geologically young faults, and prediction is based on identification of them. Such 

faults can be identified by the kind of topography 

along them, the displacement of young geologic 

units, and the occurrence of earthquakes on them. 

Faults that are young and active commonly disturb 

the land surface by creating scarps (small steps or 

cliffs), troughs, or ridges and by diverting streams. 

These and other characteristic features of faults can 

be recognized by geologic studies. When a geologic 

study shows that a fault displaces young geologic 

units, this indicates that the fault has been active 

since the units were formed. Faults which generate 

earthquakes can be considered to be active, but 

some active faults may be deeply buried and not 

capable of rupturing the ground surface. Thus, 

young faults reaching the ground surface are the 

ones most likely to produce surface faulting.

Prediction of future faulting is important in reducing losses from earthquakes. Sudden displacements on faults are the cause of damaging earthquakes, and, therefore, the locations of active faults 

(faults that can undergo movement in the immediate geologic future) give an indication of where 

earthquakes may originate. Also, the severity of 

damage from ground shaking is greatest near the 

fault. Not all active faults generate destructive 

earthquakes. Displacement on the gulf coast faults 

in Texas and Louisiana, for example, is a slow process that does not generally generate earthquakes.

Areas in the United States where young surface 

faults are known to exist have been mapped. These 

maps show faults in two general categories those 

that have had displacement within the last 10,000 

years and those that have had displacement within 

the last 2 million years. These time periods are 

long by human standards, but short with regard to

geologic processes. Faults can lie dormant for many 

thousands of years between periods of vigorous 

activity, and, therefore, their behavior over a substantial part of their recent history must be considered. These maps also give an indication of the 

relative risk of surface faulting; surface faulting is 

most likely to take place in the areas of Holocene 

faulting (during the last 10,000 years), less likely 

in the areas of Quaternary faulting (last 2 million 

years), and least likely in the remaining area.

In looking at a map of young surface faulting, 

keep in mind that, for various reasons, more areas 

of young faulting probably exist than are indicated. 

Experience has shown that, as detailed geologic 

work is done at specific sites, additional young faulting is often recognized. In California, several 

known faults have recently been found to have 

young displacements on them, and some previously unknown young faults also have been discovered. Only in the last few years has evidence 

been found to prove surface faulting in the last 

10,000 years in the area of the very severe New 

Madrid, Missouri, earthquakes of 1811-12. Many 

faults in the United States have not been examined 

thoroughly with regard to the time of latest displacement on them and other faults have geologic 

conditions that make recognition of young displacements very difficult.

Useful estimates can be made of the maximum 

surface length and displacement that a specific fault 

is capable of producing. These estimates are based 

on statistical data obtained from field studies following past earthquakes and theoretical considerations. The type of faulting to be expected is the 

same as has occurred previously on the fault. The 

type of earlier faulting can be learned from geological, seismological, and historical data. The size 

and the type of faulting are important factors in 

evaluating the damage that might be caused in the 

future.

B20 SURFACE FAULTING
containing faults 

displacement in the 

Holocene

containing faults w 

displacement in the 

Quaternary

Maps of young surface faulting. A, Conterminous United States (this page). B, Alaska and Hawaii (following page).

FACING HAZARDS B21
EXPLANATION

Area containing 

faults with 

displacement 

in the Holocene

Area containing 

faults with 

displacement 

in the 

Quaternary

Principal Islands of 

HAWAII

0 100 200 300 100 MILES

I .i.l __i___.1_

What Can Be Done To Reduce

Losses From Surface

Faulting?

Avoidance and engineering design to accommodate the differential displacements are the primary actions that will reduce losses from surface 

faulting. Avoidance requires accurate location of 

the fault and an assessment of its history of activity 

through a detailed geologic examination. For public safety or economic reasons in some areas, certain 

types of structures are not built across particular 

faults. For example, in California, various State, 

county, and city laws regulate the construction of

schools, hospitals, and homes in areas susceptible 

to surface faulting. However, structures, such as 

pipelines, dams, bridges, and aqueducts, frequently cannot be built without crossing active 

faults. Some of these structures have been designed 

and constructed to accommodate fault displacements in an earthquake. These designs will probably be successful, but they have not yet been 

tested by actual large fault displacements.

B22 SURFACE FAULTING
Earthquake-Induced 

Ground Failures

The broad subject of ground failures is discussed 

in Chapter 4. Because certain types of ground failures are frequently associated with earthquakes, 

they will be discussed in this chaper for continuity.

What Is the Economic Impact

of Earthquake-Induced

Ground Failures?

Throughout the world, ground failures induced 

during earthquakes have caused many thousands 

of casualties and millions of dollars in property damage. Loss of life has been especially high in other 

countries. For example, soil-flow failures induced 

during the 1920 Kansu, China, earthquake killed 

an estimated 200,000 people. Fortunately, the impact in the United States has been limited mainly 

to economic loss. During the 1964 Prince William 

Sound, Alaska, earthquake, ground failures caused 

about 60 percent of the estimated $500 million total 

damage. In this earthquake, five landslides caused 

about $50 million damage in the city of Anchorage; 

lateral spread failures damaged highways and severely disrupted use of railway grades and bridges, 

requiring about $50 million in repairs; and flow 

failures in three Alaskan ports carried away docks, 

warehouses, and adjacent transportation facilities 

costing about $15 million.

Aerial view, looking east, of part of Turnagain 

Heights slide, Anchorage, shortly after Alaska earthquake, March 27, 1964
What Types of Ground

Failures Are Caused by

Liquefaction?

Liquefaction is not a type of ground failure; it is 

a physical process that takes place during some 

earthquakes that may lead to ground failure. As a 

consequence of liquefaction, clay-free soil deposits, 

primarily sands and silts, temporarily lose strength 

and behave as viscous fluids rather than as solids. 

Liquefaction takes place when seismic shear waves 

pass through a saturated granular soil layer, distort 

its granular structure, and cause some of the void 

spaces to collapse. Disruptions to the soil generated 

by these collapses cause transfer of the groundshaking load from grain-to-grain contacts in the soil 

layer to the pore water. This transfer of load increases pressure in the pore water, either causing 

drainage to occur or, if drainage is restricted, a 

sudden buildup of pore-water pressure. When the 

pore-water pressure rises to about the pressure 

caused by the weight of the column of soil, the 

granular soil layer behaves like a fluid rather than 

like a solid for a short period. In this condition, 

deformations can occur easily.

Liquefaction is restricted to certain geologic and 

hydrologic environments, mainly areas where sands 

and silts were deposited in the last 10,000 years 

and where ground water is within 30 feet of the 

surface. Generally, the younger and looser the sediment and the higher the water table, the more 

susceptible a soil is to liquefaction.

Liquefaction causes three types of ground failure: lateral spreads, flow failures, and loss of bearing strength. In addition, liquefaction enhances 

ground settlement and sometimes generates sand 

boils (fountains of water and sediment emanating 

from the pressurized liquefied zone). Sand boils 

can cause local flooding and the deposition or accumulation of silt.

LATERAL SPREADS

Lateral spreads involve the lateral movement of 

large blocks of soil as a result of liquefaction in a 

subsurface layer. Movement takes place in response to the ground shaking generated by an 

earthquake. Lateral spreads generally develop on 

gentle slopes, most commonly on those between 

0.3 and 3 degrees. Horizontal movements on lateral 

spreads commonly are as much as 10 to 15 feet,

but, where slopes are particularly favorable and the 

duration of ground shaking is long, lateral movement may be as much as 100 to 150 feet. Lateral 

spreads usually break up internally, forming numerous fissures and scarps.

Damage caused by lateral spreads is seldom catastrophic, but it is usually disruptive. For example, 

during the 1964 Prince William Sound, Alaska, 

earthquake, more than 200 bridges were damaged 

or destroyed by lateral spreading of flood-plain deposits toward river channels. These spreading deposits compressed bridges over the channels, buckled decks, thrust sedimentary beds over abutments, and shifted and tilted abutments and piers.

Lateral spreads are destructive particularly to 

pipelines. In 1906, a number of major pipeline 

breaks occurred in the city of San Francisco during 

the earthquake because of lateral spreading. Breaks 

of water mains hampered efforts to fight the fire 

that ignited during the earthquake. Thus, rather 

inconspicuous ground-failure displacements of less 

than 7 feet were largely responsible for the devastation to San Francisco in 1906.

FLOW FAILURES

Flow failures, consisting of liquefied soil or 

blocks of intact material riding on a layer of liquefied soil, are the most catastrophic type of ground 

failure caused by liquefaction. These failures commonly move several tens of feet and, if geometric 

conditions permit, several tens of miles. Flows 

travel at velocities as great as many tens of miles 

per hour. Flow failures usually form in loose saturated sands or silts on slopes greater than 3 degrees.

Flow failures can originate either underwater or 

on land. Many of the largest and most damaging 

flow failures have taken place underwater in coastal 

areas. For example, submarine flow failures carried 

away large sections of port facilities at Seward, 

Whittier, and Valdez, Alaska, during the 1964 

Prince William Sound earthquake. These flow failures, in turn, generated large sea waves that overran parts of the coastal area, causing additional 

damage and casualties. Flow failures on land have 

been catastrophic, especially in other countries.

GROUND FAILURES _
For example, the 1920 Kansu, China, earthquake 

induced several flow failures as much as 1 mile in 

length and breadth, killing an estimated 200,000 

people.

LOSS OF BEARING STRENGTH

When the soil supporting a building or some 

other structure liquefies and loses strength, large 

deformations can occur within the soil, allowing 

the structure to settle and tip. The most spectacular

Sketch of water-saturated group of sand grains illustrating the process of liquefaction. Shear deformations 

(indicated by large arrows) brought about by earthquake 

ground shaking distort the granular structure causing 

some loosely packed groups to collapse as indicated by 

the small arrows. Each collapse transfers stress from 

grain-to-grain contacts to the pore water, increasing the 

pressure in that water. When pore-water pressures reach 

a critical level (grain-to-grain contact stresses approach 

zero), the granular material suddenly behaves as a liquid 

rather than as a solid. At this point, liquefaction has 

taken place.

SHEAR DEFORMATION

example of bearing-strength failures took place during the 1964 Niigata, Japan, earthquake. During 

that event, several four-story buildings of the 

Kwangishicho apartment complex tipped as much 

as 60 degrees. Most of the buildings were later 

jacked back into an upright position, underpined 

with piles, and reused.

Soils that liquefied at Niigata typify the general 

subsurface geometry required for liquefaction-caused 

bearing failures: a layer of saturated, cohesionless 

soil (sand or silt) extending from near the ground 

surface to a depth of about the width of the building.

Group of sand boils generated during the 1979 Imperial 

Valley, California, earthquake. Sand boils are caused 

by water laden with sediment venting from subsurface 

layers of sand or silt in which artesian pore-water pressures develop during the liquefaction process.

SHEAR DEFORMATION

FACING HAZARDS B25
Alaska railroad bridge compressed and buckled by lateral spreading of flood-plain deposits toward the river 

channel during the 1964 Prince William Sound earthquake. Lateral spreads arc caused by loss of strength in 

a subsurface soil layer because of liquefaction. This loss 

of strength allows overlying sediments to move laterally 

down very gentle slopes (usually between 0.3 and 3 degrees) in response to a combination of gravitational and 

earthquake loads.

*4&<*j»£t^fiaft

u& *=r».i^« « * ;,***£"*' ? *»

Photograph of San Francisco, California, showing areas 

where liquefaction and lateral spreading took place during the 1906 earthquake. Lateral spreads disrupted many

buildings and severed several water mains. Lack of water 

hampered efforts to contain the fire that broke out during 

the earthquake.

B26 GROUND FAILURES
Seward, Alaska, before (A.) and after (B) the 1964 Prince William Sound earthquake 

showing loss of docks, warehouses, and railroad facilities carried into the sea by a 

flow failure. Flow failures take place when loose sands and silts liquefy, lose strength, 

and flow down slope. Flows seldom are found on slopes less than 3 degrees. The view 

is to the south in the top photograph and to the northwest in the bottom photograph. 

Seventh Street is nearest the docks and runs from the oil tanks toward the sea in the 

top photograph and approximately left to right in the bottom photograph.

Four-story apartment buildings in Niigata, Japan, that settled and tipped because of 

loss of strength in supporting soils during the 1964 Niigata earthquake. Loss of bearing 

strength was caused by liquefaction of a sand layer that extends 10 to 15 feet below 

the base of the foundations.

FACING HAZARDS B27
What Can Happen to

Quick Clays

During Ground Shaking?

B28 GROUND SHAKING

Most clays lose strength when disturbed by 

ground shaking, and, if the loss of strength is large, 

some clays, called "quick" or "sensitive," may fail. 

The five large landslides that affected parts of Anchorage during the 1964 Prince William Sound, 

Alaska, earthquake are examples of spectacular failures of sensitive clays. The failure zones of these 

slides passed through layers of the Bootlegger Cove 

Formation which contained layers of quick clay as 

well as lenses of saturated sand and silt. Loss of 

strength that took place in the clay layers and liquefaction that took place in the sand and silt lenses 

within the Bootlegger Cove Formation because of 

the severe ground shaking were major factors contributing to the landsliding. These landslides caused 

an estimated $50 million in damage. Drilling a borehole.

i r
i."V .T-t . A (, 

Li* i-W

Ground disruption and damage at the upper part of one 

of five major landslides that developed in Anchorage, 

Alaska, during the 1964 Prince William Sound earthquake. The landslides were induced by a combination 

of loss of strength in sensitive clay layers and liquefaction 

of sand and silt lenses in the Bootlegger Cove Formation.

FACING HAZARDS B29
What Types of Landslides

Are Induced by

Earthquakes?

Past experience has shown that several types of 

landslides take place in conjunction with earthquakes. The most abundant types of earthquakeinduced landslides are rock falls and slides of rock 

fragments that form on steep slopes. Shallow debris 

slides forming on steep slopes and soil and rock 

slumps and block slides forming on moderate to 

steep slopes also take place, but they are less abundant. Reactivation of dormant slumps or block 

slides by earthquakes is rare.

Large earthquake-induced rock avalanches, soil 

avalanches, and underwater landslides can be very 

destructive. Rock avalanches originate on oversteepened slopes in weak rocks. One of the most 

spectacular examples occurred during the 1970 

Peruvian earthquake when a single rock avalanche 

killed more than 18,000 people; a similar, but less 

spectacular, failure in the 1959 Hebgen Lake, Montana, earthquake resulted in 26 deaths. Soil avalanches occur in some weakly cemented finegrained materials, such as loess, that form steep 

stable slopes under nonseismic conditions. Many 

loess slopes failed during the New Madrid, Missouri, earthquakes of 1811-12. Underwater landslides commonly involve the margins of deltas 

where many port facilities are located. The failures 

at Seward, Alaska, during the 1964 earthquake are 

an example.

The size of the area affected by earthquake-induced landslides depends on the magnitude of the 

earthquake, its focal depth, the topography and 

geologic conditions near the causative fault, and 

the amplitude, frequency composition, and duration of ground shaking. In past earthquakes, landslides have been abundant in some areas having 

intensities of ground shaking as low as VI on the 

Modified Mercalli Intensity Scale.

What Can Be Done To Reduce 

Losses From EarthquakeInduced Ground Failures?

Actions for reducing losses from lateral spreading 

include zoning to limit building in susceptible 

areas, stabilization to prevent liquefaction and 

ground failure, and construction of displacementresistant foundations. Engineering techniques for 

stabilizing sites against liquefaction include compaction, grouting, or drainage of susceptible soils. 

These techniques are generally expensive and, 

therefore, are not economically feasible unless critically important facilities are being built. Construction of displacement-resistant foundations is presently beyond the state-of-the-art for ground-failure 

displacements greater than 1 foot.

Avoidance and engineering design are the primary actions for reducing losses from flow failures. 

Avoidance of sites susceptible to flow failure is frequently possible. Sometimes, small areas can be 

stabilized by engineering techniques to prevent 

liquefaction. However, no practical means exist for 

stabilizing large geographic areas, such as those 

that failed in Alaska, against flow failure.

Actions for reducing damage due to loss of bearing strength include site selection to avoid the hazard, stabilization of liquefiable layers to prevent 

loss of strength, and use of deep foundations (such 

as piles) to transfer loads to stable layers underlying 

potentially liquefiable ones.

Avoidance, land-use zoning, and excavation are 

the primary actions for stabilizing areas susceptible 

to landslides. Specific actions are discussed in more 

detail in Chapter 4.

830 GROUND SHAKING
Rock falls and slides that took place on steep slopes of 

the eastern Sierra Nevada Mountains during the 1980 

Mammoth Lakes, California, earthquakes are examples 

of the most abundant type of earthquake-induced landslides. Debris from the falls and slides accumulated in 

piles of loose fragments at bases of slopes or formed thin 

rock-and-snow avalanches that moved 0.5 mile down 

snow-covered slopes. Rock falls and slides are common 

on slopes greater than 30 degrees. Almost all types of 

rock are affected, although failures have been most common in rocks that are weathered, thinly bedded, poorly 

cemented, or closely jointed.

Sixty-five-million-cubic-yard rock-fall avalanche that cascaded from Mount Huascaran during the 1970 Peruvian 

earthquake, burying most of the city ofYungay and part 

of the city ofRanrahirca and killing 18,000 people. Massive rock-fall avalanches of this type, which took place 

in the 1959 Hebgen Lake, Montana, earthquake, usually 

form on high slopes oversteepened by recent or active 

erosion. Rock units most vulnerable to failure are those 

weakened by weathering, shearing, or unfavorably oriented beds, foliation, or joints.

FACING HAZARDS B31
Tsunamis

What Causes a Tsunami? Tsunamis are water waves that are caused by 

sudden vertical movement of a large area of the sea 

floor during an undersea earthquake. Tsunamis are 

often called tidal waves, but this term is a misnomer. Unlike regular ocean tides, tsunamis are not 

caused by the tidal action of the Moon and Sun. 

The height of a tsunami in the deep ocean is typically about 1 foot, but the distance between wave 

crests can be very long, more than 60 miles. The 

speed at which the tsunami travels decreases as 

water depth decreases. In the mid-Pacific, where 

the water depths reach 3 miles, tsunami speeds can 

be more than 430 miles per hour. As tsunamis reach

shallow water around islands or on a continental 

shelf, the height of the waves increases many times, 

sometimes reaching as much as 80 feet. The great 

distance between wave crests prevents tsunamis 

from dissipating energy as a breaking surf; instead, 

tsunamis cause water levels to rise rapidly along 

coast lines.

Tsunamis and earthquake ground shaking differ 

in their destructive characteristics. Ground shaking 

causes destruction mainly in the vicinity of the 

causative fault, but tsunamis cause destruction both 

locally and at very distant locations from the area 

of tsunami generation.

B32 TSUNAMIS
Sequence of photographs showing the arrival of the 

March 9, 1957, Oahu, Hawaii, tsunami at Laie Point. 

The time interval between the second and third photographs is about 5 minutes. If this were a tsunami having 

wave heights of 50 feet or greater, the man in the first 

photograph would have very little chance to escape. 

(Photographs by Henry Helbush.)

FACING HAZARDS B33
Where Have Tsunamis 

Occurred Historically?

EAST COAST

Historically, no tsunamis have been generated 

on the east coast, a consequence of the low level 

of seismic activity and the lack of vertical fault displacement. No tsunami occurred during the 

Charleston, South Carolina, earthquake of 1886, 

one of the largest earthquakes in the United States. 

In addition, none of the tsunamis occurring in the 

Atlantic Ocean region has significantly affected the 

east coast of the United States. The only tsunami 

known to have been recorded on the Atlantic Coast 

of the United States was generated by an earthquake off the Burin Peninsula of Newfoundland on 

November 18, 1929; it caused a wave height of 1 

foot.

WEST COAST

Tsunamis generated by earthquakes in South 

America and the Aleutian-Alaskan region have 

posed a greater hazard to the west coast of the 

United States than locally generated tsunamis. For 

example, the 1946 Aleutian tsunami produced 

waves heights of 12 to 16 feet at Half Moon Bay, 

Muir Beach, Arena Cove, and Santa Cruz, California. The 1960 Chilean tsunami produced wave 

heights of 12 feet at Crescent City, California. The 

1964 Alaskan tsunami generated waves of more 

than 20 feet at Crescent City, California, where it 

caused $7.5 million in damage and 11 deaths. It 

also produced waves ranging from 10 to 16 feet 

along parts of the California, Oregon, and Washington coasts. In contrast, for example, the 1906 

San Francisco, California, earthquake produced 

local tsunami waves of only about 2 inches. The 

largest known locally generated tsunami on the 

west coast was caused by the 1927 Point Arguello, 

California, earthquake that produced waves of 

about 7 feet in the nearby coastal area.

ALASKA

The combination of seismic activity in the Aleutian-Alaskan trench where the Pacific and North 

American tectonic plates collide and the vertical 

displacements of faults make this region of Alaska 

a source of tsunamis. The earliest recorded tsunami 

in this region was in 1788. Four major tsunamis 

were generated in 1946, 1957, 1964, and 1965; the 

1964 Alaskan tsunami caused over $80 million in 

damage and killed 107 people.

HAWAII

The Hawaiian Islands have experienced many 

destructive tsunamis because of their location in 

the Pacific Ocean where about 90 percent of all 

recorded tsunamis take place. Since 1819, more 

than 100 locally and distantly generated tsunamis 

have been recorded in the Hawaiian Islands with 

16 of them causing significant damage. More than 

one-half of all tsunamis recorded in the Hawaiian 

Islands were generated in the Kuril-KamchatkaAleutian regions of the northern and northwestern 

Pacific. Tsunamis generated in that area produce 

the greatest waves on the northern side of the islands. About one-fourth of the historic tsunamis 

affecting Hawaii were generated along the western 

coast of South America. Tsunamis generated in the 

island areas of the Philippines, Indonesia, the New 

Hebrides, and Tonga-Kermadec have been recorded in the Hawaiian Islands, but they have not 

been damaging. The worst locally generated tsunamis were generated in 1869 and 1975 on the southeastern coast of the big island of Hawaii; they 

caused destructive waves of as much as 59 feet.

B34 TSUNAMIS
What Is the Economic Impact

of Tsunamis?

Tsunamis have produced great destruction and 

loss of life in Hawaii and along the west coast of 

the United States. Since 1945, more people have 

been killed as a result of tsunamis than as a direct 

result of earthquake ground shaking. For example, 

the 1946 Aleutian tsunami killed 173 people in 

Hawaii and caused $26 million in property damage 

in the city of Hilo. The 1960 Chilean tsunami killed 

61 people in Hawaii and caused $23 million in property damage. The 1964 Alaskan tsunami, the most 

recent major tsunami to affect the United States,

Damage to the Scotch Cap lighthouse from the Aleutian 

tsunami of April I, 1946, Umimak Island, Alaska. A,

killed 107 people in Alaska, 4 in Oregon, and 11 

in Crescent City, California. This tsunami caused 

more than $100 million in damage on the west 

coast.

Destruction to structures and other facilities is 

a consequence of the time between successive wave 

crests, the wave heights at the shoreline and inland 

locations, and the wave and current velocities. The 

effects of tsunamis include structural failure, scouring at foundations, erosion, flooding, and movement of stone and debris.

Before. B, After. (Photographs bij U.S. National Oceanic 

and Atmospheric Administration.)

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FACING HAZARDS B35
What Can Be Done To Reduce 

Losses From Tsunamis?

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Because tsunamis cannot be prevented, a warning system that can give many hours of advance 

notice is the primary way used to reduce losses. 

The Tsunami Warning System was established in 

response to the Aleutian tsunami of April 1, 1946, 

which caused great damage and loss of life in the 

Hawaiian islands. This system, the responsibility 

of the U.S. National Oceanic and Atmospheric 

Administration, consists of 22 seismic observatories 

and 53 tide stations. The first major tsunami in the 

Pacific following establishment of the warning system was caused by the November 4, 1952, Kamchatka earthquake. Advance warnings were proEXPLANATION

LOCATION NUMBER ZONE 

3

2

3

2

3

4

3

2

3

4

H AW A I I

26\

25 1

2411

23)

22 «

21

^PUU OHAU

 A

20~1 NAPOOPOO

19T

18*

!7t HOOKENA 

16>

^/KAULUOA PT

130 

[ KAAPUNA

12»

11|»HOOPULOA \

io4

9 rKANEWAA PT

8

B

Y

7^ KAUNA PT

6^^* PUUIKI 

5 4\^

3^~-«

\

&>«4

At, ^JtKSR

A* *J ^/ C/ >* 0_^~^1 45

AO fl47

 (148

Vl49

fl50 KAHILIPALI PT

»*151

iJ6 

^»V87

<^v

? ^i

^

<3

,*

^89

,93&92,

* > 93-99

.o5xjoo-iii £/ m ^^ 

<. l^^al^LELEIWI PT #" N**1'^ A113 I Nj Wv ' J

^> HILO J114PAPAI

^ 115 Hl16

V? KALOLI PT*Wj 78 

*\119

MAKUU XJ20 2i

MAKAUKIU PT>122

KAPOHO PT J 

POHOIKI ^24

OPIHIKAO«/ 5

KEHENAftf^26

KALAPANA -7l28

»^\29

_ *rtt30

154 TO 4

4 TO 11

11 TO 13

13 TO 17

17 TO 19

19 TO 27

27 TO 31

31 TO 48

48 TO 61

61 TO 62 

62 TO 64

64 TO 66 

66 TO 69

69 TO 70 

70 TO 77

77 TO 82

82 TO 110

110 TO 124

124 TO 127

127 TO 130

130 TO 134

134 TO 141

141 TO 154

'33

^

<*

7152 KAALUALU 

ri53 KAMAOA

vided to communities in the path of this tsunami; 

the result was a reduction of damage and no casualties. The 1957 Aleutian tsunami, the second 

major Pacific tsunami following the establishment 

of the warning system, caused $3 million damage 

in the Hawaiian Islands, but no loss of life.

Land-use zoning of coastal areas is another way 

used to reduce losses from tsunamis. Such zoning 

is based on the heights of tsunami waves expected 

for exposure times of 20, 50, and 100 years. Tsunami hazard maps, such as shown in the text, are 

used in zoning.

Map of the tsunami hazard for the island of Hawaii (from 

Houston and others, 1977). Various locations on the island are classified as being, in Zones 2 through 5. Zone 

2 denotes wave heights of 5 to 15 feet; Zone 3, 75 to 30 

feet; Zone 4, 30 to 50 feet; and Zone 5, 50 feet or greater. 

There is a 10-percent probability that these wave elevations will be exceeded in a 50-year period. Shading 

denotes areas in Zone 5. The Island of Hawaii does not 

have a Zone 1.

154

 B36 TSUNAMIS
Selected References General

Hays, W. W., ed., 1980, Earthquake prediction information: U.S. Geological Survey Open-File Report 

80-843, 328 p.

Press, Frank, 1976, Earthquake prediction: Scientific 

American, v. 232, no. 5, p. 14-23.

Robinson, G. D., and Spieker, A. M., eds., 1978, Nature 

to be commanded: U.S. Geological Survey Professional Paper 950, 95 p.

Scientific American, 1971, Continents adrift: San Francisco, W. H. Freeman and Company, 172 p.

U.S. National Atmospheric and Oceanic Administration, 

1970, Earthquake history of the United States: Environmental Data Services Report 41-1, 78 p.

Ground Shaking

Algermissen, S. T., and Perkins, D. M., 1976, A probabilistic estimate of maximum acceleration in rock in 

the contiguous United States: U.S. Geological Survey 

Open-File Report 76-416, 45 p.

Borcherdt, R. D., ed., 1975, Studies for seismic zonation 

of the San Francisco Bay region: U.S. Geological Survey Professional Paper 941-A, 102 p.

Federal Emergency Management Agency, 1981, An assessment of the consequences and preparations for a 

catastrophic California earthquake Findings and actions taken: M & R-2, Washington, D.C., 59 p.

Hays, W. W., 1980, Procedures for estimating earthquake ground motions: U.S. Geological Survey Professional Paper 1114, 77 p.

Nuttli, O. W., 1973, The Mississippi Valley earthquakes 

of 1811 and 1812; intensities, ground motion and magnitudes: Seismological Society of America Bulletin, v. 

63, no. 1, p. 227-248.

J. H. Wiggins Co., 1979?, Building losses from natural 

hazards Yesterday, today, and tomorrow: Redondo 

Beach, Calif., 20 p. (Publicly disseminated under National Science Foundation Grant No. ENV-77-08435. 

Available from J. H. Wiggins Co., 1650 South PacificCoast Highway, Redondo Beach, Calif.).

Surface Faulting

Blair, M. L., and Spangle, W. E., 1979, Seismic safety 

and land-use planning Selected examples from California: U.S. Geological Survey Professional Paper 

941-B, 82 p.

Bonilla, M. G., 1979, Surface faulting and related effects, 

in Wiegel, R. L., ed., Earthquake engineering: Englewood Cliffs, N.J., Prentice-Hall Inc., p. 47-74.

Cluff, L. S., and Bolt, B. A., 1969, Risk from earthquakes 

in the modern urban environment, with special emphasis on the San Francisco Bay area, in Danehy, E.

A., and Harding, R. C., eds., Urban environmental 

geology in the San Francisco Bay area: Sacramento, 

Calif., Association of Engineering Geologists, p. 25-64.

Hart, E. W., 1977, Fault hazard zones in California: 

California Division of Mines and Geology Special Publication 42, 24 p.

Howard, K. A., Aaron, J. M., Brabb, E. E., Brock, M. 

R., Cower, H. D., Hunt, S. J., Milton, D. J., Muehlberger, W. R., Nakata, J. K., Plafker, G., Prowell, D. 

C., Wallace, R. E., and Witkind, I. J., 1978, Preliminary map of young faults in the United States as a 

guide to possible fault activity: U.S. Geological Survey 

Miscellaneous Field Studies Map MF 916.

Kockelman, W. J., 1980, Examples of the use of earthscience information by decisionmakers in the San 

Francisco Bay Region, California: U.S. Geological Survey Open-File Report 80-124, 88 p.

Russ, D. P., 1979, Late Holocene faulting and earthquake recurrence in the Reelfoot Lake area, northwestern Tennessee: Geological Society of America 

Bulletin, pt. 1, v. 90, no. 11, p. 1013-1018.

Verbeek, E. R., 1979, Surface faults in the Gulf Coastal 

Plain between Victoria and Beaumont, Texas (abs.): 

Tectonophysics, v. 52, no. 1-4, p. 373-375.

Earthquake-Induced Ground Failures

Keefer, D. K., Wieczorek, G. F., Harp, E. L., and 

others, 1978, Preliminary assessment of seismicallyinduced landslide susceptibility: International Conference on Microzonation, 2d, San Francisco, Calif., 

v. 1, p. 271-290.

Seed, H. B., 1970, Soil problems and soil behavior, in 

Weigel, R. L., ed., Earthquake engineering: Englewood Cliffs, N. J., Prentice-Hall, Inc., p. 227-252.

Youd, T. L. and Hoose, S. N., 1978, Historic ground 

failures in northern California triggered by earthquakes: U.S. Geological Survey Professional Paper 

993, 177 p.

Tsunamis

Houston, J. R., ed., 1980, Tsunamis and flood waves, 

hazard definition and design considerations: Interagency Committee on Seismic Safety in Construction, 

Subcommittee 8 Report (draft), 234 p.

Houston, J. R., Carver, R. D., and M., Dennis, G., 

1977, Tsunami wave elevation frequency of occurrence 

for the Hawaiian Islands: U.S. Army Engineer Waterways Experiment Station, Technical Report H-77-16, 

45 p.

Wiegel, R. L., 1970, Tsunamis, in Weigel, R. L., ed., 

Earthquake engineering: Englewood Cliffs, N. J., 

Prentice-Hall, Inc., p. 253-306.

FACING HAZARDS B37
(T)

i C

EXPLANATION 

Oambreak flood 

Tidal flood

Flash flood

Oy Flood wave generated in 

Lake Okeechobee by 

hurricane

', [_____| Area affected by riverine 

* floods. Variation of

boundaries indicates 

* incidents of overlap

©

Numbers correspond to 

those in table, p. B40

> -

Map showing distribution ofgreat floods in the conterminous I'nitetl States

B38 FLOOD HAZARDS
3. Hazards From Floods

Floods

What is the Cause 

of Flooding?

Aerial view of western part of Fairbanks, Alaska, near 

crest of flood on Chena River, August 15, 1967. River 

flows from right to left. (Photograph by U.S. Bureau of 

Land Management.)

Floods have been and continue to be one of the 

most destructive natural hazards facing the Nation. 

Moreover, the probability exists that a greater flood 

will take place than any experienced in the past.

A flood is any abnormally high streamflovv that 

overtops the natural or artificial banks of a stream. 

Flooding is a natural characteristic of rivers. Flood 

plains are normally dry-land areas which act as a 

natural reservoir and temporary channel for flood 

waters. If more runoff is generated than the banks 

of a stream channel can accommodate, the water 

will overtop the stream banks and spread over the 

flood plain causing social and economic disruption 

and damage to crops and structures. The ultimate 

factor of damage, however, is not the quantity of 

water being discharged, but the stage or elevation

of the water surface. Damage from high stages in 

streams having relatively low discharge can be 

caused by backwater from ice, channel constrictions, or a concurrent flood on another stream. 

Furthermore, floods can form where there is no 

stream, as, for example, when abnormally heavy 

precipitation falls on flat terrane at such a rate that 

the soil cannot absorb the water or the water cannot 

run off as fast as it falls.

Floods take place in the United States in all seasons. Winter floods due to the rainfall and temperature pattern take place in the east progressing 

northward from the Gulf Coast States in January 

to the Ohio River valley in March. Winter floods 

caused by general cyclonic storms take place along 

the western slopes of California, Oregon, and 

Washington.

Spring floods are common in the Northwestern 

States, the Great Lakes area, the Missouri River 

basin, the eastern slopes of Washington and Oregon, and the mountains in California and Arizona. 

The floodwater comes from the melting of snows 

that accumulated during winter. Ice jams also frequently cause flooding. Early spring floods in the 

lower Mississippi River basin are caused by the 

seasonal rainfall pattern. Late spring floods in the 

mountain plateaus mainly result from melting of 

the snows at high altitudes.

Summer floods are likely to take place in any 

part of the United States except on the west coast. 

Although some summer floods have been caused 

by general cyclonic storms, most summer floods, 

except for those associated with hurricanes which 

occur in late summer and autumn, are caused by 

thunderstorms that affect small areas.

On the basis of present knowledge, the size, 

time, and place of floods cannot be predicted much 

in advance.

Storage, either natural or artificial, has a distinct 

influence on floods. Floods are seldom a problem 

downstream from a swampy region or a region containing numerous lakes and ponds because much 

of the runoff is retarded by storage.

FACING HAZARDS B39
Great floods in the United States since May 1889

[Adapted from Climatological Data, National Summary, 1977, National Oceanic and Atmospheric Administration, vol. 28, no. 13, and by information furnished from the

Federal Disaster Assistance Administration]

Number*

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

10

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

Type of 

flood

b

d

d

b,e

d

d

d

b

f

d

d

d

d

d

d

d

d

d

d

d

d

d

c,d

b

c,d

d

d

c,d

b

b,e

d

Date

Mav 1889 ___________________

September 8, 1900 - ___ ..

Mav-June 1903 .___________ 

March 1913 _________________

September 14, 1919 _________

June 1921 ___________________

September 1921 _____________

Spring of 1927 ______________

November 1927 __ _ ____

March 12-13, 1928 __________

September 13, 1928 _________

Mav-June 1935 ___ _ __ __

March April 1936 ___________

March 1938 _______ _ ________

September 21. 1938 .___..___

|ulv 1939 __________...._____

Mav-Julv 1947 ______________

June Julv 1951 __......___ 

August 1955 -. ______

June 27-30, 1957 ____________

June 1965 ________ .._____._

September 10. 1965 _________

August 17 18, 1969 __________

Julv 30-August 5, 1970 ______

February 1972 ______________

June 1972 ___________________

June 1972 .______________ __

Spring 1973 ____________

June-lulv 1975 ______________

September 1975 .. _. _____

Julv 1976 __._____ __-_

April 1977 __________________

Julv 1977 ___________________

April 1979 _______________ _ _

September 12-13, 1979 ______

Location

TGXUS rivers

Black Hills, South Dakota ___ - ___ _ - _____ - _

Lives 

lost

3,000

6,000

100

467

600-900

120

215

313

88

450

1,836

110

107

137

79

600

78

29

28

187

61

390

40

16

75

60

256

11

125

237

105

33

<10

50

11

139

22

78

<10

13

Estimated 

damages 

(millions of 

dollars)

30

40

147

22

25

19

284

46

14

26

18

270

418

25

306

2

235

923

714

155

150

416

415

1,420

399

1,421

453

10

165

4,020

1,155

273

470

1,000

30

424

330

500

2,000

' Number corresponds to those in the frontispiece of this chapter. 

' Dam break flood. 

Tidal flood.

'' Riverine flood.

'' Flash flood.

1 Flood wave generated in Lake Okeechobee by hurricane.

B40 FLOODS
What Is Man's Role in 

Aggravating Flooding?

Floods are natural and recurrent events. They 

become a hazard when man competes for the use 

of flood plains. The natural function of a flood plain 

is to carry away excess water in time of flood. Man's 

failure to recognize this function has led to rapid 

and haphazard development on flood plains and a 

consequent increase in flood hazards. Flood-plain 

occupancy and use are often based on the economic 

advantages of level ground, fertile soils, ease of 

access, and available water supplies without full 

consideration of flood risk (Waananen and others, 

1977).

In recent years, the Federal Government has 

assumed more responsibility for providing relief 

and partial indemnification for property losses resulting from floods. In addition to relief, since 1936 

the Federal Government has spent more than $9 

billion on flood-protection works.

In spite of the flood-protection programs since 

1936, the average annual flood hazard has become

greater than before such programs began because 

people have moved to and built in flood-prone areas 

faster than flood-protection works have been constructed. The increased loss is apparently not due 

to greater floods but to increased encroachment of 

man on flood plains. Many factors have been responsible for man's development of flood-prone 

areas the general growth of population, income, 

and wealth, among others; but it is also clear that 

the substantial separation of costs from benefits, 

whereby the general public bears most of the costs 

of flood-protection works while individual members primarily receive the benefits, has been a 

major factor encouraging such development. Many 

people in high-flood-risk areas are seriously uninformed about the risks which they face. Either they 

are grossly over-optimistic about the probability 

that their property will not be flooded or they expect public help to bail them out when the inevitable flood strikes (U.S. Congress, 1973).

A first magnitude flow from Floridian aquifer, St. Mark's Spring, Leon County, Florida, 1974.
What Is the Impact of Flooding NATURAL

About 3 million miles of streams exist in the

on the Economy? conterminous United States, and about 6 percent 

of the land area is prone to flooding. A proportionate percentage of the Nation's population and tangible property is concentrated in flood-prone areas. 

More than 20,800 communities have flood problems. About 6,100 of these communities have populations greater than 2,500. (U.S. Water Resources 

Council, 1977).

Floods are a source of great personal hardship. 

They threaten loss of life, cause suffering, damage 

property, destroy crops, and disrupt commerce.

The average annual flood loss in the United 

States (in current dollars), not to mention the suffering and death caused by floods, has increased 

from less than $100,000 at the beginning of the 

century to more than $3 billion today. By the year 

2000, potential annual flood loss is expected to be 

greater than $4 billion on the average even with 

moderate application of flood-plain management 

measures.

The nature of flood damage varies widely. Some 

damage, or at least inconvenience, begins as soon 

as a stream overtops its normal banks and water 

begins to occupy the flood plain. A further rise in 

stage may cause flooding of ground floors of residences and other buildings. Eventually, as the velocity of the water in the flood plain increases, 

houses may be swept off foundations, and automobiles and other movable property may be carried away by the swift current. During most great 

floods of record, loss of human life takes place. 

Large floods in rural areas generally destroy crops 

and livestock and frequently render the land unfit 

for use, at least temporarily, by erosion in some 

places and deposition of sand and mud in other 

places.

DAM BREAK FLOODS

Disastrous floods caused by failure of dams, although not in the category of "natural events," have 

caused great loss of life and property damage. Dam 

failures frequently are associated with intense rainfalls and prolonged flood conditions. However, 

some dam breaks have taken place during dry periods as a result of progressive erosion of an embankment, which developed from seepage leaks.

Flood discharges decrease as the flood wave from 

a dam break moves downstream. The greatest 

threat from dam breaks, therefore, is usually in 

areas immediately below a dam. The following examples illustrate their impact.

Johnstown, Pennsylvania

The May 31, 1889, flood in the vicinity of Johnstown, Pennsylvania, which resulted from the failure of a large dam on the South Fork of the Little 

Conemaugh River, 13 miles upstream from Johnstown, was a major catastrophe in the Nation's history. More than 3,000 lives were believed lost. The 

day before the dam failed, streams were swollen 

as a result of 6 to 8 inches of storm rainfall. The 

water level behind the South Fork Dam rose far 

above normal and leaks formed in the earth embankment. About 3:00 p.m. (May 31), the dam suddenly burst. The resulting flood wave was estimated at 30 to 39 feet. Seven small towns were 

destroyed as the wave traveled the 13 miles to 

Johnstown in about 15 minutes. Johnstown, at the 

mouth of the Little Conemaugh River and directly 

in the path of the flood wave, was almost totally 

destroyed (Hoxit and others, in press).

Buffalo Creek From Saunders to Man, 

West Virginia

On February 26, 1972, at approximately 8:00 

a.m., a coal mine refuse dam collapsed on Middle 

Fork, a tributary to Buffalo Creek. The resulting 

failure released some 0.7 million cubic yards of impounded water and sediment into the Buffalo 

Creek valley. The flood swept through the 16 miles 

of valley at an average speed of 3 miles per hour 

and reached the town of Man at the mouth of Buffalo Creek about 11:00 a.m. The traveltime for the 

16 miles was about 3 hours. During these 3 hours, 

at least 118 lives were lost, 500 homes were destroyed, 4,000 people were left homeless; property 

damage exceeded $50 million, and highway damage exceeded $15 million (Davies, Bailey, and 

Kelly, 1972).

B42 FLOODS
The Flood in Southeastern Idaho Caused by the 

Teton Dam Failure on June 5, 1976

The failure of the Teton Dam near Newdale, 

Idaho, June 5, 1976, caused a flood of unprecedented magnitude on the Teton River, lower Henrys Fork, and Snake River upstream from American 

Falls Reservoir. Eleven lives were lost. Water 

spread over more than 180 square miles, and damages reportedly were about $400 million.

The Teton Reservoir behind the 130-foot-high 

earth dam was being filled for the first time during 

spring 1976. About 7:30 a.m., June 5, two seepage 

leaks were observed. Another leak developed 

about 10:00 a.m., and the downstream dam embankment eroded progressively as the flow from 

this leak increased. About 11:00 a.m., a whirlpool 

formed in the reservoir. The crest of the embankment fell into the water at 11:55 a.m., and, at 11:57 

a.m., the water breached the dam and cascaded 

into the canyon. As the fast-moving flood waters 

emerged from the canyon mouth 5 miles downstream, flood waves spread rapidly downgrade over 

the widening flood plain of the Teton River. The 

flooded area in the vicinity of Rexburg, Idaho, was 

more than 6 miles wide. The communities of Wilford and Sugar City were devastated by a 16-foothigh wall of water. Most of the city of Rexburg was 

inundated to depths of about 7 feet within a few 

minutes by rampaging flood waters carrying large 

trees and other floating debris. Water was about 

7 feet deep throughout the town of Roberts and 

covered parts of Menan, Idaho Falls, Firth, and 

Blackfoot. By midnight on June 7, 1976, the crest 

of the main flood reached American Falls Reservoir 

about 100 miles downstream from Teton Dam. 

American Falls Reservoir stored the entire flow.

Flood warnings by the U.S. Water and Power 

Resources Service (formerly U.S. Bureau of Reclamation), local radio stations, and law enforcement 

agencies enabled nearlv all of the people to vacate 

the flood plain; this action undoubtedly saved many 

lives (Ray and Kjelstrom, 1978).

Wrecked house and other debris, including large mobile 

home, lodged on a damaged private bridge 0.5 mile 

downstream from Drake, Colorado. Flood of August 

1976, Big Thompson River, Colorado.

YEAR 1975 YEAR 2000

PUBLIC LANDS 

AND BUILDINGS. 

MINING, UTILITIES, 

AND RURAL

OTHER AREAS

INCLUDING: 

PUBLIC LANDS 

AND

BUILDINGS. 

MINING, 

UTILITIES, AND 

RURAL 

INDUSTRIES

AGRICULTURAL

$3,400,000,000 $4,300,000,000 (1975 DOLLARS) 

Trends in the distribution of annual flood losses. 1975-2000. (Modified from U.S. Water Resources Council, 1978.)

-, T- ~* _ "* - »vJuf" ^*wirw*>--z? ~ i> ;- ->* -. -- - &&'&^-&&Ps**

*%*^*

^%,*&* -."t^/BeWZ ' ^"^KL- *^-*^$g^
Looking upstream at Teton Dam near Newdale, Idaho, after collapse, June 6, 1976, 

2:11 p.m. (Photograph by U.S. Army Corps of Engineers.)

B44 FLOODS
Mouth of Teton River canyon afternoon of June 6, 1976, 5 miles downstream from 

collapsed Teton Dam. Devastated community of Wilford, Idaho, in distance.(Photograph by Perks Photo Service, Idaho Falls, Idaho.)

FACING HAZARDS B45
Flash Flooding 

What Causes Flash Floods?

What Are the Physical

Characteristics of

Flash Floods?

Where Do Flash Floods

Occur?

Flash floods, which have taken many lives and 

caused great property damage, are local floods of 

great volume and short duration. A flash flood generally results from a torrential rain or "cloudburst" 

on a relatively small drainage area. Cloudbursts, 

associated with severe thunderstorms, take place 

mostly in the summer.

Flash floods also result from the failure of a dam 

or from the sudden breakup of an ice jam. Each 

can cause the release of a large volume of flow in 

a short time.

Violent thunderstorms or cloudbursts usually 

develop in a short time and produce floods on relatively small and widely dispersed streams. Runoff 

from intense rainfalls result in high flood waves that 

can destroy roads, bridges, homes, buildings, and 

other community developments. Discharges quickly 

reach a maximum and diminish almost as rapidly. 

Flood flows frequently contain large concentrations 

of sediment and debris collected as they sweep 

channels clean.

The disastrous nature of flash floods is illustrated 

by the Big Thompson River flood of July 31-August 

1, 1976, in Colorado. As much as 20 inches of rain 

fell on about 60 square miles of the drainage area, 

causing a devastating flood on the Big Thompson 

River and its tributaries between Estes Park and 

Loveland, Colorado. The flood lasted only a few 

hours, but, during that time, it caused many deaths 

and much destruction; at least 139 lives were lost, 

and damages were estimated at more than S35 

million. The flood crest on the Big Thompson River 

moved through the 7.6 miles between Drake and

Flash floods can take place in almost any area of 

the country, but they are particularly common in 

the mountainous areas and desert regions of the 

West. Flash floods are a potential source of destruction and a threat to public safety in areas 

where the terrane is steep, surface runoff rates are 

high, streams flow in narrow canyons, and severe 

thunderstorms prevail.

the canyon mouth in about 30 minutes, for an average speed of about 15 miles per hour.

Although the rainfall and flood discharges were 

unusually large, they are not unprecedented for 

some areas along the eastern foothills and plains 

of Colorado. The May 1935 and June 1965 floods 

on some streams along the eastern plains were 

much greater than the 1976 peaks in the storm area 

(McCain and others, 1979).

The eruption of Mount St. Helens Volcano 

(Chapter 5) in southwestern Washington, May 18, 

1980, produced major debris and mudflovvs and 

severe flooding on the Ton tie River and lower Cowlitz River. Runoff from melted glaciers and snow 

on the volcano's north slope, supplemented by outflow from Spirit Lake, was the source of the flow. 

The great volume of sediment and thousands of 

logs transported during the flood on the Toutle 

River destroyed most of the bridges. Much of the 

sediment was carried downstream into the Cowlitz 

and Columbia Rivers where it formed a shoal that 

blocked the shipping channel for several days.

B46 FLASH FLOODING^
Flood damage along the Big Thompson River near center 

of Drake, Colorado, August 1976.

Flooding along Cowlitz River May 19, 1980, at Castle 

Rock, Washington, following massive mudflows initiated 

by the eruption of Mount St. Helens Volcano. From 3 

to 5 feet of sediment were deposited over the Castle Rock 

fairground. The business section of Castle Rock (in background) was protected by a levee.
Riverine Floods 

What Causes Riverine Floods? Riverine floods are caused by precipitation over 

large areas or by the melting of the winter's accumulation of snow or both. Riverine floods differ 

from flash floods in their extent and duration. 

Whereas flash floods are of short duration on small 

streams, riverine floods take place in river systems 

whose tributaries may drain large geographic areas 

and encompass many independent river basins. 

Floods on large river systems may continue for 

periods ranging from a few hours to many days.

What Are the Physical

Characteristics of 

Riverine Floods?

B4S FIVEPINE FLOODS

Flood flows in large river systems are influenced 

primarily by variations in the intensity, amount, 

and distribution of precipitation. The condition of 

the ground amount of soil moisture, seasonal variations in vegetation, depth of snow cover, and imperviousness due to urbanization directly affects 

flood runoff.

Three characteristics of river channels, channel 

storage, changing channel capacity, and timing, 

control the movement of riverine flood waves (Leopold and Langbein, 1960). As a flood moves down

the river system, temporary storage in the channel 

reduces the flood peak. As tributaries enter the 

main stream, the river gets larger and larger downstream. Tributaries are not of the same size nor are 

they spaced uniformly; therefore, their flood peaks 

reach the main stream at different times. The difference of timing tends to modify peaks as a flood 

wave moves downstream.

Sediment sampling of a stream from a highway bridge. 

Note the water washing over the floor of the bridge.
Where Do Riverine 

Floods Occur?

Riverine floods take place throughout the United 

States. Although annual precipitation amounts vary 

widely, heavy rains can occur at various times in 

both arid and humid regions.

The catastrophic effects of riverine flooding are 

illustrated by the flood of June-July f972 in the 

Middle Atlantic States resulting from the heavy 

rains associated with Hurricane Agnes. The storm 

originated in the Caribbean Sea region in midJune, crossed the Florida panhandle coastline on 

June f9, L972, and brought heavy rains from the 

Carolinas northward to New York. Many streams 

in the affected area experienced peak flows several 

times greater than the previous maxima of record. 

Suspended-sediment loads of most flooded streams 

were unusually high. The widespread flooding 

claimed 117 lives and caused damage of more than 

$3 million in 12 States (Bailey and others, 1975).

Hurricane Agnes storm trackjune 14-22, 1972. (From 

U.S. National Weather Service.)

Washington, D. 

Roanoke 

Norfolk

Atlanta 

o

August 

Macono

EXPLANATION 

O Position at 0100 EST 

 Position at 0700 EST 

A Position at 1300 EST 

A Position at 1900 EST 

19 Date of occurrence

Jacksonville 

Tallahassee

20°

FACING HAZARDS B49
Tidal Floods 

What Causes Tidal Floods? Tidal floods are overflows of coastal lands bordering an ocean, an estuary, or a lake. These coastal 

lands, such as bars, spits, and deltas affected by 

the coastal current, occupy the same protective 

position relative to the sea that flood plains do to 

rivers.

Coastal flooding primarily is due to landward 

flows caused by high tides, waves from high winds, 

surges from distant storms, tsunamis (long waves

produced by submarine earthquakes see Chapter 

2), or a combination of these events. Along shores, 

damage also can be caused by ice driven ashore by 

wind or wave action (U.S. Water Resources Council, 1972). Tidal floods can also be caused by the 

combination of waves generated by hurricane 

winds and flood runoff resulting from the heavy 

rains that accompany hurricanes.

What Are the Physical

Characteristics of

Tidal Floods?

Tidal floods may extend over large distances 

along a coastline. The duration of tidal floods is 

usually short, being dependent upon the elevation 

of the tide which rises and falls twice daily in most 

places. However, maximum tide elevations can be

identical on consecutive days. In the case of tidal 

floods associated with hurricanes, the high velocities of hurricane winds often produce wave heights 

about 3 feet higher than the maximum level of the 

prevailing high tide.

Where Do Tidal Floods Occur? Most of the severe tidal floods are caused by tidal 

waves generated by high winds superimposed on 

the regular cyclic tides. Tropical hurricanes are the 

primary sources of the extreme winds. Each year, 

several hurricanes enter the United States mainland, striking along the coasts of the Gulf of Mexico 

and the Atlantic Ocean.

Hurricane Frederic, one of the most intense hurricanes of record, struck the coastal areas of Mississippi, Alabama, and Florida on September 12-13. 

1979. Maximum wind speed recorded at Dauphin 

Island, Alabama, was 144 miles per hour. Maximum prevailing flood elevations were about 11 feet 

at Mobile, Alabama, and about 14 feet at Gulf 

Shores, Alabama. The maximum wave height 

above the prevailing tide was about 8 feet. At least 

13 lives were lost, and the total damage exceeded 

$2 billion (Bohman and Scott, 1980).

B50 TIDAL FLOODS
Most beachfront homes in the Gulf Shores, Alabama, 

area were either demolished or severely damaged by high 

winds and tidal surge from Hurricane Frederic, September 12-13, 1979. (Photograph by U.S. Army Corps of 

Engineers, Mobile District.)

FACING HAZARDS B51
What Can Be Done

To Reduce Losses

From Floods?

Substantial efforts have already been made by 

Federal and State Governments and the private 

sector to reduce losses from floods. In spite of these 

efforts, man's uses of flood plains and flood-prone 

areas continue to increase and, in direct relation, 

so have losses from floods. The message is clear 

as long as flood plains and other flood-prone areas 

are occupied, the Nation will face continuing costs 

associated with losses and flood protection.

The key action in effective reduction of losses 

from floods lies in the intelligent planning for and 

regulation of the use of land exposed to the flood 

hazard (U.S. Congress, 1966). The National Program for Flood Plain Management, started in 1969, 

is directed toward this goal, among others. The 

U.S. Geological Survey and the U.S. Army Gorps 

of Engineers have important responsiblities in the 

program. Since 1969, the Geological Survey, as its 

part in the program, has identified flood-prone 

areas throughout the Nation on more than 13,000 

topographic maps. Currently, the Survey effort is

directed toward mapping areas downstream from 

dams and reservoirs, areas having high potential 

for flash floods, and areas of potential future urban 

development. Prediction of the si/.e, time, and 

place of floods is not yet an effective loss-reduction 

action because of limited knowledge.

The U.S. Flood Insurance Administration directs 

a program to make flood insurance available to 

property owners on a nationwide basis through cooperative efforts of the Federal Government and 

private industry. This program also encourages 

State and local governments to adopt sound programs of flood-plain management to reduce or eliminate future flood losses. Flood insurance studies 

in more than 6,000 of the 20,800 communities having known flood problems have been prepared for 

the Flood Insurance Administration by other government agencies and by consultants in the private 

sector. Flood-loss prevention and reduction measures are discussed in Waananen and others (1977).

Selected References Bailey, J. F., Patterson, J. L., and Paulhus, J. L. II., 

1975, Hurricane Agnes rainfall and floods June-July 

1972: U.S. Geological Survey Professional Paper 924, 

403 p.

Bohman, L. R., and Scott, J. C., 1980, Hurricane Frederic tidal floods of September 12-13, 1979, along the 

Gulf Coast, Grand Ba>, Alabama: U.S. Geological Survey Hydrologic Investigation Atlas HA-622.

Davies, W. E., Bailey. J. F., and Kelly, D. B., 1972, 

West Virginia's Buffalo Creek flood Study of the hydrology and engineering geology: U.S. Geological Survey Circular 667, 32 p.

Hoxit, L. R., and others, in press, Johnstown-Western 

Pennsylvania storm and floods of July 19-20, 1977: 

U.S. Geological Survey Professional Paper 1211.

McCain, J. F., Hoxit, L. R., Maddox, R. A., Chappell, 

C. F., Caracendos, F., Shroba, R. R., Schmidt, P. 

W., Crosby, E. J., Hansen, W. R., and Soule, J. M., 

1979, Storm and flood of July 31-August 1, 1976, in 

the Big Thompson River and Cache la Poudre River 

basins, Larmier and Weld Counties, Colorado: U.S. 

Geological Survey Professional Paper 1115, 152 p.

Leopold, L. B., and Langbein, W. B., I960, A primer 

on water: U.S. Geological Survey Special Publication, 

50 p.

Ray, H. A., and Kjelstrom, L. C., 1978, The flood in 

southeastern Idaho from the Teton Dam failure of June 

5, 1976: U.S. Geological Survey Open-File Report 

77-765, 82 p.

U.S. Congress, 1966, Task force on Federal flood control 

policy, A unified national program for managing flood 

losses: House Document 465, U.S. 89th Congress, 2d 

session, 47 p.

 Flood protection act of 1973: Senate Committee 

on Banking, Housing, and Urban Affairs, U.S. 93d 

Congress, 1st session, 29 p.

U.S. Water Resources Council, 1972, Policy Development Committee, Flood hazard evaluation guidelines 

for Federal executive agencies, 1972: U.S. Government Printing Office, 22 p.

 1977, Estimated flood damages Nationwide 

analysis report: Appendix B, Flooding Technical Committee, U.S. Government Printing Office, 27 p.

 1978, The Nation's water resources 1975-2000:

Summary, Second National Water Assessment, U.S. 

Government Publications Office, v. 1, 86 p. 

Waananen, A. O., Limerino, J. T, Kockelman, W. J., 

Spangle, W. E., and Blair, M. L., 1977, Flood-prone 

areas and land-use planning Selected examples from 

the San Francisco Bay Region, California: U.S. Geological Survey Professional Paper 942, 75 p.

HAZARDS FROM FLOODS
100°39'30" 100°38'

Portion of flood-hazard map of Sonora, Texas, showing 

approximate areas subject to inundation by a flood on j ^ 

Dry Devils River and Lowery Draw having an annual 32' \ \ r 

probability of 1 percent. 3CT*-^i *

FACING HAZARDS B53
» -~ f :* */ 

I

J.

.:*'.

I » \ 1 .jft«

> > V " ' //'^t

> ' >-* / ,1
4. Hazards From Ground

Failures

Ground failures involving landslides, expansive 

soils, and subsidence are a major threat each year 

to man and his works. Reduction of losses from 

each of these types of ground failures requires earth 

science information of the type discussed below.

Landslides

How Significant Are 

Landslides?

Landslides, a general term covering a wide variety of mass-movement land forms and processes 

involving the downslope transport of soil and rock 

material under gravitational influence, are a significant hazard in virtually every State. Although 

individual landslides generally are not as spectacular or as costly as some other geologic and hydrologic hazards, they are more widespread. Collectively, they cause major economic loss and 

casualties. In addition, landslides take place in conjunction with other hazards such as earthquakes 

(Chapter 2), floods (Chapter 3), and volcanoes 

(Chapter 5).

Photograph of complex landslide on the valley wall of 

the Columbia River approximately 40 miles upriverfro?n 

Pasco, Washington. This slide began as a slump due to 

weakening of the 300-foot-high slope by irrigation water; 

the lower portion of the slide mobilized into an earth 

flow into the river. Slides of this type were common along 

this stretch of the river during the 1970's. These slides 

have caused only minor direct economic losses, but they 

have increased sedimentation (shown by discoloration 

of the water) in the river, have diminished water quality, 

and have impacted the spawning of steelhead trout and 

other game fish. (Photograph by Rockwell Hanfnrd Operations, Richland, Washington.)

FACING HAZARDS B55
What Is the Economic Impact

of Landslides?

Because damages from landslides vary from subtle to dramatic over both short and long periods of 

time, an accurate estimate of their cost is very difficult to make. For this reason, direct and indirect 

costs are estimated. Direct costs relate to losses 

incurred in actual damages to installations or property. Indirect costs include loss of tax revenues on 

properties devalued as a result of landslides, reduced real estate values in areas threatened by 

landslides, loss of productivity of agricultural or 

forest lands affected by landslides, and loss of industrial productivity because of interruption of 

transportation systems by landslides. Indirect costs 

of landslides may be substantially larger than direct 

costs.

The Highway Research Board (Smith, 1958) 

made the first attempt to estimate costs of landslide 

damage in the United States. This study reported 

that "the average yearly cost of landslides in the 

United States runs to hundreds of millions of dollars," an estimate that was probably realistic at that 

time. J. P. KrohnandJ. E. Slosson( 1976) estimated 

the annual loss from landslide damage to buildings 

and their sites in the United States to be $400 

million (1971 dollars). Combining the above estimates with indirect costs and estimated damages 

to facilities not classed as buildings gives losses of 

more than $1 billion per year (Schuster, 1978) as 

a reasonable estimate of present-day direct and indirect costs of landslides in the United States.

More accurate cost estimates can be made for 

individual landslides. For example, the Portuguese 

Bend landslide in Palos Verdes Hills, California, 

is estimated to have cost more than $10 million in 

damage to roads, houses, and other structures between 1956 and 1959. The filling of the reservoir 

behind Grand Coulee Dam in the State of Washington cost taxpayers and private property owners 

at least $20 million to avoid and correct the damage 

due to landslides that occurred between 1934 and 

1952. A single landslide in Cincinnati, Ohio, has 

cost in excess of $22 million between 1974 and 

1980.

As the result of a survey conducted by the U.S. 

Federal Highway Administration in 1973, R. G. 

Chassie and R. D. Goughnour (1976) concluded 

that total annual damages to highways due to landslides in the United States are well over $100 million. Landslide damages also are costly in urban 

areas. The annual per capita costs of damages have 

been estimated at $2.50 in Allegheny County (Pittsburgh), Pennsylvania, $5.80 in Hamilton County 

(Cincinnati), Ohio, $3.00 in Los Angeles, California, and $1.30 in the nine-county San Francisco 

Bay area, California (Fleming and Taylor, 1980).

Landslides in the United States have not resulted 

in major loss of life because most catastrophic slope 

failures have taken place in nonpopulated areas. 

However, a notable exception occurred in conjunction with the 1959 Hebgen Lake, Montana, 

earthquake which induced the Madison Canyon 

landslide. This landslide had a volume of about 37 

million cubic yards and buried 26 people who were 

camped along the banks of the Madison River. 

Krohn and Slosson (1976) estimated that the total 

loss of life in the United States from landslides is 

more than 25 lives per year.

 B56 LANDSLIDES
Areas highly 

susceptible to 

landsliding

This building in Cincinnati, Ohio, was virtualhj destroyed by a landslide which crushed the rear of the 

building. Slide debris, including trees and rubble, has 

been pushed across the floor. This landslide is typical 

of many that occur in the metropolitan area; they involve 

a layer of surficial material sliding on bedrock of shale 

and limestone. The sketch map, which was compiled from 

various sources, shows areas in the highly urbanized 

part of Cincinnati that are very susceptible to ground 

failures of this type. Location of the photograph is shown 

on the map. (Photograph by J. O. Maberry.)

Sliding of unstable earth materials undermined this canyon rim home on a coastal terrace in the Pacific Palisades 

area of southern California. (Photograph by J. T. 

McGill.)
m

House on Oak Park Drive in San Francisco, California, 

damaged by landsliding on June 1, 1979. House is on the 

toe of the slide, and the major part of the landslide is 

out of the view to the right. (Photograph byE.E. Brabb.)

Failure of fill on Interstate Highway 80, east of San 

Francisco, California. (Photograph by F. A. Taylor.)

B58 LANDSLIDES
Aerial view of the Madison Canyon landslide in southwestern Montana. Rocks from the mountaintop dropped 

about 1,300 feet and reached a speed of about 100 miles 

per hour before striking the valley bottom and riding up 

the opposite valley wall. The lake filling the valley is 

Earthquake Lake. (Photograph by J. R. Stacy.)

Slope failures on the south face of an open-pit copper 

mine in Arizona. The slide is on the right half of the 

photograph.

FACING HAZARDS B59
What Are the Types and

Processes of Slope

Movement?

Landslides can be classified in many ways, each 

having some usefulness to planners in emphasizing 

features pertinent to recognition and reduction of 

losses from landslides. Two criteria, types of movement and types of material, are typically used. 

Types of movement include falls, topples, slides, 

spreads, flows, and combinations of two or more 

of these five types. Types of material include two 

classes bedrock and soils, with soils being divided 

into debris and earth.

B60 LANDSLIDES

Summary classifications and definitions of slope movements (from Varnes, 1978)

[See figure for definitions. |

T\pe of material

Type of movement

Bedroek

Soils

Coarse-grained (debris) Fine-grained (earth)

Falls .______...

Topples __-_. 

Slides _______..

Rotational --.

Translational 

Lateral spreads 

Flows --__-___. 

Complex ----..

Rock fall .._..___-.__..... .. Debris fall ____.....

Rock topple ---------_----___--_ Debris topple ______

Rock-block slide _______-..__ Debris-block slide __

Rock slump ----____------______ Debris slump _______

Rock slide _---__-________-__-__- Debris slide ________

Rock spread ________---------- Debris spread ______

Rock flow ______________________ Debris flow ________

Combination of two or more of the above

Earth fall 

Earth topple 

Earth-block slide 

Earth slump 

Earth slide 

Earth spread 

Earth flow

A rock topple and fall halted traffic on this highway in the central Santa Monica Mountains of southern California. 

The sequence of failures began at the left with a small block slide on bedding surfaces dipping out of the roadcut. 

This slide removed support that resisted overturning of joint-bounded blocks higher in the roadcut, which then 

tumbled one after another onto the road. (Photograph courtesy of the Department of the County Engineer, Los 

Angeles County, California.)
TOPPLE (debris topple)

FALL (rock fall)

FALL Mass travels most of the distance in free fall, by leaps 

and bounds, and rolling of bedrock or soil fragments.

Clayey gravel f^'^^TiiV/jb. 

Clean sand

TOPPLE An overturning movement that, if unchecked, will 

result in a fall or slide.

SLIDE Movement of material by shear displacement along 

one or more surfaces or within a relatively narrow zone.

SLIDE (rock slump)

SandstoneShaleROTATIONAL SLIDE Movement ...._.. js turning about a 

point (surface of rupture is concave upward).

SLIDE (rock slide)

TRANSLATIONAL SLIDE Movement is predominantly along 

planar or gently undulatory surfaces. Movement frequently is 

structurally controlled by surfaces of weakness, such as 

faults, joints, bedding planes, and variations in shear 

strength between layers of bedded deposits, or by the contact 

between firm bedrock and underlying detritus.

FLOW (earth flow)

LATERAL SPREAD (earth spread)

Firm clay 

Soft clay with 

water-bearing silt 

and sand layers

Firm clayey g

LATERAL SPREAD Lateral extension movement of a fractured 

mass; some spreads are without a well-defined basal shear 

surface; others include extension of rock or soil resulting 

from liquefaction or plastic flow of subjacent material.

COMPLEX (slump-earth flow)

(Modified from Zaruba and Mencl, 1969) 

FLOW Movement of mass such that the form taken 

by moving material or the apparent distribution 

of velocities and displacements resembles that 

of viscous fluids; velocity ranges from slow to 

extremely rapid.

COMPLEX Landslide incorporating two 

or more types of movement.

Examples of landslides by type of movement. (Modified from Varnes, 1978.)

FACING HAZARDS B61
What Is the Distribution of

Landslides?

The map of landsliding in the conterminous 

United States provides an overview of the distribution and relative severity of landslide hazards. 

The map shows two important aspects of landsliding incidence and susceptibility. Incidence of 

landsliding refers to areas where landslides have 

actually occurred. For example, areas of high incidence contain more than 15 percent slope failures. Areas of moderate incidence contain 1.5 to 

15 percent failed slopes. Susceptibility to landsliding refers to the strength of the earth materials in 

the area. Areas of high susceptibility are underlain 

by very weak or fractured materials.

Areas in the Appalachian Mountains, RockyMountains, and the coastal ranges along the Pacific

Ocean have the most severe landslide problems. 

All types of slope movements occur in these areas, 

including debris flows, the most dangerous form 

of slope movement with respect to human life. The 

large areas in the midcontinent region colored blue 

are underlain by relatively weak shales. In these 

areas, landsliding is prominent on moderate to 

steep slopes, and, because the materials are weak, 

large excavations commonly produce landslides 

even in flat areas.

Although not shown on the map, large parts of 

Alaska and Hawaii also are severely affected by 

landslides.

What Causes Landslides? All slides involve the failure of earth materials 

under shear stress. The initiation of the process ' 

can, therefore, be thought of in terms of the factors 

that contribute to increased shear stress and the 

factors that contribute to low or reduced shear 

strength. Although a single action, such as addition 

of water to a slope, may contribute to both an increase in stress and a decrease in strength, it is 

helpful to separate the various physical results of 

such an action.

The principal factors contributing to increased 

shear stress are:

 Removal of lateral support by such means as erosion by streams and rivers, glaciers, or waves 

and longshore or tidal currents; previous slope 

failure; and results of construction, especially 

where cuts, quarries, pits, and canals are established, retaining walls and sheet piling are 

removed, or lakes and reservoirs are created 

and their levels altered.

 Loading by such natural or human means as 

weight of rain, hail, snow; accumulation of 

loose rock fragments or accumulated volcanic 

material; stockpiles of ore or rock; waste piles; 

and weight of buildings and other structures.

 Vibrations from earthquakes, blasting, machinery, traffic, and even thunder. 

The principal factors contributing to low or reduced shear strength include:

 The initial state or inherent characteristics of the 

material its composition, texture, structure, 

slope geometry.

 Changes due to weathering and other physicochemical reactions.

 Changes in direct water content and pore pressure and in structure.

B62 LANDSLIDES
Overview map of landslide problems in the conterminous United States; scale is 1:21,000,000. The severity is highest in areas colored with red and decreases in 

order of yellow, green, blue, and purple. Areas which 

may contain landslides or be susceptible to landsliding 

on a scale too small to be shown are not colored 

(modified from Radbruch-Hall and others, 1976). The 

map is based on computer techniques described by 

Radbruch-Hall (1979) and Edwards and Batson 

(1980).

FACING HAZARDS B63
What Can Be Done To Reduce 

Losses From Landslides?

A community faced with a landslide is primarily 

interested in preventing the harmful effects of the 

slide. Many times, the physical cause of the slide 

cannot be removed, so it may be more economical 

to reduce losses continually, or intermittently, 

without actually removing the physical cause.

Various studies have shown that the most damaging landslides are closely related to the activities 

of man and that substantial loss reduction can be 

achieved by regulating land use before man's activities take place. Effective regulation, involving 

measures such as land-use controls, drainage or 

runoff controls, and improved grading ordinances, 

requires close cooperation among geologists, engineers, and planners to evolve a process that begins with research, continues with synthesis of information and communication with others who 

generally are not trained in earth science and engineering, and ends with action on the part of an

individual, a group, or a governmental organization. The benefits warrant this effort. For example, 

J. T. Alfors and others (1973), in a study by the 

California Division of Mines and Geology, estimated that losses of $9.9 billion expected in California from landslides from 1970 to 2000 could be 

reduced 90 percent or more by a combination of 

actions involving geologic investigations, good engineering practice, and effective enforcement of 

legal restraints on land use.

With respect to highways, R. G. Chassie and R. 

D. Goughnour (1976) showed that the application 

of scientific methods and engineering principles to 

the acquisition and use of earth materials could 

significantly reduce the landslide hazard in New 

York State. They noted that improved procedures 

in the 7 years prior to 1976 had already reduced 

landslide repair costs by as much as 90 percent.

B64 LANDSLIDES
Trenching operation at a recent earthflow to learn more 

about subsurface slippage, Greene County, Pennsylvania. (Photograph by J. S. Pomeroy.)

An earth-moving equipment operator receiving instructions from a geological engineer at an earthflow site, 

Greene County, Pennsylvania. (Photograph by J. S. 

Pomeroy.)
Expansive Soils

What Are the Parent Materials 

of Expansive Soils?

Soils and soft rocks which tend to swell or shrink 

due to changes in moisture content are commonly 

known as expansive soils. In the United States, two 

major groups of rocks serve as parent materials of 

expansive soils. Both groups are more common in 

the Western United States than in the Eastern 

United States. The first group consists of ash, glass, 

and rocks from volcanic eruptions. The aluminum 

silicate minerals in these volcanic materials often 

decompose to form expansive clay minerals of the 

smectite group, the best known of which is montmorillonite. The second group consists of sedimentary rocks containing clay minerals, examples 

of which are the shales of the semiarid West-Central United States.

What Is the Economic Impact 

of Expansive Soils?

As of 1973, expansive soils in the United States 

were estimated to cause over $2 billion per year 

in damages to homes, commercial buildings, highways and streets, buried utilities, and other structures. Some sources now estimate the annual costs 

of expansive soils to be as high as $7 billion (Krohn 

and Slosson, 1980). Of the more than 250,000 new 

homes built annually on expansive soils in the 

United States, 10 percent undergo significant damage during their useful lives some beyond repair and 60 percent undergo minor damage (Jones 

and Holtz, 1973).

Annual structural damage in the United States due to 

expansive soils (Jones and Holtz, 1973)

[In millions of dollars]

Single family homes ._____...

Commercial buildings __________

Multistory buildings __--___-__-

Walks, drives, and parking areas

300

360

80

110

High\ and streets ___.__._.______..___ 1.140

Buried utilities and services 

Airport installations __--_-__ 

Involved in urban landslides 

Other ......_.___._.....__.

100

40

25

100

Total annual damage ___-_-...._____..._ $2,255

B66 EXPANSIVE SOILS
1 V

Differential heave of concrete floor in a school building caused by expansive soils. 

Floating floor slab inside has risen 4 inches. (Photograph by the Colorado Geological 

Survey.)

Vertical displacement of I inch along crack in concrete floor slab caused by expansive 

soils. (Photograph by the Colorado Geological Survey.)

Crack in wall of public building caused by expansive 

soils. (Photograph by the Colorado Geological Survey.)
What Is the Distribution of 

Expansive Soils?

The general distribution in the United States of 

smectite-rich parent materials which serve as 

sources of expansive soils is shown in the map. 

Smectites are regionally abundant in geologic formations throughout the Rocky Mountains, most of 

the Great Plains, much of the Gulf Coastal Plain, 

the lower Mississippi River Valley, and the Pacific 

Coast. They are locally abundant in geologic formations along the Atlantic and Gulf Coastal Plains 

and in the Great Basin region. They are a very 

minor constituent of geologic formations in the rest 

of the United States, but they may be abundant 

locally in surficial deposits along both coasts and 

in the western and west-central parts of the Nation 

(Tourtelot, 1974).

In situ weathering of expansive clay-shale beds in the 

Monterey Shale, Santa Monica Mountains, southern 

California. Adjacent interbeds of more dolomitic or 

siliceous composition are disrupted increasingly as they 

approach the ground surface.

 BS8 EXPANSIVE SOILS
Overview map of expansive soils in the conterminous 

United States; scale is 1:21,000,000. Expansive soils are 

most abundant in areas colored red and decrease in 

order of blue and purple. Areas which may contain expansive soils on a scale too small to be shown are not 

colored (modified by D. H. Radbruch-Hall from 

Patrick and Snethen, 1976). The map is based on computer techniques described by Radbruch-Hall (1979) 

and Edwards and Batson (1980).

FACING HAZARDS B69
What Is the Physical

Mechanism Causing

Expansive Soils?

All clayey soils swell or shrink when subjected 

to moisture change; some simply undergo greater 

volume change than others. The physicochemical 

processes which cause moisture-induced volume 

changes, both swelling and shrinking, are complex 

and will be described simplistically here. Volume 

change in clays caused by change of moisture content has two components interparticle volume 

change and intracrystalline volume change (Tourtelot, 1974). Because interparticle swelling and 

shrinking are functions of physical properties such 

as particle size of the clays imvolved rather than 

their mineralogical characteristics, interparticle 

volume changes, which are reversible processes, 

occur in all clay minerals. Intracrystalline volume 

change occurs within individual clay lattice systems.

FaCtOrS CailSe ^wo recluirernents must be met before swelling 

II. J cii i o or shrinking of soil or soft rock can take place. First, 

Swelling and Shrmkingr the soil or rock must have the potential for volume

change. Second, in the case of swelling, water must 

be available and must be able to move freely to the 

expansive soil.

The potential for volume change of a soil or rock 

depends on the type and amount of clay minerals 

it contains. The potential of a clay to change volume 

is considerably greater for smectites than for other 

clay minerals such as kaolinites and illites. Because 

pure smectite is seldom encountered, the potential 

for volume change of a soil or soft rock depends 

also on the percentage of clay minerals it contains.

In situations where the external load is constant, 

volume will not change without moisture change. 

Changes that man makes in the environment often 

will cause moisture changes. For example, the discharge of roof gutters and downspouts on houses 

in semiarid areas often causes swelling by concentrating rain water into foundation soils. In areas 

having wet climates, planting trees may reduce the 

available soil moisture causing shrinking; conversely, the removal of existing forest cover can 

result in increased soil moisture and swelling of 

expansive foundation soils.

The geometry of the mass of expansive soil is 

also important when determining the potential for 

volume change. A thick layer of expansive soil has 

the potential for greater total volume change than 

a thin layer, if each has the same physical properties, availability of water, and confining pressure. 

An increased load on a soil causes increased pressure and a reduction in the amount of swelling, or, 

if the load is great enough, it can cause consolidation of the soil. A clay-rich soil or soft rock at the 

surface of the ground has a greater tendency for 

swelling than one at depth, because the latter is 

subject to confining pressure from the material 

above it.

Can Expansive Soils 

Be Recognized?

Expansive soils can be recognized either in the 

field or by means of laboratory analyses. Shales, 

clay shales, weathered volcanic rocks, and residual 

soils containing smectite will often have a characteristic "popcorn" texture, especially in semiarid 

areas. The most successful methods of recognizing 

expansive soils involve laboratory analysis of the 

clay-mineral content in soil and soft rock. These

methods are X-ray diffraction, differential thermal 

analysis, and microscopic examination. The most 

common laboratory methods used for identifying 

expansive soils on the basis of physical characteristics related to volume change are free swell test, 

Atterberg limits (a test of soil plasticity), and direct 

measurement of volume change.

B70 EXPANSIVE SOILS
Characteristic soft puffy expansive soil showing desiccation cracks. Note pen for scale. (Photograph by the 

Colorado Geological Survey.)

FACING HAZARDS B71
What Can Be Done

To Reduce Losses From

Expansive Soils?

The best means of preventing or reducing damage from expansive soils is to avoid them. However, 

when no other choice is possible except to place 

a structure on a potentially expansive soil, engineering procedures such as the following are necessary: (1) removal of the soil, (2) application of 

heavy loads, (3) preventing access of water, (4) 

prewetting, and (5) stabilization.

Removal of expansive soils and replacing them 

with nonexpansive soils is sometimes possible. 

Usually, however, the expansive soil extends to 

such a great depth that complete removal and hackfill are not economical. Thus, the amount of excavation and backfill needed to prevent the occurrence of destructive volume change must he 

determined. Backfill of nonexpansive material must 

be placed to a sufficient depth to provide the necessary weight to restrain the uplift of the remaining 

expansive soil.

Swelling can be prevented by loading an expansive soil so that the confining pressure is greater 

than the swelling pressure developed by the soil. 

Load can be applied to a foundation soil by means 

of an embankment or blanket of nonexpansive soil 

or by construction of large buildings.

The water entering expansive soil units is usually 

surface water that has moved downward into the

expansive soil. However, in semiarid areas, water 

often moves upward by means of capillary flow from 

the ground-water table to expansive soils at the 

surface. Methods for isolating expansive soils from 

moisture include installation of ditches or pipes to 

carry away surface water, use of sand and gravel 

to break the continuity of the capillary flow, and 

enveloping expansive soil masses with impermeable membranes.

Concrete slabs and bituminous pavements on 

clay soils in semiarid areas inhibit the normal evaporation of capillary water; this increases the moisture content near the surface. The increased moisture causes swelling of expansive soils and 

subsequent damage to the slabs and pavements. 

The potential for damage can be reduced by prewetting the underlying soils to the moisture contents expected while the slab and pavement are in 

service.

Chemical stabilization has also been used successfully to prevent or minimize volume change of 

expansive soils. The ionic character of the soil and 

water combination can be modified by the addition 

of certain chemicals, such as hydrated lime, Ca(OH)2, 

to prevent volume change. This action is based on 

studies that show that the ionic character of water 

has a major effect on volume change.

B7S EXPANSIVE SOILS
Subsidence

What Is the Economic Impact 

of Subsidence?

Subsidence, the lowering or collapse of the land 

surface either locally or over broad regional areas, 

has taken place in nearly every State. Although 

subsidence is usually not spectacular or catastrophic, it causes several tens of millions of dollars 

in damages annually in the United States. Losses 

amounting to $109 million from 1943 to 1973 in the 

Houston-Baytown area of Texas are an example of 

the magnitude of costs caused by subsidence resulting largely from withdrawal of fluid. In addition, damages of $30 million per year are estimated 

to be caused by subsidence over abandoned coal 

mines. Loss of life due to subsidence is rare; however, a catastrophic mine collapse in South Africa 

that caused the deaths of 29 people in 1962 serves 

to remind of the potential danger. To reduce the 

potential losses that can result from subsidence, 

planners and decisionmakers must be aware of and 

understand the effects of this hazard.

This 324-foot-wide and 100-foot-deep sinkhole in Winter Park. Florida, collapsed on May 8 and 9, 1981. The 

collapse was caused in part by the prevailing drought. Economic loss is estimated to exceed $2 million. The 

losses include a house, several cars, portions of several business establishments, streets, and the city swimming 

pool. View to the south. (Photography by A. S. Navoy.)

FACING HAZARDS B73
What Are the Locations and 

Causes of Subsidence?

Subsidence is caused by a large number of natural and man-made activities. They are discussed 

below.

NATURAL SUBSIDENCE

Natural processes causing subsidence include 

the dissolving of limestone and other soluble materials, earthquakes, and volcanic activity. Large 

areas of the United States are underlain by limestone and other soluble minerals. As underground 

water percolates through such materials, soluble 

minerals dissolve, forming cavities or caverns. 

Land overlying these caverns can collapse suddenly, forming sinkholes of 100 feet or more in 

depth and 300 feet or more in width. Other times, 

the land surface can settle slowly and irregularly. 

The landscape created by such subsidence is called 

karst terrane. This type of subsidence usually 

causes extensive damage to structures located over 

pits formed by dissolving of the soluble minerals; 

sometimes it has even caused deaths. Although the 

formation of sinkholes is a natural phenomenon, 

the process can be accelerated by man's practices 

with regard to ground-water withdrawal, land development, and disposal of water.

The major locations of karst terrane and caverns 

in the United States, as shown by the maps, are 

in parts of many of the Southeastern and Midwestern States. Sinkholes also are found in some 

of the Western and Northeastern States. Alabama, 

where soluble limestone and other rocks are present in nearly one-half of the State, has thousands 

of sinkholes that pose serious problems for highways and construction.

Earthquake-related subsidence has taken place 

mainly in Alaska, California, and Hawaii and to a 

lesser extent in other States. This type of subsidence can result from vertical movement on faults 

and may effect broad areas. This process took place 

in 1964 in southern Alaska in conjunction with the 

Prince William Sound, Alaska, earthquake. More 

than 70,000 square miles were tilted downward 

more than 3 feet and subsequently flooded. Subsidence resulting from intense earthquake ground 

shaking involves somewhat smaller areas than that 

resulting from regional vertical faulting. Intense

ground shaking generated during the 1811-12 New 

Madrid, Missouri, earthquakes caused subsurface 

sand and water to be ejected to the surface. This 

ejection left voids in the subsurface, causing local 

compaction of subsurface materials and settling of 

the ground.

Volcanic-related subsidence is a potential problem in parts of Alaska, California, Hawaii, Oregon, 

and Washington. Subsidence usually is caused by 

local collapses above shallow tunnels formed by 

flow of lava. Collapses over much broader areas can 

also occur as magma chambers are emptied by volcanic eruptions.

MAN-INDUCED SUBSIDENCE

The withdrawal of oil, gas, and water has increased dramatically since 1940. Because underground fluids fill intergranular spaces and support 

sediment grains, removal of such fluids results in 

a loss of grain support, reduction of intergranular 

void spaces, and compaction of clays. The land surface commonly subsides wherever widespread subsurface compaction has taken place, causing damage to canals, aqueducts, sewer systems, and pipelines and increasing the probability of flooding in 

some areas.

The most dramatic examples of subsidence caused 

by withdrawal of oil, gas, and water are along the 

Gulf Coast of Texas, in Arizona, and in California. 

The harbor at Long Beach, California, has subsided 

as much as 27 feet from withdrawal of gas and oil. 

The Houston-Galveston area of Texas has experienced as much as 7.5 feet of subsidence locally. An 

area of about 2,500 square miles has subsided 1 

foot or more. Subsidence in the Houston-Galveston 

area appears to have been caused mainly by the 

withdrawal of large amounts of ground water, although some areas of local subsidence have been 

caused by the extraction of gas and oil. Coastal 

towns in Texas, such as Baytown and Seabrook, 

have subsided about 3 feet and are now susceptible 

to flooding from storm surges and hurricanes.

Recent research suggests that subsidence caused 

by withdrawal of ground water can also cause fissuring or renewal of surface movement in some 

areas cut by preexisting faults. Fissuring is the forB74 SUBSIDENCE
mation of open cracks. Surface faulting and fissuring associated with withdrawal of ground water are 

believed to have either taken place or to be a potential problem in the vicinity of Las Vegas, Nevada, as well as in parts of Arizona, California, 

Texas, and New Mexico (Holzer, 1977).

Underground mining, especially shallow coal 

mining, is another significant cause of subsidence. 

The rocks above mine workings may not have adequate support and can collapse from their own 

weight, either during mining or long after mining 

is completed. Subsidence in areas of underground 

mining has caused hazardous conditions in parts of 

Pennsylvania and other Appalachian States, Colorado, North Dakota, Wyoming, New Mexico, 

Washington, Iowa, and Illinois. Subsidence-related damage to surface structures is common in 

the area around Pittsburgh, Pennsylvania, where 

coal has been mined extensively. Subsidence 

depressions and pits, forming above abandoned 

underground mines, are a hazard in the Sheridan, 

Wyoming, area.

Solution mining also can cause subsidence. In 

solution mining, water-soluble minerals such as 

salt, gypsum, and potash are dissolved and pumped 

to the surface so that the water can be evaporated. 

Huge underground cavities are formed, causing 

surface subsidence. Examples of this hazard, whose 

locations are not well known, include the sudden 

collapse of a street at Grand Saline, Texas, in 1976, 

into an abandoned salt mine cavity created between 

1924 and 1949, the subsidence in 1974 near Hutchinson, Kansas, and subsidence in 1971 near Detroit, Michigan.

Additional activities of man that either cause subsidence or increase the potential for subsidence 

include leaching of intergranular elements such as 

uranium and copper from subsurface rocks, coal 

gasification, and oil shale retorting.

Hydrocompaction, or the settling of sediments 

after water is added, is another significant cause of 

subsidence, especially in the arid to semiarid Western and Midwestern States. The areas of known 

compaction include San Joaquin Valley, California, 

Heart Mountain-Chapman Beach and Riverton, 

Wyoming, areas, Hysham Bench, Montana, Columbia Basin, Washington, Denver, Colorado, 

Washington-Hurricane area in southwest Utah and 

central Utah, and Missouri River Basin. Hydrocompaction takes place when dry surface or subsurface deposits are extensively wetted for the first 

time since their deposition as, for example, when

arid land is irrigated for crop production or an irrigation canal is built on loose dry uncompacted 

sediments. Wetting causes a reduction in the cohesion between sediment grains, allowing the grains 

to move and to fill in the naturally occurring intergranular openings. The result is a lowering of 

the land surface of from 3 to 6 feet, although subsidence of as much as 15 feet has been recorded. 

The effects of hydrocoinpaction on the land are 

usually uneven, causing depressions, cracks, and 

wavy surfaces. As a result, canals, highways, pipelines, buildings, and other structures can be seriously damaged.

House in sinkhole. (Photograph by U.S. Federal Housing Authority.)

FACING HAZARDS B75
Principal Islands of ' 

HAWAII \ IOO 200 3OO 400 MILES

Map of karst terrane. (From Davies, 1970.)

B76 SUBSIDENCE
EXPLANATION

 Zl

a 

EI

LIMESTONE AND 

DOLOMITE TERRAIN

Shafts and sinkholes on plains and valley floors; undissected 

plateau uplands

Shafts and sinkholes with extensive limestone ledges on plains 

and valley floors

Sinkhole ponds and lakes on plains 

Sinkholes on ridges and dissected uplands

Collapsed sinkholes on dissected plateaus and plains

GYPSUM AND 

SALT TERRAIN

Sinkholes and shafts on plains

PSEUDOKARST TERRAIN 

Sinkholes on lava plains 

Basins on plains of weathered lava

Sinkholes and basins on gravel and sand plains, and plateaus 

Small shallow sinkholes on granite and diorite uplands 

Limestone ledges outside of areas of other karst types

The Portage area, Alaska. Widespread flooding caused 

by regional subsidence took place as a result of the 1964 

Prince William Sound, Alaska, earthquake.

FACING HAZARDS B77
EXPLANATION

Each symbol identifies a cave or

group of caves 

Solution caves in limestone, 

dolomite, and other carbonate rocks

Solution caves in gypsum 

Lava tubes

Other caves in sandstone, talus, 

granite, metamorphic rocks, and 

unconsolidated deposits; littoral 

caves

0 100 200 100

Map of cavern areas. (From Davies, 1970.)

B78 SUBSIDENCE
Withdrawal of ground water in south-central Arizona 

has caused as much as 12 feet of local subsidence and 

numerous fissures like the ones shown here. (Photograph 

by T. L. Holzer.)

,M,- --JFHj} *;:;*,* *v"*vv** v*/

FACING HAZARDS B79
Subsidence in coastal Baytotcn, Texas, has caused this 

housing, development to be flooded by daily tides. (Photograph by T. L. Holzer.)

SUBSIDENCE
EXPLANATION 

Average heat value

Lignite 

6,700 Btu/lb

Location of coal fields in the United States (modified from Rickert and others, 1979).

FACING HAZARDS B81
Subsidence pits resulting from collapse of the surface into voids left by underground 

coal mining in Wyoming. The mine was abandoned in 1914. (Photograph by F. W. 

Osterwald.)

B82 SUBSIDENCE
What Can Be Done To Reduce 

Losses From Subsidence?

Use of geological and geophysical information to 

identify areas susceptible to subsidence is a key to 

devising loss-reduction actions. Such information 

often is available from the U.S. Geological Survey, 

State surveys, other geologic-related agencies of 

States and communities, geology departments of 

local colleges and universities, and consultants in 

the private sector. Because subsidence can be 

caused by many combinations of natural and manmade conditions or processes, potential hazards 

must be identified on the basis of a site-by-site 

evaluation. City or county engineers, consulting 

engineers, engineering geologists, and geologists 

may be sources of the types of site-specific information needed. Mine development plans on file 

with Federal, State, or local agencies and local 

mining firms may indicate areas of past, present, 

or future underground openings. In regions of karst 

terrane or where mine plans do not exist, it may 

be necessary to undertake geophysical exploration 

and drilling programs to insure that a site is free 

of caverns, cavities, or mine openings.

Restriction of man's activities in areas identified 

as being potentially susceptible to subsidence is 

the best overall strategy for reducing losses. For 

example, to prevent compaction, the water needs 

of a community may have to be supplied from surface sources rather than withdrawn from the 

ground. Underground coal mining can be limited 

to areas where future subsidence will not endanger 

property or lives, or mining methods can be used 

that will not leave areas susceptible to future subsidence.

If the potential for subsidence is known to exist 

in an area, land-use zoning can protect property 

and lives. Nonintensive land uses such as parks or 

golf courses can be planned for areas where subsidence is prevalent.

Engineering methods can be used to help stabilize the land in subsidence-prone areas where 

development has already taken place or cannot be 

avoided. For example, subsidence caused by the 

withdrawal of underground oil, gas, or water can 

be dealt with by several methods. In the Los Angeles Long Beach, California, area, where oil and

gas have been withdrawn from beneath the harbor 

district for several decades, subsidence was reversed by the injection of water into sediments to 

replace the oil being withdrawn. In areas such as 

the San Joaquin and Santa Clara Valleys in California, subsidence was either reduced or stopped 

when additional imported water was returned to 

subsurface sediments to replace the ground water 

that was withdrawn.

Subsidence associated with coal mining can be 

prevented best by taking actions during or immediately following the mining operation. If mining is carried out in such a way that enough coal 

is left to support the roof of mine workings, the 

chance of later subsidence is reduced. Also, the 

voids remaining when coal is removed can be filled 

with compacted mine waste or other industrial 

wastes. This action will prevent or significantly reduce surface subsidence but may introduce the risk 

of contaminating ground water unless noxious substances in the wastes are neutralized.

Legislative measures have been enacted in several States to reduce losses. For example, the Mine 

Subsidence Insurance Act became effective in 

Pennsylvania in 1962. This act allowed some property owners to buy insurance from the Commonwealth at rates established after an inspection of 

the premises. In 1966, Pennsylvania enacted the 

Bituminous Mine Subsidence Land Conservation 

Act, which entitles property owners, where coal 

has not yet been mined, to purchase the coal under 

their property at a fair price. The potential for local 

subsidence associated with mining is thereby controlled by the property owner. House Bill 525 was 

passed by the Texas Legislature in 1979, creating 

the Harris-Galveston Coastal Subsidence District. 

The purpose was to control the subsidence and 

thereby prevent coastal flooding in the area. The 

Subsidence District's Board of Directors (15 members, representing city and county governments, 

industry, agriculture, and science) has the power 

to regulate the amount of water withdrawn. This 

legislation, along with an increase in the use of 

surface water, has sharply reduced subsidence in 

this part of coastal Texas.

FACING HAZARDS B83
Selected References Landslides

Alfors, J. T., Burnett, J. L., and Cay, T. E., Jr., 1973, 

The nature, magnitude, and costs of geologic hazards 

in California and recommendations for their mitigation: California Division of Mines and Geology Bulletin 198, 112 p.

Briggs, R. P., Pomeroy, J. S., and Davies, W. E., 1975, 

Landsliding in Allegheny County, Pennsylvania: U.S. 

Geological Survey Circular 728, 18 p.

Chassie, R. G., and Goughnour, R. D.. 1976, States 

intensifying efforts to reduce highway landslides: Civil 

Engineering, v. 46, no. 4, p. 65-66.

Edwards, Kathleen, and Batson, R. M., 1980, Preparation and presentation of digital maps in raster format: 

The American Cartographer, v. 7, no. 1, p. 39-49.

Fleming, R. W., and Taylor, F. A., 1980, Estimating 

the cost of landslide damage in the United States: U.S. 

Geological Survey Circular 832, 21 p.

Krohn, J. P., and Slosson, J. E., 1976, Landslide potential in the United States: California Geology, v. 29, 

no. 10, p. 224-231.

Nilsen, T. II., and Turner, B. L.. 1975. Influence of 

rainfall and ancient landslide deposits on recent landslides (1950-71) in urban areas of Contra Costa 

County, California: U.S. Geological Survey Bulletin 

1388, 18 p.

Radbruch-Hall, D. H., 1979, Environmental aspects of 

engineering geological mapping in the United States: 

Bulletin of International Association of Engineering 

Geology, no. 19, p. 351-358.

Radbruch-Hall, D. H., Colton, R. B., Davies, W. E., 

Skipp, B. A., Lucchitta, Iva, and Varncs, D. J., 1976, 

Preliminary landslide overview map of the conterminous United States: U.S. Geological Survey Miscellaneous Field Studies Map MF-771.

Schuster, R. L., 1978, Introduction, in Schuster, R. L., 

and Krizek, R. J., eds., Landslides, analysis and control: National Research Council, Transportation Research Board Special Report 176, p. 1-10.

Smith, Rockwell, 1958, Landslides and engineering 

practice, in Eckel, E. D., ed.. Economic and legal 

aspects: Washington, D.C., Highway Research Board 

Special Report 29, p. 6-19.

Varnes, D. J., 1978, Slope movement types and processes, in Schuster, R. L., and Krizek, R. J., eds., 

Landslides, analysis and control: National Research 

Council, Transportation Research Board: Special Report 176, p. 11-33.

Zaruba, Quido, and Mend, Vojtech, 1969, Landslides 

and their control: New York, Elsevier, 205 p.

Expansive Soils

Holtz, VV. G., and Hart, S. S., 1978, Home construction 

on shrinking and swelling soils: Colorado Geological 

Survey Special Publication 11, 18 p.

Jones, D. E., Jr., and Holtz, VV. G., 1973, Expansive 

soils The hidden disaster: Civil Engineering, American Society of Civil Engineers, v. 43, no. 8, p. 49-51.

Krohn, J. P. and Slosson, J. E., 1980, Assessment of 

expansive soils in the United States: Proceedings of 

the 4th International Conference on Expansive Soils, 

American Society of Civil Engineers, v. 15, p. 596-608.

Patrick, D. M., and Snethen, D. R., 1976, Expansive 

earth materials A survey by physiographic areas of 

their occurrence and distribution: U.S. Army Engineers Waterways Experiment Station, Vicksburg, 

Miss., 34 p.

Tourtelot, H. A., 1974, Geologic origin and distribution 

of swelling clays: Association of Engineering Geologists Bulletin, v. 11, no. 4, p. 259-275.

Subsidence

Alien, A. S., 1969, Geologic settings of subsidence, in 

Varnes, D. J., and Kiersch, G., eds., Reviews in engineering geology: New York, Geological Society of 

America, v. 2, p. 305-342.

Davies, W. E., 1970, Karst lands and caverns: U.S. 

Geological Survey, National Atlas of the United States 

of America, 417 p.

Davies, W. E., Simpson, J. H., Ohlmacher, J. E., Kirk, 

W. S., and Newton, E. G., 1976, Engineering aspects 

of karst in the United States: U.S. Geological Survey 

Open-File Map 76-623, scale 1:7,500,000.

Dunrud, C. R., 1976, Some engineering geologic factors 

controlling coal mine subsidence in Utah and Colorado: U.S. Geological Survey Professional Paper 969, 

39 p.

Dunrud, C. R., and Osterwald, F. W., 1980, Effects of 

coal mine subsidence in the Sheridan, Wyoming, area: 

U.S. Geological Survey Professional Paper 1164, 49

PGilluly. James, and Grant, U. S., 1949, Subsidence in 

the Long Beach Harbor area, California: Geological 

Society of America Bulletin, v. 60, no. 3, p. 461-529.

Holzer, T. L., 1977, Ground failure in areas of subsidence due to ground-water decline in the United States, 

in International Symposium on Land Subsidence: AnB84 HAZARDS FROM GROUND FAILURES
aheim, Calif., 1976, Proceedings International Association of Hyclrological Sciences Publication 121, p. 

423-433.

Jones, L. L., and Larson, James, 1975, Economic effects 

of land subsidence due to excessive groundwater withdrawal in the Texas Gulf Coast area: Texas Water Resources Institute Technical Report 67, 33 p.

Lofgren, B. E., 1969, Land subsidence due to the application of water, in Varnes, D. J., and Kiersch, G., 

eds.. Reviews in engineering geology: Boulder, Colo., 

Geological Society of America, v. 2, p. 271 303.

Newton, J. G., 1976, Early detection and correction of 

sinkhole problems in Alabama, with a preliminary 

evaluation of remote sensing applications: State of 

Alabama Highway Department HPR Report 76, 83 p.

Poland, J. F., and Davis G. H., 1969, Land subsidence 

due to withdrawal of fluids, in Varnes, 1). J., and 

Kiersch, G., eds., Reviews in engineering geology: 

Boulder, Colo., Geological Society of America, v. 2, 

p. 187-269.

Poland, J. F., and Green, J. H., 1962, Subsidence in the 

Santa Clara Valley, California A progress report: 

U.S. Geological Survey Water Supply Paper 1619-C, 

16 p.

Rickert, D. A., Ulman, W. J., and Hampton, E. R., 

eds., 1979, Synthetic fuels development Earth-science considerations: U.S. Geological Survey, Washington, D.C., 45 p.

U.S. Bureau of Mines, 1975, Surface subsidence control 

in mining regions: Draft Environmental Statement, 

47 p.

House and street damaged by small movement on landslide caused by the San Fernando, California, earthquake. (Photograph by Les

L - -.-,. . .< 

FACING HAZARDS B85
MAY 18 ASHFALL 

About 1.2-2.8 inches

MAY 25 

ASH FALL : v 

About O.l^-/. 

2.8 inchea-^g-'" 

B{ owiri g ash wa s ob.sfi.ryed. across' . 

.Montana and.North Dakota:as the 

. cloud .below .1D./OO.Q feet rnoyed 

east. ' -.- .-- 

Abom Q:4~T-2 inctie's 

' : About. "O^

. .. .

St. Helens - Bl«?V»/e-.'. - ! .'Missqiif

O

TV

O v.

oc»

O

 .- . _- : Mrtrs.0. 0 " 1. .... .;,?..- : < - .- **- -.^»

K-**.S*;JS.V-.. .. .;.. /..;. ;: . . . /. ::.-.-. ; -.--,V-.'»T.U .X-. X.\V;>-"^"

" V/^ :'''"-'-'Sv-7;:^.V^:^r -"°"

JJ- » ,

.^-'^ X

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i C O - T

T E V ; » ".'-s-"- ' ':: /'- -? - . >-' A-'r"'-7'.-.-.-;.-r ;: "_ .- .|"; K.'> .Y.Y^1-. : '-'.'.'.

' : ; -- :: :-v.y.-V-V:V iVd^*:-.':^-;^^ '. ' -'. .". » '.'." .. ."j., .. - . ..; <.^- ':: -V <' \ '!-" -'. -' ' .' '..' '. " " -i:3^" .".'.-.' . --'.'.'.'. . " "\ ' \

 '." '-^i- ^"^"S-riSSf^s."."'/*~iSi' .' ' ' -. ' . \ t-f ' . §«xTV V ..'' ' _' ' 

 /' ."-..'. - _ . ' ". ''-.' '. . '*"

0 100 200 300 400 500 MILES

Ash data from the United States Geological Survey 

and National Oceanic and Atmospheric Administration

The ash cloud above 53,000 feet 

moved slowly southeast over 

Texas and the Gulf coast

ESS HAZARDS FROM VOLCANIC ERUPTIONS

Map showing distribution of ash from the May 18, 1980, 

eruption of Mount St. Helens. Some communities were 

covered by as much as 3 inches of ash. (Adapted from 

National Geographic, January 1981.)
5. Hazards From 

Volcanic Eruptions

Volcanic Eruptions

What Are the Hazards From 

Volcanic Eruptions?

Volcanic hazards events and conditions that 

result from volcanic eruptions include airfall debris called tephra, lateral blasts, hot avalanches 

called pyroclastic flows, mudflows, and lava flows. 

Except for ashfalls, volcanic hazards are restricted 

to regions around active volcanoes. Volcanic hazards can occur suddenly with little or no warning. 

In addition, volcanic eruptions can induce earthquakes (Chapter 2), floods (Chapter 3), and landslides (Chapter 4).

What Is the Economic Impact 

of Volcanic Hazards?

Eruptions take place infrequently, and, relative 

to other hazards, such as those from earthquakes, 

floods, and ground failures, they cause low annual 

losses. Nevertheless, an eruption can have a significant short-term economic impact. For example, 

the total cost of the Mount St. Helens eruptions, 

from March through August 1980, is expected to 

reach about $2 billion to $3 billion. Volcanic eruptions have been infrequent in the conterminous 

United States during the last 2 centuries, and few 

would be expected to take place during a person's 

lifetime. Moreover, the most destructive effects of 

eruptions, other than very infrequent ones which 

affect very large areas, are usually limited to areas 

within several tens of miles downvalley or downwind from the volcano. Loss from each type of 

volcanic hazard generally decreases as distance 

from the volcano increases.

Although volcanic eruptions have happened infrequently in the Western United States during the 

19th and 20th centuries, eruptions will surely occur 

in the future at some volcanoes in the United States 

now dormant. The severity of the hazards caused 

by these potential eruptions is great enough that 

loss from them is being considered in land-use decisions in some communities by both public and 

private interests. Land-use decisions are being 

made on the basis of such earth-science considerations as the distance and direction of a community 

and its stream drainage system from volcanoes, the 

history of eruptive behavior of these and other volcanoes, and the potential impact of each type of 

volcanic hazard on land use.

FACING HAZARDS B87
Lava flows

Description of volcanic hazards

Hot avalanches, mudflows, and floods

Origin and characteristics.

Location

Size of area affected by single 

event.

Effects

Predictability of location of areas 

endangered by future 

eruptions.

Frequency, in conterminous 

United States as a whole.

Degree of risk in affected area.

Result from nonexplosive eruptions of molten

lava. 

Flows are erupted slowly and move relatively

slowly; usually no faster than a person can walk.

Flows are restricted to areas downslope from 

vents; most reach distances of less than 6 miles. 

Distribution is controlled by topography.

Flows occur repeatedly at central-vent volcanoes, 

but successive eruptions may affect different 

flanks. Elsewhere, flows occur at widely scattered sites, mostly within volcanic "fields."

Most lava flows cover no more than a few square 

miles. Relatively large and rare flows probably 

would cover only hundreds of square miles.

Land and objects in affected areas subject to burial, and generally they cause total destruction 

of areas they cover. Those that extend into areas 

of snow, may melt it and cause potentially dangerous and destructive floods and mudflows.

May start fires.

Relatively predictable near large, central-vent 

volcanoes. Elsewhere, only general locations 

predictable.

Probably one to several small flows per century 

that individually cover less than 10 square 

miles. Flows that cover tens to hundreds of 

square miles probably occur at an average rate 

of about one every 1,000 years. (In Hawaii, 

eruption of many flows per decade would be 

expected.)

To people, low. 

To property, high.

Hot avalanches can be caused directly by eruption 

of fragments of molten or hot solid rock; mudflows and floods commonly result from eruption 

of hot material onto snow and ice and eruptive 

displacement of crater lakes. Mudflows also 

commonly caused by avalanches of unstable 

rock from volcano.

Hot avalanches and mudflows commonly occur 

suddenly and move rapidly, at tens of miles per 

hour.

Distribution nearly completely controlled by topography.

Beyond volcano flanks, effects of these events are 

confined mostly to floors of valleys and basins 

that head on volcanoes. Large snow-covered 

volcanoes and those that erupt explosively are 

principal sources of these hazards.

Deposits generally cover a few square miles to a 

few hundreds of square miles. Mudflows and 

floods may extend downvalley from volcanoes 

many tens of miles.

Land and objects subject to burning, burial, dislodgement, impact damage, and inundation by 

water.

Relatively predictable, because most originate at 

central-vent volcanoes and are restricted to 

flanks of volcanoes and valleys leading from 

them.

Probably one to several events per century caused 

directly by eruptions.

Probably only about one event per 1,000 years 

caused directly by eruption at "relatively inactive" volcanoes.

Moderate to high for both people and property 

near erupting volcano. Risk relatively high to 

people because of possible sudden origin and 

high speeds. Risk decreases gradually downvalley and more abruptly with increasing height 

above vallev floor. _________________

Volcanic ash (tephra) and gases

Produced by explosion or high-speed expulsion 

of vertical to low-angle columns or lateral blasts 

of fragments and gas into the air; materials can 

then be carried great distances by wind. Gases 

alone may issue nonexplosively from vents.

Commonly produced suddenly and move away 

from vents at speeds of tens of miles per hour.

Distribution controlled by wind directions and 

speeds, and all areas toward which wind blows 

from potentially active volcanoes are susceptible. Zones around volcanoes are defined in 

terms of whether they have been repeatedly 

and explosively active in the last 10,000 years.

An eruption of "very large" volume could affect 

tens of thousands of square miles, spread over 

several States. Even an eruption of "moderate" 

volume could significantly affect thousands of 

square miles.

Land and objects near an erupting vent subject 

to blast effects, burial, and infiltration by abrasive rock particles, accompanied by corrosive 

gases, into structures and equipment. Blanketing and infiltration effects can reach hundreds 

of miles downwind. Odor, "haze, and acid 

effects may reach even farther.

Moderately predictable. Voluminous ash originates mostly at central-vent volcanoes; its distribution depends mainly on winds. Can be 

carried in any direction; probability of dispersal 

in various directions can be judged from wind 

records.

Probably one to a few eruptions of "small" volume 

every 100 years. Eruption of "large" volume 

may occur about once every 1,000 to 5,000 

years. Eruption ol "very large" volume, probably no more than once everv 10,000 years.

Moderate risk to both people and property near 

erupting volcano; decreases gradually downwind to verv low.
What Areas Are Likely To Be

Affected by Future

Volcanism?

With the exception of ashfalls, which can cover 

very broad regions downwind from volcanoes, the 

parts of the United States that could be severely 

affected by a volcanic eruption are probably no 

more than a few percent of either the Western 

counterminous United States or Hawaii. Areas considered most likely to be affected by future eruptions can be divided into several hazard zones on 

the basis of evidence from past eruptions in the 

Cascade Range of the Northwestern United States

(Crandell, 1976; Mullineaux, 1976, 1980). These 

zones chiefly denote differences in the kinds and 

extents of volcanic hazards, but thev also mark differences of anticipated severity and frequency of 

occurrence. For example, zones defined at some 

volcanoes outline the extent of the ashfall hazard; 

areas most likely to be affected by ashfall are certain 

sectors downwind from explosive relatively active 

volcanoes.

Preliminary map of potential volcanic hazards in the 

conterminous United States (from Mullineaux, 1976). 

The map shows areas subject to lava flows, tephra, 

ashfall, lateral blasts, mudflows, and hot avalanches. 

The seventy is highest in the areas colored red and 

decreases in order of green and purple. The map is 

based on computer techniques described in RadbruchHall (1979) and Edwards and Batson (1980).

FACING HAZARDS B89
What Are the Kinds of

Volcanic Eruptions and

Their Effects?

Volcanic eruptions can be broadly classed as nonexplosive or explosive. Nonexplosive eruptions are 

generally caused by an iron and magnesium-rich 

magma (molten rock) that is relatively fluid and 

allows gas to escape readily. Lava flows that are 

common on the island of Hawaii are the characteristic product of nonexplosive eruptions. Explosive eruptions, in contrast, are violent and are derived from a silica-rich magma that is not very fluid; 

these eruptions are common at volcanoes in the 

Cascade Range and in the volcanic chain of Alaska. 

Explosive eruptions produce large amounts of fragmental debris in the form of airfall ash, pyroclastic 

flows, and mudflows on and beyond the flanks of 

the volcanoes.

Tephra is one of the products of an eruption. 

Tephra is a term used to describe rock fragments 

of all sizes erupted into the air above a volcano, 

often in a vertical column that reaches into the 

outer layer of the stratosphere. Large rock fragments generally fall back onto, or near, the volcano. 

Small fragments are carried away by wind and fall 

to the ground at a distance determined by the grain 

size and density, the height to which the fragments 

are erupted, and the velocity of the wind. Eruption 

of a large volume of tephra will cause a distinct 

layer of ash to accumulate. The spatial distribution 

of ash accumulation is generally in the form of a 

lobe which is thickest directly downwind from the 

volcano and thinnest toward the boundaries; the 

thickness decreases as distance from the volcano 

increases. Tephra can endanger lives and damage 

property at considerable distances from a volcano 

by forming a blanket at the ground surface and by 

contaminating the air with abrasive particles and 

corrosive acids. Close to a volcano, people can be 

injured or killed by breathing tephra-laden air; 

damage to property is caused by the weight of tephra and its smothering and abrasive effects.

For a tephra eruption of any given volume and 

height, the probability that ash will affect a specific 

location in the tephra-hazard zones depends on the 

frequency that winds above the volcano blow in 

the direction of that location. The severity of the 

tephra hazard, however, depends on the thickness

of ash deposited, which is governed by the volume 

of the eruption and the wind speeds. The tephra 

hazard zones depict potential thicknesses of ash at 

different distances and directions from the volcanoes; the shapes and extents of these zones are 

based on wind speeds in various directions.

Hot fragments and gases can be ejected laterally 

at high speed from explosive volcanoes and can be 

extremely dangerous. Lateral blasts, the term for 

this phenomenon, commonly leave deposits that 

are no more than 3 to 6 feet thick near their source 

vent; these deposits thin rapidly as distance from 

the vent increases. They generally do not extend 

more than several miles from the vent, but occasionally, as at Mount St. Helens, a blast can reach 

as far as about 15 miles. Lateral blasts endanger 

people chiefly because of their heat, rock fragments 

carried, and high speed which may not allow sufficient time for them to escape or to find adequate 

cover. Damage to structures results chiefly from 

impact and high-speed "wind." Lateral blast phenomena can grade outward to pyroclastic flows that 

move down slopes. The effects of the two events 

are similar.

Pyroclastic flows are masses of hot dry rock debris that move like a fluid. They owe their mobility 

to hot air and other gases mixed with the debris. 

They often form when large masses of hot rock 

fragments are suddenly erupted onto a volcano's 

flanks. Pyroclastic flows can move downslope at 

speeds of as much as 100 miles per hour and tend 

to follow and bury valley floors. Clouds of hot dust 

generally rise from the basal coarse part of the flow 

and may blanket adjacent areas, especially downwind. Because of their great mobility, pyroclastic 

flows can affect areas 15 miles or more from a volcano. The principal losses from a pyroclastic flow 

are caused by the swiftly moving basal flow of hot 

rock debris, which can bury and incinerate everything in its path, and the accompanying cloud of 

hot dust and gases, which can extend beyond the 

basal flow and cause asphyxiation and burning of 

the lungs and skin. Losses from pyroclastic flows 

decrease as height above the valley floor and the 

distance from the volcano decrease.

B90 VOLCANIC ERUPTIONS
Mudflows are masses of water-saturated rock 

debris that move down slopes in a manner resembling the flowage of wet concrete. The debris is 

commonly derived from masses of loose unstable 

rock deposited on the flanks of a volcano by explosive eruptions; the water may be provided by rain, 

melting snow, a crater lake, or a lake or reservoir 

adjacent to the volcano. Mudflows can also be induced by a pyroclastic flow or a lava flow moving 

across snow and melting it. Mudflows can be either 

hot or cold, depending on the presence or absence 

of hot rock debris. The speed of mudflows depends 

mostly on their fluidity and the slope of the terrane; 

they sometimes move 50 miles or more down valley 

floors at speeds exceeding 20 miles per hour. Mudflows may reach even greater distances than pyroclastic flows, about 60 miles from their sources. 

Losses from mudflows decrease rapidly as the 

height above the valley floor increases and gradually as distance from the volcano increases. The 

chief threat to man is burial Structures can be 

buried or swept away by the vast carrying power 

of the mudflow.

Vertical eruption column from Mount St. Helens, July 

22, 1980, viewed from southwest. Ash clouds from pyroclastic flows in the crater and on the north flank drift 

eastward at low altitudes. (Photograph byR.P. Hoblitt.)

Lava flows are generally erupted quietly, although they are often preceded by explosive volcanic activity. Lava flows from volcanoes like 

Mount Rainier, Mount St. Helens, and Mount 

Shasta, for example, typically appear only after an 

eruption has been in progress for hours, days, or 

a few weeks, rather than at the outset of the eruption. However, flows from small basaltic volcanoes 

like many near Bend, Oregon, often take place soon 

after an eruption begins. The fronts of lava flows 

usually advance at speeds ranging from barely perceptible to about as fast as a person can walk. Lava 

flows typically cause no direct danger to human 

life, but they generally cause total destruction in 

the areas they cover. Lava flows that extend into

areas of snow may melt it and cause floods and 

mudflows; lava flows that extend into vegetated 

areas can start fires. On large central-vent volcanoes, such as Mount St. Helens and Mount Shasta, 

lava flows generally are short; therefore, lava-flow 

hazard zones include only the flanks of the volcano 

and the nearest 1 to 2 miles of adjacent valleys and 

basins. In many parts of the Western United States, 

a slight risk exists from lava flows and small-volume 

ashfalls that are not associated with large volcanoes. 

Flood-hazard zones extend considerable distances down some valleys. For some volcanoes in 

the western Cascade Range, these zones reach the 

Pacific Ocean.
Thick pumice layers about 4 miles northeast of center 

of Mount St. Helens. Maximum height of outcrop is about 

30 feet. Entire sequence above shovel (lower center) was 

erupted within the last 4,500 years.

Thin white volcanic ash from Crater Lake, southern 

Oregon, interbedded with valley-floor deposits 15 miles 

west ofWenatchee, Washington, and 300 miles north of 

the source of the ash. River citibank is about 16 feet 

high.

B92 VOLCANIC ERUPTIONS
Logging truck, 7.5 miles north-northwest of Mount St. 

Helens, destroyed by lateral blast of May 18,1980. Truck 

was overturned and propelled downslope until restrained by heavy cable installed to anchor other logging 

equipment. (Photograph by R. P. Hoblitt.)

View to west over ridge 6 miles northwest of Mount St. 

Helens, swept by lateral blast of May 18, 1980. Blast 

moved from left to right. (Photograph by C. D. Miller.)
View of pyroclastic flows with phreatic explosion pits, 

Mount St. Helens, May 8,1981, Rapid erosion of recent 

deposits by stream action has occurred over the past 

year.

B94 VOLCANIC ERUPTIONS
122°

47C

EXPLANATION

Boundary of 

physiographic 

provinces

Osceola mudflow 

5000 years old

Electron mudflow 

-500 years old

Sketch map showing the extent of the Osceola and Electron Mudflows. The photograph shows the mudflow exposed in the face of a terrace 2 miles southeast of Greenwater, Washington, in the White River Valley. The 

largest boulder in the center of the photograph is about 

6 feet in diameter. (Photograph by D. R. Crandell.)

 **v,' '** -> - «7»£3u^ . * *f c

.^ ^Hfif.^?!*!

FACING HAZARDS B95
Mudflow-damaged house along the Toutle River about 

25 miles west-northwest of Mount St. Helens. End section 

of house was torn free and lodged against trees. Mudflow 

height is recorded by mud coatings on tree trunks. (Photograph by D. R. Crandell.)

Southwest flank of Mount St. Helens in late summer 

1979. Dark lava flows cover the south flank of volcano 

to right; "mitten" flow at right center and "two-finger" 

flow at left center reach approximately to the base of the 

volcano. (Photograph by R. P. Hoblitt )

B96 VOLCANIC ERUPTIONS

What Is the Frequency of

Occurrence and Volume of

Volcanic Eruptions?

An inverse relation exists between the size of 

eruptions (volume of material erupted) and how 

often they occur. Small eruptions occur much more 

frequently than large ones. The volumes and frequencies of past eruptions provide the major criteria for defining the various hazard zones. For 

example, tephra-hazard zones are defined on the 

basis of eruptions of four different volumes: small, 

moderate, large, and very large. Evidence suggests 

that an eruption of small volume may be expected 

at some place in the Cascade Range once every 100

years, one of large volume about once every 1,000 

to 5,000 years, and an eruption of very large volume 

about once every 10,000 years.

During the last 2 million years, a few eruptions 

having very large volume have occurred in the 

Western United States. Locations of these eruptions include in and near Yellowstone National 

Park, Wyoming, at Long Valley, California, and in 

the Jemez Mountains of New Mexico. These eruptions deposited ash over very large regions of the 

Western United States.

Can Future Volcanism Be

Forecast?

B98 VOLCANIC ERUPTIONS

Each volcano or volcanic area has its own unique 

record of eruptive activity; it may have remained 

relatively constant for thousands of years, or it may 

have gradually or suddenly ungergone dramatic 

physical changes. The record of activity in prehistoric and historic time provides a basis for defining 

the hazards of specific volcanic eruptions. This record tells what kinds of eruptions have occurred, 

what areas have been affected, and how often they 

have taken place. Although not an infallible guide, 

such a record provides the best data for forecasting 

the kinds, distribution, frequency, and effects of 

future volcanism.

Scientific instruments are used to forecast the 

possible locations and times of future eruptions. 

The seismometer is one of the most effective instruments used to monitor a volcano. It is used to 

detect earthquakes which typically increase in size 

and number just before a volcanic eruption. In 

Hawaii, the number of shallow earthquakes at 

depths of about 1 mile usually increases greatly 

several days to several hours prior to an eruption. 

Ground deformation also commonly precedes volcanic eruptions and can be used to forecast the 

locations of possible eruptions.

Looking south into the crater of Mount St. Helens shows prominent growth of the new dome and past eruption 

snow cover. (Photograph taken on May 8, 1981.)
What Can Be Done To Reduce 

Losses From Volcanism?

Losses from a volcanic eruption can be reduced 

in several ways (Warrick, 1979). These include (1) 

use of knowledge of the past eruptive activity of a 

volcano to define the potential kinds, scales, locations, extents, effects, and severity of future 

eruptions and to define hazard zones, (2) establishment of monitoring systems to forecast an impending eruption and to provide warning, (3) disaster 

preparedness and emergency evacuation, (4) protective measures, (5 ) risk assessment and land-use 

planning, (6) insurance, and (7) relief and rehabilitation.

Activities associated with forecasting depend on 

the reliable detection and monitoring by scientific 

instruments of physical changes preceding an eruption. Phenomena that can be reliably monitored 

include earthquakes, ground deformation, changes 

in composition of volcanic gases, and changes in 

electrical and magnetic fields.

Forecasts of impending eruptions are most reliable at frequently active volcanoes such as Kilauea 

Volcano in Hawaii. Kilauea has been carefully monitored for many years at the Hawaiian Volcanic 

Observatory operated by the U.S. Geological Survey. Too little is known about volcanoes in Alaska 

and the Cascade Range to anticipate a reliable capability for forecasting eruptions there in the near 

future. Seismic monitoring of Mount St. Helens by 

the U.S. Geological Survey and the University of 

Washington detected the earthquakes preceding 

the volcanic eruption that began March 27, 1980. 

Advance warning, based on the earthquakes together with knowledge of the potential hazards, 

provided an opportunity for State and local officials 

to prepare and implement emergency plans.

Disaster preparedness and emergency evacuation plans can provide substantial loss reduction 

when the locations and types of hazards for a particular volcanic eruption are taken into consideration. These plans should be based on volcanic-hazard zones showing the relative severity, extents, 

and effects of specific volcanic eruptions.

Protection from volcanic hazards can be effective 

in reducing losses for some events, but not for 

others. Relatively simple actions can reduce losses.

For example, providing high-efficiency dust masks 

and goggles can protect individuals from respiratory damage and eye irritation; changing oil and air 

filters can reduce loss of vehicles from ashfalls.

Insurance can reduce the economic impact. 

However, the sales record of earthquake insurance 

in California and flood insurance throughout the 

Nation suggests that insurance for volcanic hazards 

would not be widely purchased, even if it were 

widely available at reasonable cost. Rate schedules, 

keyed to areas of the highest probable loss, such 

as the floor of a valley that heads on a volcano, 

might focus public attention and limit development 

of the most hazardous areas.

Risk assessment, coupled with land-use planning, provides a strategy for reducing losses from 

volcanic hazards. Assessment of the risk involves 

weighing the value of the community resources in 

danger, the vulnerability of the valued resources, 

and the probability that a volcanic eruption of a 

certain volume will take place within a certain period of time. On the basis of the risk assessment, 

a community can make decisions about land use 

that are consistent with their needs and goals of 

public safety.

Relief and rehabilitation actions are adjustments 

after the volcanic hazard has happened. Together 

with predisaster planning, such actions keep the 

cost of recovery from being too much of a burden 

on the community. The Federal Government now 

provides partial relief and rehabilitation assistance, 

but such assistance is not a substitute for preparedness and planning.

FACING HAZARDS B99
Selected References Crandell, D. R., 1976, Preliminary assessment of potential hazards from future volcanic eruptions in Washington: U.S. Geological Survey Miscellaneous Field 

Studies Map MF-774, scale 1:1,000,000.

Edwards, Kathleen, and Batson, R. M., 1980, Preparation and presentation of digital maps in raster format: 

The American Cartographer, v. 7, no. 1, p. 39^49.

Hoblitt, R. P., 1980, Observations of pyroclastic flows 

of July 22 and August 7, 1980, Mount St. Helens, 

Washington: Transactions of American Geophysical 

Union, EOS, v. 61, no. 46, p. 1137-1138.

Mullineaux, D. R., 1976, Preliminary overview map of 

volcanic hazards in the 48 conterminous United States: 

U.S. Geological Survey Miscellaneous Field Studies 

Map MF-786, scale 1:7,500,000.

National Geographic, 1981, Mount St. Helens: v. 159, 

no. 1, p. 3-65.

Radbruch-Hall, D. H., 1979, Environmental aspects of 

engineering geological mapping in the United States: 

Bulletin of International Association of Engineering 

Geology, no. 19, p. 351-358.

U.S. Geological Survey, 1980, Volcanoes: Earthquake 

Information Bulletin, v. 12, no. 4.

Warrick, R. A., 1979, Volcanoes as hazards-an overview, 

in Sheets, P. D., and Grayson, D. K., eds., Volcanicactivity and human ecology: New York, AcademicPress, p. 161-194.

The late Dr. David Johnston, U.S. Geological Surve 

using a correlation spectrometer which measures ultr* 

violet radiation as an indicator of the sulfur dioxide co\ 

tent of gases ejected from Mount St. Helena, Washington

Tree protected by snow bank, Toutle station, Mount S 

Helens, May 1980. (Photograph by Peter W. Lipman.

B100 HA/ARDS FROM VOLCANIC ERUPTIONS
FACING HAZARDS B101
\ V

Vteu; of Salt Lake City, Utah, looking southeast toward the Wasatch Mountains. About 

85 percent of the State's population lives within 6 miles of the 230-mile-long Wasatch 

fault zone that trends along the mountain front. No moderate to large earthquakes 

have taken place since the settlement of the area in 1847. However, geologic evidence

suggests that a moderate to large earthquake happens on the average about once every 

50 to 430 years along the Wasatch fault zone. Because of the possibility of a damaging 

earthquake, the State created a Seismic Safety Council in 1977 to recommend public 

policy and loss-reduction programs to the Governor and Legislature. (Photograph by 

Richard Vanhorn.)

B102 IMPROVING DECISIONMAKING
6. Suggestions for Improving 

Decisionmaking To Face 

Geologic and Hydrologic

Hazards

What Is the Cost of Not

Attempting To Reduce

Losses From Geologic and

Hydrologic Hazards?

Decisionmakers must weigh the cost of potential 

losses from geologic and hydrologic hazards against 

the cost of loss-reduction actions. Economic loss 

is one measure of the cost of geologic and hydrologic hazards to the Nation. The table gives estimates of both the average annual loss and the potential for sudden loss as a result of earthquakes, 

floods, ground failures, and volcanic eruptions. 

Economic loss, however, is only a fraction of the 

true impact because these hazards cause considerable hardship to individuals and communities, 

including death and physical injury, psychological 

trauma, disruption of lives, and a reduction in the 

overall stability of the community. For example, 

even though the 1972 Hurricane Agnes is recognized in terms of economic impact as the greatest 

natural disaster in the United States, its total impact should also include more than 118 deaths, 

more than 250,000 families uprooted in Pennsylvania, upturned graves and markers, reduced salinity in the Chesapeake Bay which affected the 

shellfish industry, and interrupted power and 

transportation.

The history of the Nation's long-term response 

to natural disasters suggests that public policy and 

action are not always adequate to reduce the very 

large and increasing economic and social costs. 

Hence, public officials and the populace must be 

continuously reminded of the threat and the magnitude of the economic and social disruptions that 

will accompany future earthquakes, floods, ground 

failures, and volcanic eruptions.

Recent information (Schiff, 1980; Moorhouse and 

others, 1980; Smith, 1980) suggests that significant 

loss reduction can be achieved for some hazards 

with the use of simple low-cost practices. Such 

practices must be identified and implemented for 

all geologic and hydrologic hazards.

Estimates of average annual losses and the potential 

for sudden loss from geologic and hydrologic hazards

in the United States

[Note: Some loss estimates may be too high or too low 

by a factor of 2]

Hazard

Sudden loss

Annual loss potential

(in billion (in billion

dollars) dollars)

Earthquakes ground shaking, 0.6 50

surface faulting earthquakeinduced ground failures,

tsunamis). 

Floods flash floods, riverine 3 5

floods, tidal floods. 

Ground failures landslides, 4 6

expansive soils, subsidence. 

Volcanic eruptions tephra, * 3

lateral blasts, pyroclastic

flows, mud flows, lava

flows.

1 Past data are too limited to determine average annual losses for 

volcanic eruptions. The 1978 eruption of Mount St. Helens in the State 

of Washington will provide a reference for the future.

FACING HAZARDS BIOS
What Is the Difference

Between Good and Bad

Decisions?

B104 IMPROVING DECISIONMAKING

Earth-science information is only one of several 

types of information needed to devise methodologies for reducing losses from geologic and hydrologic hazards. Decisionmaking on matters concerning a community's vulnerability to earthquakes, 

floods, ground failures, and volcanic eruptions typically involves choosing from a variety of different 

alternatives. Decisionmakers have been described 

"as sociopolitical men who must bargain with diverse clients, knowing that the "public good" is 

defined in many conflicting ways by intensely 

competitive and self-interested groups. Such decisionmakers know that goals are fluid, multiple,

inconsistent, multidimensional, and incommensurable. They also know that no fixed solutions are 

possible, regardless of their technical or economic 

elegance" (Alston and Freeman, 1975).

In spite of the difficulties in reaching decisions 

which meet the various definitions of the public 

good, the process of decisionmaking to reduce 

losses from natural hazards must include information about the land surface form and drainage 

pattern, soil and rock properties, and its historical 

record of responding to natural hazards and man's 

activities. To neglect this type of information has 

proved to be costly and unwise.

View of San Fernando Valley, California, looking south from the Pacoima Dam.
What Can Communities Do? Decisionmaking to avoid or to reduce losses from 

geologic and hydrologic hazards is restricted by 

economic, social, and public policy factors. The 

principal restraint is stated by the question, "How 

much will it cost?" If a community decides to attempt to reduce losses from geologic and hydrologic hazards, its planners and decisionmakers must 

face the possibility of increased costs and decide 

what actions are conservative and prudent.

As communities accept the premise that costs 

associated with specific loss-reduction actions such 

as avoidance, land-use zoning, engineering design, 

and insurance are prudent, the question that will 

be asked is, "How much are we willing to pay?" 

An initial requirement for answering this question 

is for the community to determine:

 The physical causes of each natural hazard and 

the probability of each hazard occurring locally.

 The current local annual loss and the potential 

for sudden loss from each hazard.

 The local distribution of levels of relative severity 

expected from each hazard.

 The potential loss as a function of time and lossreduction actions.

The table is a summary of earth-science information 

needed in planning and decisionmaking to reduce 

losses from hazards.

Earth science information needed to reduce losses 

from geologic and hydrologic hazards

Reduction decision Technical information needed about 

the hazards from earthquakes, 

floods, ground failures, and volcanic eruptions

Avoidance Where has the hazard occurred 

in the past? Where is it occurring now? Where is it predicted to occur in the future?

What is the frequency of occurrence?

Land-use zoning Where has the hazard occurred 

in the past? Where is it occurring now? Where is it predicted to occur in the future?

What is the frequency of occurrence?

What is the physical cause?

What are the physical effects of 

the hazard?

How do the physical effects vary 

within an area?

What zoning within the area will 

lead to reduced losses to certain types of construction?

Engineering design Where has the hazard occurred 

in the past? Where is it occurring now? Where is it predicted to occur in the future?

What is the frequency of occurrence?

What is the physical cause?

What are the physical effects of 

the hazard?

How do the physical effects vary 

within an area?

What engineering design methods and techniques will improve the capability of the site 

and the structure to withstand 

the physical effects of a hazard 

in accordance with the level 

of acceptable risk?

Distribution of losses < Where has the hazard occurred 

in the past? Where is it occurring now? Where is it predicted to occur in the future?

What is the frequency of occurrence?

What is the physical cause?

What are the physical effects of 

the hazard?

How do the physical effects vary 

within an area?

What zoning has been implemented in the area?

What engineering design methods and techniques have been 

adopted in the area to improve the capability of the 

structure to withstand the 

physical effects of a hazard in 

accordance with the level of 

acceptable risk?

What annual loss is expected in 

the area?

What is the maximum probable 

annual loss?

FACING HAZARDS B105
What Is the Benefit-Cost Ratio

of Reducing Losses From

Geologic and Hydrologic

Hazards?

No widely accepted method exists for determining benefit-cost or risk-benefit ratios for specific 

loss-reduction actions. However, the following excerpt from The Nature, Magnitude, and Costs of 

Geologic Hazards in California and Recommendations for Their Mitigation (1973) provides some 

insight into benefit-cost analysis of the groundshaking hazard:

Given a continuation of present conditions, it is estimated that losses due to earthquake shaking will 

total $21 billion (in 1970 dollars) in California between 1970 and 2000. Most of the damage and loss 

of life will occur in zones of known high seismic 

activity; structures that do not comply with the Field 

and Riley Acts, passed in 1933, will be especially 

vulnerable. If the present-day techniques for reducing losses from earthquake shaking were applied 

to the fullest degree, life loss could be reduced up 

to 90 percent, and the total value of losses could be 

reduced by as much as 50 percent. Total costs for 

performing the loss reduction work would be about 

10 percent of the total project loss, which with 50 

percent effectiveness provides a benefit to cost ratio 

of 5:1.

According to Terry Margerum (1980), "for most geologichazards, the loss amount is generally reduced well over 

90 percent when construction codes are applied.

Damage to fish shop in Japan caused by rock fall. (Photograph by Kapsuyoshi Uchiyama.)

B106 IMPROVING DECISIONMAKING
What Are the Local-StateFederal Roles in Reducing 

Losses From Geologic and 

Hydrologic Hazards?

A program for reducing losses from geologic and 

hydrologic hazards is likely to be more effective if 

plans are formulated and conducted by local governments. Because geologic and hydrologic hazards 

may well extend beyond the jurisdiction of a single 

local government, neighboring governments must 

avoid conflicting plans and policies. This consideration is particularly important in flood-plain management; for example, one community's plan to 

zone flood-prone land for aesthetic and recreational 

use may be jeopardized by another community's 

plan to reserve the opposite stream bank for industrial uses. Alternatively, an upstream community could conceivably adopt a policy of urbanization and structural flood protection that would lead 

to an increased probability of floods for a downstream community.

R. A. Platt and others (1980) suggest that the 

substate regional level is the most appropriate one 

for coordinating local plans and policies. Regional 

coordination units are more effective if they are 

able to offer incentives to the communities that 

enter into agreements and coordination of loss-reduction plans.

A team of interdisciplinary experts from all levels 

of government and the private sector can provide 

guidance for planning and decisionmaking to reduce losses. Each expert and representative of government and the private sector contributes whatever they do best.

Suggested contributions for government

Federal

State

Local

Provide national and regional 

earth-science information on 

each hazard.

Develop federal legislation and 

policy to support short- and 

long-term loss-reduction programs.

Provide technical assistance.

Provide advice on preparedness.

Encourage short- and long-term 

planning to reduce losses.

Provide support for research on 

scientific, engineering, anu 

socioeconomic problems.

Conduct postdisaster surveys.

Formulate preparedness plans.

Adopt legislation.

Enforce State laws and regulations designed to reduce losses 

from specific hazards.

Create councils of interdisciplinary experts, such as the California Seismic Safety Commission and the Utah Seismic 

Safety Advisory Council, to 

recommend public policy and 

loss-reduction programs.

Aid in identifying risks to communities.

Aid in identifying sources of 

funding and technical expertise to use in loss reduction 

programs.

Provide support and, whenever 

possible, funding for research.

Identify a community leader to 

rally local support for loss-reduction actions, favoring shortterm solutions.

Collect, archive, and update 

earth science information 

which affects loss-reduction 

actions.

Modify land-use and development ordinances to reflect the 

best available knowledge of 

geologic and hydrologic hazards.

Increase public awareness and 

encourage individual preparedness.

FACING HAZARDS B107
Selected References Alfors, J. T., Burnett, J. L. and Gay, T. E., Jr., 1973, 

The nature, magnitude, and costs of geologic hazards 

in California and recommendations for their mitigation: California Division of Mines and Geology Bulletin 198, 112 p.

Alston, R. M and Freeman, D. M., 1975, The natural 

resources decision-maker as political and economic 

man Toward a synthesis: Journal of Environmental 

Management, v. 3., p. 167-183.

Burton, I., Kates, R. W., and White, G. F., 1978, The 

environment as hazard: New York, Oxford University 

Press, 240 p.

Davenport, S. S., and Waterstone, Penny, 1979, Hazard 

awareness guidebook: planning for what comes naturally: Austin, Texas Coastal and Marine Council, 41 p.

Margerum, Terry, 1980, The big quake What local governments can do: Berkeley, Calif., Association of Bay 

Area Governments, 36 p.

Moorhouse, D. C., James, S. E., and Patwardhan, A. 

S., 1980, Loss reduction planning for earthquakes and 

earthquake prediction A preliminary model for businesses, in Hays, W. W., ed., Earthquake prediction 

information: U.S. Geological Survey Open-File Report 80-843, p. 262-278.

Platt, R. A., McMullen, G. M., Paton, R., Patton, A., 

Grahek, M., English, M. R., and Kusler, J. A., 1980, 

Intergovernmental management of floodplains: Boulder, Colo., Institute of Behavioral Science, University 

of Colorado, Monograph 30, 317 p.

Schiff, A. J., 1980, Pictures of earthquake damage to 

power systems and cost-effective methods to reduce 

seismic failures of electric power equipment: West 

Lafayette, Ind., Center for Earthquake Engineering 

and Ground Motion Studies and School of Mechanical 

Engineering, Purdue University, 49 p.

Smith, S. M., 1980, Earthquake predictions and their 

effects on preparedness A public education perspective, in Hays, W. W., ed., Earthquake prediction information: U.S. Geological Survey Open-File Report 

80-843, p. 307-328.

Ward, D. B., 1978, Communicating seismic safety information for public policy development, in Hays, W. 

W., ed., Communicating earthquake hazard reduction 

information: U.S. Geological Survey Open-File Report 78-933, 426 p.

BIOS IMPROVING DECISIONMAKINC
<r U.S. GOVERNMENT PRINTING OFFICE: 1981 341-594/109
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