. he National EarthqJ.Jake 

azards Reduction Programentific Status 

The National Earthquake Hazards 

Reduction ProgramScientific Status 

By THOMAS C. HANKS 

U.S. GEOLOGICAL SURVEY BULLETIN 1659 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Library of Congress Catalog-Card No. 85-600610 

UNITED STATES GOVERNMENT PRINTING OFFICE 1985 

For sale by the 

Superintendent of Documents 

U.S. Government Printing Office 

Washington, D.C. 20402 

COVER 

The main fault scarp of the M = 7.0 Borah Peak, Idaho, earthquake of October 28, 1983 (view 

is to the east, from a point just north of Doublespring Pass Road). The Lost River Range, in the 

background, rises to Borah Peak (elevation 12,655 ft), the highest point in Idaho, just to the right 

of the field of view. Over millions of years, the range has been uplifted to its present elevation and 

backtilted to the east along west-facing normal faults, such as the one which produced the scarp 

pictured here. The scarp cuts an alluvial fan of Pinedale age (12,000 to 15,000 years old). The beveled 

surface that intersects the 1983 scarp surface from above was, prior to the 1983 earthquake, the 

degraded fault scarp of a single earthquake that occurred several thousand years ago. Subsurface 

expression of this prehistoric earthquake was studied in detail on the walls of a trench excavated 

in 1976. This trench, located by the slightly darker and more erodable backfill material in the scarp 

face near the right margin of the cover, was displaced during the 1983 earthquake. Photograph 

courtesy of R. E. Wallace, U.S. Geological Survey. 
CONTENTS 

Introduction 1 

Acknowledgments 3 

Earthquake studies and NEHRP 5 

Trends of the science 6 

The measure of an earthquake 6 

The earthquake cycle 7 

Structural and tectonic setting 9 

Inhomogeneous faulting 11 

Specific accomplishments 12 

Crustal deformation 12 

Late Quaternary faulting geology 17 

Seismicity studies 18 

Magmatic seismicity 21 

Induced seismicity 22 

Stress measurements 24 

Earthquakes in the "eastern" United States 25 

Strong ground motion 28 

Calculation of risk 32 

Seismic zonation 33 

Scientific progress and program perspectives 37 

Bibliography 40 

FIGURES 

1. Photograph showing failure of the lower Van Norman Dam, as a result of the 1971 

San Fernando, California, earthquake 2 

2. Map showing locations of damaging earthquakes in the United States, from colonial 

times through 1970 3 

3. Lower-hemisphere, equal-area projection showing teleseismic P waves and momenttensor inversion of the 1982 New Brunswick, Canada, earthquake 7 

4. Map illustrating seismic potential of the major circum-Pacific plate boundaries in 

1980 8 

5. Sketch maps showing seismicity of the San Francisco Bay region before and after 

the 1906 earthquake 9 

6. Map illustrating seismicity of the Earth, 1960-80 10 

7. Diagrams showing idealized tectonic blocks of southern California 11 

8. Sketch map showing generalized rates of surface faulting in the Great Basin 

province 13 

9. Sketch maps summarizing earthquake characteristics for the Wasatch and East 

Cache fault zones in Utah and Idaho 14 

10. Index map showing locations of gravity stations, leveling baselines, and trilateration 

networks in southern California, with plots of temporal changes in gravity, elevation, 

and areal strain 15 

11. Sketch map showing locations of trilateration networks in the western United States 

and the principal strain rates measured in each 16 

12. Plot of shear-strain rate as a function of time since the 1857 and 1906 earthquakes 

on the San Andreas fault 16 

13. Cross section of the San Andreas fault exposed in a trench at Pallett Creek, 

California 18 

Contents Ill 
14. Photograph, profile, and model calculation of the Great Wall southwest of 

Shizuishan, Ningxia-Hui Autonomous Region, People's Republic of China 20 

15. Map illustrating seismicity of western Nevada and California in 1980 21 

16. Northwest-southeast cross section through Long Valley caldera, showing principal 

geologic units 23 

17. Plots of number of earthquakes per day, cumulative number of earthquakes, and 

water level in the Monticello Reservoir, South Carolina, as functions of time 24 

18. Plot of horizontal shear stress as a function of depth and distance from the San 

Andreas fault near Palmdale, California 25 

19. Map showing orientations of maximum principal horizontal stress for the North 

American plate 26 

20. Sketch map illustrating seismicity and faulting in the New Madrid seismic 

zone 27 

21. Accelerograms of the 1982 Gaza, New Hampshire, earthquake and of an aftershock 

of the 1975 Oroville, California, earthquake 28 

22. Plots of seismic moments, source dimensions, and stress drops for six sets of 

earthquakes 29 

23. Sketch maps showing ground velocity in the 230° direction excited by the 1979 

Imperial Valley, California, earthquake 30 

24. Observed and synthetic ground velocity records of the 1979 Imperial Valley 

earthquake and the distribution of offset on the Imperial fault 31 

25. Plots of moment-magnitude data for central California earthquakes 31 

26. Map showing horizontal velocities at rock sites that have a 90-percent probability of 

not being exceeded in 50 years for the 48 coterminous United States 32 

27. Sketch map of California, showing 30-year cumulative probabilities of occurrence of 

earthquakes along segments of the San Andreas fault system 34 

28. Sketch maps showing probabilities of ground shaking of JMA intensity V or greater 

for the islands of Honshu and Shikoku, Japan 35 

29. Plot of maximum intensity of ground shaking expected for any earthquake in the 

San Francisco Bay region 36 

30. Detail of figure 29 38 

TABLES 

1. Locations, dates, and moment magnitudes of earthquakes mentioned in the 

text 4 

2. Objectives of the National Earthquake Hazards Reduction Plan, as written in Public 

Law 95-124 4 

IV Contents 
The National Earthquake Hazards Reduction ProgramScientific Status 

By Thomas C. Hanks 

INTRODUCTION 

In the early morning of December 16, 1811, a major earthquake occurred 

near New Madrid, Missouri , just across the Mississippi River from the KentuckyTennessee state line. It was felt in Boston, some 1,800 km distant, and frightened 

people in Washington, D.C. In the same source area, comparable-size shocks 

occurred again on january 23 and yet again on February 7, 1812. Structural 

damage and loss of life resulting from these three earthquakes were slight, since 

the strongly shaken area was sparsely populated-then. Ground-failure effects, 

however, including sunken land, landslides, and liquefaction, occurred over an 

area of some 100,000 km2 . Navigation on the central part of the Mississippi River 

was uncertain for some time after the earthquakes, as a result of sunken islands, 

landslides and cave-ins, and uprooted timber. Only in the past decade has much 

progress been made in understanding the cause and effect of these earthquakes 

and the processes that will give rise to succeeding events and their attendant 

hazards in the New Madrid seismic zone. 

At 0600 p.s.t. February 9,1971, the northern Los Angeles Basin was strongly 

shaken by the San Fernando, California, earthquake. Although the collapse of 

two structures (built in 1925) at the San Fernando Veterans Hospital complex was 

responsible for most of the 58 people killed in the earthquake, modern construction, with only a few exceptions, fared very well-surely the result of 

earthquake-resistant design and construction procedures initiated in the Los 

Angeles area as a result of the 1933 Long Beach , California, earthquake. Only 

luck and a low water level, however, prevented the inundation of 80,000 people 

living in the flood plain beneath the lower Van Norman Reservoir; the dam itself 

failed, in any mechanical sense of the word (fig. 1). Even at an early date, an 

enormous amount of information on this earthquake and its effects was available, including a study of the psychological imprint the event left on small 

children awakened in the most intensely shaken areas. 

Earthquakes have tormented the psyche and works of 

man from time immemorial, but nationally integrated and 

funded programs leading to meaningful earthquake-hazards 

reduction are only recent developments. In the United States, 

national interest in such a program began to develop during 

the early 1970's. The San Fernando earthquake itself was a 

major impetus, focusing attention again on earthquake hazards and the point that luck alone is no sure protection against 

them. At the same time, there was much scientific and technical reason to believe that meaningful reduction of earthquake hazards could be achieved through a national program: 

the scientific and engineering disciplines related to earthquakes and their effects had progressed rapidly through the 

1960's, in part owing to the stimulus of the great 1964 Alaska 

earthquake. 

A growing awareness that potentially damaging and 

destructive earthquakes are, indeed, a nationwide problem 

had also taken hold by the early 1970's. Figure 2 shows the 

locations of potentially damaging and destructive earthquakes that have occurred in the United States from colonial 

times through 1970. Although the "earthquake problem" is 

readily associated with Alaska, Hawaii, California, and 

Nevada, large earthquakes have also shaken parts or all of 

Washington, Oregon, Idaho, Montana, and Utah several 

times or more in the 20th century. Neither is the eastern 

seaboard any stranger to such earthquakes, although they 

occur less frequently here; in that region, the major seismic 

events of the historical record are the St. Lawrence River 

valley earthquakes of 1663 and 1925 , the Cape Ann, Massachusetts, earthquake of 1755 , and the Charleston, South 

Introduction 
Carolina, earthquake of 1886. Geographic locations, dates, 

and magnitudes of all the earthquakes mentioned in the text 

are listed in table I . 

Two earthquakes in 1975 provided additional dimensions to the developing interest in a national program of 

earthquake-hazards mitigation. The first of these was the 

Haicheng earthquake in northeastern China and its spectacular prediction by Chinese scientists. Closely following 

promising reports of earthquake-prediction research in the 

Soviet Union, the prediction of the Haicheng earthquake 

provided much additional impetus for earthquake-prediction 

efforts in the United States. The second event was the 

Oroville, California, earthquake adjacent to the Oroville 

Dam, at the headwaters of the California Aqueduct. This 

earthquake reminded us anew that people can condition the 

occurrence of earthquakes by manipulation of water supplies, 

both through reservoir impoundment and through deep-level 

pumping for petroleum extraction or fluid-waste disposal. 

These developments, together with the attendant public 

and political concern, culminated in the Earthquake Hazards 

Reduction Act of 1977 (Public Law 95-124). This act 

directed the President to establish a National Earthquake 

Hazards Reduction Program (NEHRP), with objectives as 

given in table 2. Public Law 95-124 authorized funding of 

NEHRP for three years; since fiscal year (FY) 1980, Congress 

has reauthorized NEHRP on an annual basis. As of FY 1984, 

Figure 1. Lower Van Norman Dam (looking west) on the morning of February 9, 1971, several hours after the San Fernando 

earthquake. A massive slope failure of most of the upstream face has downdropped the damcrest into the reservoir, leaving 

just a few feet of freeboard on the remnant face. From R. F. Scott, "San Fernando Earthquake, 9 February 1971, Preliminary 

Soil Engineering Report," in jennings, P.C., ed., "Engineering Features of the San Fernando Earthquake, February 9, 1971" 

(Pasadena, California Institute of Technology, Earthquake Engineering Research Laboratory Report EERL 71-D2, p. 299-331), 

1971. 

2 The National Earthquake Hazards Reduction Program-Scientific Status 
four Federal agencies are funded directly through NEHRP: 

the U.S. Geological Survey (USGS; $35.3 million of which 

$8.4 million supports a grants and contracts program external 

to USGS), the National Science Foundation (NSF; $25.9 

million), the Federal Emergency Management Agency 

(FEMA; $4.1 million), and the National Bureau of Standards 

(NBS; $0.5 million). Generally speaking, the USGS has 

responsibility for the scientific elements of the program, and 

the NSF has responsibility for the engineering elements, 

although the NSF Division of Earth Sciences supports basic 

research related to the NEHRP as well. The FEMA is charged 

with planning and coordinating the entire program, as well as 

implementing mitigation and preparedness strategies. The 

NBS has responsibility for developing building codes and for 

testing earthquake-resistant construction. 

The purpose of this report is to examine the scientific 

status of NEHRP, that is, to describe and assess our present 

body of scientific understanding and data bases pertinent to 

the objectives of NEHRP. It is worth emphasizing, however, 

that the objectives of NEHRP cannot be met with scientific 

achievements alone. Such developments must sooner or later 

become part of rational implementation plans, land-use45' 

40' 

35' 

30' 

130' 

• 

160' 

,.o 

125' 120' 

155' 

l~======d'"·-

•• • 170'E .-... , __ ...... 160' "'' 170'W 

115' 110' 105' 100' 

planning policies, building codes, and emergency-preparedness plans, for example, which can do so much to reduce, if 

not obviate, potential hazards from earthquakes. We do not 

speak much to such implementation plans here, our point of 

view being the obvious but narrow one: these implementation 

plans may not mean much if they are not based on the best 

scientific knowledge and data available. Equally provincially, 

in speaking of "scientific understanding and data bases," we 

shall be concerned with those related to the earth sciences, 

not the engineering sciences. Although much of the scientific 

knowledge discussed herein is of great relevance to-and is 

routinely used in-problems of engineering concern, the 

reader will find little here about the many accomplishments 

of the earthquake engineering fields appropriate, say, to 

objective 1 in table 2. 

ACKNOWLEDGMENTS 

This report has its origins in a set of letters in which the 

following scientists outlined their assessment of the significant scientific accomplishments related to the National Earthquake Hazards Reduction Program: C.R. Allen and H. 

95' 

95' 

90' 85' 

EXPLANATION 

• Intensity V-VII (ex«pl Colifornio) 

• Intensity VII-VIII 

e Intensity VIII-IX 

® Intensity I X-X 

~ Intensity X-XII 

90' 

80' 75' 70' 65' 

·• . -

National Oceanic and Atmosphenc Administration 

Environmental Data Service 

Revised 1970 Ed1tion 

85' ao• 75' 

45' 

40' 

35' 

30' 

25' 

20' 

Figure 2. United States, showing locations of earthquakes of modified Mercalli intensity V or greater from colonial times 

through 1970. From "Earthquake History of the United States" (U.S. Department of Commerce, National Oceanic and 

Atmospheric Administration and U.S. Geological Survey Publication 41-1 revised edition (through 1970), reprinted 1982 with 

supplement (1971-80)). 

Acknowledgments 3 
Table 1. 

Locations, dates, and moment magnitudes of earthquakes 

mentioned in the text 

Location Date 

Honshu, Japan ______________ Sept. 13, 1984 

Morgan Hill, California ________ Apr. 24, 1984 

Borah Peak, Idaho ___________ Oct. 28. 1983 

Coalinga, California __________ May 2. 1983 

Gaza, New Hampshire _________ Jan. 18. 1982 

New Brunswick, Canada_ _______ Jan. 9, 1982 

Aswan, Egypt-------------- Nov. 14, 1981 

Eureka, California ___________ Nov. 8, 1980 

El Asnam, Algeria ___________ Oct. 10, 1980 

Mammoth Lakes. California _____ May 27, 1980 

Mammoth Lakes, California _____ May 25, 1980 

Mammoth Lakes, California _____ May 25, 1980 

Mammoth Lakes, California _____ May 25. 1980 

Livermore Valley, California _____ Jan. 27, 1980 

Livermore Valley, California _____ Jan. 24, 1980 

Imperial Valley, California_ ______ Oct. 15, 1979 

Coyote Lake, California ________ Aug. 6, 1979 

Santa Barbara, California _______ Aug. 13, 1978 

Klerksdorp Mining District, 

South Africa _____________ Apr. 7, 1977 

Oroville, California (aftershock) ___ Aug. 6, 1975 

Oroville, California ___________ Aug. 1, 1975 

Haicheng, People's Republic 

of China--------------- Feb. 4, 1975 

San Fernando, California_ _______ Feb. 9, 1971 

Koyna, India _______________ Dec. 10. 1967 

Parkfield, California __________ June 28, 1966 

Kremasta, Greece ____________ Feb. 5, 1966 

Prince William Sound, Alaska ____ Mar. 28. 1964 

Kariba, Zambia/Zimbabwe ______ Sept. 23, 1963 

Hsinfengkiang, People's Republic 

of China _______________ Mar. 18, 1962 

Imperial Valley, California_ ______ May 19, 1940 

Parkfield, California __________ June 8, 1934 

Long Beach, California ________ Mar. 11. 1933 

St. Lawrence River Valley, 

Canada _________________ Mar. 1, 1925 

Parkfield, California __________ Mar. 10, 1922 

San Francisco, California _______ Apr. 18. 1906 

Parkfield, California __________ Mar. 3, 1901 

Charleston, South Carolina ______ Sept. 1, 1886 

Parkfield, California __________ Apr. 10, 1881 

Owens Valley, California_ _______ Mar. 26, 1872 

Fort Tejon, California_ _________ Jan. 9, 1857 

Hayward, California __________ June 10, 1836 

New Madrid, Missouri_ ________ Feb. 7, 1812 

New Madrid, Missouri _________ Jan. 23. 1812 

New Madrid, Missouri _________ Oec. 16, 1811 

Cape Ann, Massachusetts _______ Nov. 18, 1755 

Ningxia-Hui Autonomous 

Region, People's Republic 

of China ________________ Jan. 3, 1739 

St. Lawrence River Valley, Canada__Feb. 6, 1663 

Magnitude 

6.5 

6.2 

7.0 

6.4 

4.5 

5.8 

5.5 

7.3 

6.9 

6.0 

6.0 

5.7 

6.2 

5.5 

5.8 

6.5 

5.7 

6.0 

5.5 

4.8 

5.8 

7.0 

6.6 

6.3 

6.1 

6.3 

9.2 

5.8 

6.1 

7.0 

6.0 

6.2 

6.2 

6.0 

7.7 

-::::6 

6.5-7 

-::::6 

7.8 

7.9 

6.5-7 

7-7.5 

7-7.5 

7-7.5 

6.5-7 

7.5-8 

6-6.5 

Table 2. 

Objectives of the National Earthquake Hazards Reduction 

Program, as written in Public Law 95-124 

(1) The development of technologically and economically feasible 

design and construction methods and procedures to make new 

and existing structures, in areas of seismic risk, earthquake 

resistant, giving priority to the development of such methods 

and procedures for nuclear power generating plants, dams, 

hospitals. schools, public utilities, public safety structures, 

high-occupancy buildings, and other structures that are 

especially needed in time of disaster; 

(2) The implementation, in all areas of high or moderate seismic 

risk, of a system (including personneL technology, and procedures) for predicting damaging earthquakes and for identifying, evaluating, and accurately characterizing seismic hazards; 

(3) The development, publication. and promotion. in conjunction 

with State and local officials and professional organizations, of 

model codes and other means to coordinate information about 

seismic risk with land-use policy decisions and building 

activity; 

(4) The development, in areas of seismic risk, of improved understanding of, and capability with respect to, earthquake-related 

issues, including methods of controlling the risks from earthquakes, planning to prevent such risks, disseminating warnings 

of earthquakes, organizing emergency services, and planning 

for reconstruction and redevelopment after an earthquake; 

(5) The education of the public, including State and local officials, 

as to earthquake phenomena, the identification of locations and 

structures which are especially susceptible to earthquake 

damage, ways to reduce the adverse consequences of an earthquake, and related matters; 

( 6) The development of research on-

(A) ways to increase the use of existing scientific and engineering knowledge to mitigate earthquake hazards; 

(B) the sociaL economic, legal, and political consequences of 

earthquake prediction; and 

(C) ways to assure the availability of earthquake insurance or 

some functional substitute; and 

(7) The development of basic and applied research leading to a 

better understanding of the control or alteration of seismic 

phenomena. 

Kanamori, California Institute of Technology; C.H. Scholz, 

D. W. Simpson, and L.R. Sykes, Columbia University; R.K. 

McGuire, Risk Engineering, Inc.; M.D. Zoback, Stanford 

University; R.D. Borcherdt, R.C. Bucknam, W.L. 

Ellsworth, E.R. Engdahl, J.F. Evernden, T.C. Hanks, D.P. 

Hill, K.R. Lajoie, A. McGarr, S.A. Sipkin, W. Thatcher, 

R.E. Wallace, and R.L. Wesson, U.S. Geological Survey; 

T.V. McEvilly, University of California, Berkeley; M. Wyss, 

University of Colorado; and K. Aki, University of Southern 

California. These letters were the basis of detailed discussions, roughly along the lines of the major sections of this 

report, held March 20, 21, and 22, 1983, at Asilomar, California. Attending this meeting were Borcherdt, Bucknam, 

Ellsworth, Engdahl, Evernden, Hanks, Hill, Kanamori, 

4 The National Earthquake Hazards Reduction Program-Scientific Status 
McGarr, McGuire, Simpson, Sipkin, Thatcher, Wallace, 

Wyss, and Zoback, as did J.F. Davis, California Division of 

Mines and Geology. On the basis of these discussions, these 

scientists wrote detailed contributions to each of the principal 

sections of this report. Writing from both the initial letters and 

the detailed Asilomar statements, Hanks compiled this 

report, with the additional help ofM.L. Zoback's thoughts on 

the Basin and Range province. Carol Sullivan attended to the 

innumerable minutiae that accompany the formulation, preparation, and redaction of a manuscript such as this one. 

EARTHQUAKE STUDIES AND NEHRP 

The essence of earthquake studies is their observational 

and empirical character. We know what we know about 

earthquakes almost entirely through what we observe about 

them or their laboratory analogs, not because we have any 

special insight into how the Earth works in the ways that 

generate earthquakes. Matters are likely to stay this way for 

the foreseeable future, since the processes that give rise to 

earthquakes and the material failure that results in them are 

extraordinarily complicated systems. Moreover, these are 

three-dimensional systems we must study remotely from a 

two-dimensional surface, and these processes operate on time 

scales long compared to a human lifetime. 

Consistent with these circumstances are several other 

matters that strongly condition the ways in which progress is 

made in earthquake studies. In the first place, the great 

complexity of the cause and effect of earthquakes requires 

that a wide spectrum of earth science and engineering disciplines be brought to bear on the problem. Secondly, systematic, long-term observational programs-for example, 

those spanning two or more large earthquakes along the same 

fault-are of the utmost importance. Finally, because earthquakes know no political boundaries, there is much to be 

gained from continuing, shared interest in and cooperation 

among the many domestic, foreign, and international programs in earthquake studies. 

With these considerations in mind, it will come as no 

surprise at all that a primary focus of NEHRP has been the 

systematic and comprehensive collection of fundamental 

data sets. These observations range over a wide spectrum of 

geological, geochemical and geophysical disciplines. They 

include, but are not limited to, geologic mapping on regional, 

local, and ~ 1-meter scales; assemblages of late-Quaternary 

dates and rates of deformation; global, regional, and local 

seismic monitoring of earthquakes; measurements of modem 

vertical and horizontal crustal deformation across wide 

ranges in spatial and temporal sampling; determination of 

regional crustal structure from seismic and gravity data; 

measurements of heat flow and in situ shear stress in 

tectonically active areas; recording of earthquake strong 

ground motions; and documentation of the effects of earthquakes, including surface faulting, dynamically induced 

ground failure (liquefaction and landslides), and diverse 

kinds of structural damage. 

In most cases, however, these data sets-and 

NEHRP-are what they are today because of long histories 

of shared interests. The recording of earthquake strong 

ground motion, for example, stands at the interface between 

seismology and earthquake engineering. These observations 

are used by seismologists to elucidate dynamic characteristics of the faulting process, but also by engineers to 

understand the dynamic responses of both geologic and engineering structures; in many cases, the same record has served 

both purposes. Since NEHRP has been in place, several 

significant strong-motion-data sets have been collected in 

California alone, including those for the earthquakes of Santa 

Barbara (1978), Coyote Lake (1979), Imperial Valley (1979), 

Livermore Valley (1980), Mammoth Lakes (1980), Coalinga 

(1983), and Morgan Hill (1984). Certainly, the NEHRPfunded strong motion program of the USGS has figured 

prominently in the collection, processing, and analysis of 

these observations-but by no means exclusively. In California, the U.S. Army Corps of Engineers, the U.S. Bureau of 

Reclamation, the California Division of Mines and Geology, 

the Los Angeles Metropolitan Water District, and the U.S. 

Veterans Administration, as well as the USGS, all have 

significant stakes in the present state-wide configuration of 

strong-motion accelerographs, as do various universities 

operating special-purpose research arrays (generally funded 

through the NSF) and the sum total of building owners 

required to have strong-motion instrumentation by the Los 

Angeles Building Code and, more recently, by the Uniform 

Building Code. Moreover, the present-day strong-motion 

program of the USGS has antecedents dating back to 1932. 

Such continuing and common interests of diverse disciplines 

in various observational programs are the norm, not the 

exception, in earthquake studies-and in NEHRP. 

What this means, then, is that NEHRP cannot be 

sharply circumscribed as a scientific entity in and of itself. 

NEHRP relies heavily in a temporal sense on the rich history 

of past observational experience and in a spatial sense on the 

many investigations carried on at numerous institutions. both 

at home and abroad. In the United States, many such studies 

are supported directly by NEHRP; from an international point 

of view, most of them are not. In this report, which purports to 

assess the scientific status of NEHRP as of 1984, we have 

borrowed freely from this larger experience of earthquake 

studies, although we cannot recount it entirely here. We 

certainly do not wish to imply that the scientific results 

described herein are due to NEHRP alone. Our predilections, 

indeed, are in just the opposite direction. We regard the 

continuity of observational traditions and of close cooperation with other programs and institutions as, if not exactly 

"accomplishments" of NEHRP, certainly essential features 

of its well-being. Simply stated, then, this report attempts to 

describe what "we'' (in a large and shared sense) have 

Earthquake Studies and NEHRP 5 
learned scientifically since the inception of NEHRP-in 

terms that we know are generalized but that we hope are 

manageable-about these problems of national and international concern. 

TRENDS OF THE SCIENCE 

At the heart of this science lie two basic questions: why 

do earthquakes occur, and what happens when they do? The 

latter question is associated with the gamut of earthquake 

hazards, from surface faulting, through strong ground 

motion, to ground and structural failures. The "why" of 

earthquakes can be reformulated as a set of questions related 

to where, when, how big, and by what mechanism, questions 

intimately connected with earthquake prediction, although 

they pertain to induced seismicity and earthquake control as 

well. 

Whatever their association with earthquake hazards, 

prediction, and control may be, these questions set long-term 

themes for earthquake research. In this section, we discuss 

four such themes, trends of the science in the 1980's that have 

already yielded broad and general insights concerning the 

cause and effect of earthquakes. These matters, in their 

general importance, sit somewhat above the 10 more specific 

accomplishments presented in the following section. More 

importantly, these trends hold much promise for providing 

the science with an even more complete and general understanding of earthquakes. 

The measure of an earthquake 

Most of what we know about earthquakes, specifically 

their locations (including origin times), faulting orientations, 

source strengths, and dynamic faulting characteristics, is 

derived from seismic data. Such observations tell us that an 

earthquake occurred, when and where it occurred, and, as a 

result of quite recent developments, increasingly more about 

how it occurred. Seismic data alone, however, do not tell us 

why a certain earthquake occurred, nor do they tell us when 

the next such earthquake will occur; to address such questions, we must turn to a host of other geologic and 

geophysical observations. Nevertheless, earthquake catalogs 

that are accurate and complete with respect to the above 

characteristics are essential to earthquake studies because 

earthquake-recurrence intervals, seismic-moment-release 

rates (or fault-slip rates), strong ground motion, and earthquake risk, among other things, are assembled from such 

data. 

By long custom, earthquake locations, magnitudes, 

and faulting orientations have been the principal data sets of 

earthquake seismology. After the installation of the WorldWide Standardized Seismological Network (WWSSN) in the 

early 1960's, such determinations became greatly improved 

on a global basis and played a central role in the development 

of the theory of plate tectonics and in matters relating to the 

discrimination of earthquakes from underground nuclear 

explosions. Since the early 1970's, rapid progress has been 

made in the development of broad-band, high-dynamicrange, digital instrumentation. Global, regional, and local 

deployment of arrays of such instruments are now providing 

data that are significant in many important ways. In this 

section, we describe how such data have been used to take the 

measure of an earthquake. In other sections, we describe 

results that depend heavily on these same advances, including fully computerized locations of the swarm of earthquakes 

at Mammoth Lakes, California, in January-February, 1983, 

determination of the crustal structure of the New Madrid 

seismic zone, and waveform inversion for the source mechanism of the 1979 Imperial Valley earthquake. 

(It is impossible to overstate the significance of these 

advances in digital data acquisition and in computer technology allowing for rapid and efficient digital data processing. No other science has the data problems that earthquake 

seismology has: from the smallest earthquake we can measure under normal circumstances to the largest earthquake we 

know of, signal amplitudes span a factor of 1010, across a 

frequency band of six orders of magnitude or more. These 

systems, moreover, must always be at the ready, so to record a 

partial set of the hundreds of thousands of earthquakes that 

are registered globally each year.) 

Seismic moment and the seismic moment tensor 

(which specifies the directional nature of faulting in terms of 

the slip vector and fault-surface orientations) are readily, 

accurately, and now routinely determined from such digital 

data (fig. 3). Seismic moment, M0 = J.LU A where f.L is the 

shear modulus of the source region and u is the average slip 

across the fault surface of area A, figures prominently in 

earthquake studies. Cumulative moment sums are easily 

translatable into average slip rates for any fault of interest, and 

nowadays it is almost meaningless to speak of a recurrence 

interval without specifying the M 0 of the event of interest. 

Seismic moment, moreover, is the controlling source factor in 

strong-ground-motion excitation, inasmuch as the dynamicstress differences that accompany crustal faulting appear to 

be so remarkably constant (see discussion below). The most 

important practical value of rapid and accurate determinations of the seismic-moment tensor, however, is for tsunami 

warning, in the case of major shallow earthquakes that rupture the ocean floor. Including the transit times of the seismic 

waves, seismic-moment-tensor inversions, in principle, can 

be available within tens of minutes of the earthquake occurrence, whereas tsunami-transit times are hours or more to 

distant landfalls. 

It is moreover known that M 0 does not suffer from the 

"saturation" problems that afflict all time-domain-amplitude 

magnitude scales. The recent development of a moment magnitude scale, M = 73log M0 - 10.7, is a natural progression 

to a more quantitative, more accurate, and more physically 

meaningful statement of source strength. Several recent studies have shown that M agrees well with M L (local magnitude) 

6 The National Earthquake Hazards Reduction Program-Scientific Status 
for 3 :::; ML :::; 7, withM5 (surface-wave magnitude) for 5 :::; 

M5 :::; 7 '12 , and with M w (energy magnitude) at larger magnitudes. The moment-magnitude scale, then, promises to 

provide earthquake studies with a unifying and uniformly 

valid measure of source strength. 

In addition to seismic moment, other measures of the 

earthquake source may be extracted from seismograms, 

including faulting duration, dimension and direction of rupture, various stress differences accompanying the failure process, and measures of the nature and extent of dynamically 

inhomogeneous faulting. In the past decade, much interest 

has been focused on the stress differences associated with 

crustal faulting, in part for what this tells us about the failure 

process itself and the conditions that drive it and in part for 

the strong influence such stress differences exert on the 

resulting ground motion. 

It has been known for some time that the average staticstress drops of earthquakes (that is, the difference between 

stress levels before and after faulting) are essentially constant 

over the entire range of events accessible by instrumental 

recording (10 16 :::; M0 ~ 1030 dyne-em; 0 :::; M :::; 9.5). 

Typically, these stress drops range from several bars to several 

hundred bars, and although this variance is not small on an 

absolute scale, it is indeed small in comparison with the range 

of seismic moments and source dimensions of earthquakes 

across the entire range 0 :::; M ~ 9.5. The source of this 

"stress-drop variance" is not well understood, although part 

of it is attributable to true uncertainties in the static-stressdrop determinations. At least some of this variance is thought 

ANMO_ .At. • . " 

p x2 ""~ VV\}VIA 

KEV 

~ fVV'v... A , BER 

.·• V vw· Px2 

. · .. J . '"IL GRFO 

·r-- IJ"'V""''''V Px2 

Axis Az PI ::t 0.4 [ 

p 266 20 I I I I I I I 0 .0 I 

T 62 68 

I 173 8 

M 0 = 2 .20 x 1024 dyne-e rn 

Mw = 5 .5 

0 1 2 3 4 

TIME , IN MINUTES 

Figure 3. Lower-hemisphere, equal-area projection showing moment-tensor inversion of teleseismic P waves of the 

1982 New Brunswick, Canada, earthquake. Initial analysis 

from U.S. Geological Survey, Branch of Global Seismology 

and Geomagnetism. 

to be of true tectonic significance, but it has been noteworthy 

in this regard that the stress drops of tremors induced by deep 

gold mining in South Africa are indistinguishable from those 

of naturally occurring, tectonically driven earthquakes. 

In the past decade, several estimates of the dynamicstress differences accompanying crustal faulting have 

become available, all of them obtainable from some measure 

of the recorded ground acceleration. The significance of these 

results are several. At any given depth, the dynamic-stress 

differences show less scatter than do the average static-stress 

drops. Second, very recent studies of dynamic-stress differences in California and Japan reveal that they depend on 

both depth and faulting type-dependences not discernible 

in static-stress-drop estimates, probably because of their 

larger uncertainties. These results point to a frictional model 

of faulting wherein dynamic stress differences should 

increase with depth in the seismogenic layer. Finally, because 

various measures of ground acceleration of engineering interest are directly controlled by these dynamic-stress differences, an important practical result of this approach has 

been the development of simple but accurate techniques to 

calculate ground-motion amplitudes for engineering design 

purposes, a matter to which we return in the section below 

entitled " Strong Ground Motion. " 

The earthquake cycle 

The notion of an earthquake cycle dates back to the 

Reid hypothesis, formulated by H.F. Reid on the basis of his 

observations of the 1906 San Francisco earthquake. This 

hypothesis holds that two comparable-size earthquakes rupturing the same section of fault will be separated by a period 

of time sufficient to reaccumulate strain by an amount equal 

to the elastic-strain drop accompanying the first earthquake. 

Since then, specific stages within the earthquake cycle have 

been recognized for major earthquakes: a long period of 

seismic quiescence following a major earthquake and its 

immediate aftershocks, a shorter (and varying) period of 

enhanced seismicity as elastic-strain accumulation 

approaches (and, locally, exceeds) the critical strain level, and 

a very short (hours to days) but commonly nonexistent period 

of immediate foreshocks, all followed by the next major 

earthquake. 

Segments of plate boundaries that have exhibited long 

periods (decades to centuries) of seismic quiescence are now 

known as seismic gaps. They can be mapped worldwide, and 

systematic identification of them has proceeded through the 

1970's. Initially, seismic gaps were ranked in terms of their 

"seismic potential" solely on the basis of the length of time 

since the last, gap-filling earthquake. Between 1968 and 

1980, 10 major (M ?:: 7 .5) plate-boundary earthquakes were 

correctly anticipated by means of the seismic-gap hypothesis 

(fig. 4). 

This success in forecasting the place and magnitude of 

major plate-boundary earthquakes is of great importance in 

Trends of the Science 7 
rationally anticipating, planning for, and working against 

potential earthquake hazards. The concept is also important 

to the siting and execution of experimental programs with 

which we may determine more precisely the time of the 

impending earthquake, as well as to the siting of strongmotion accelerographs to obtain ground-motion records 

close to rna jor and great earthquakes. In the United States, the 

Shumagin Islands, Alaska, seismic gap and the section of the 

San Andreas fault that broke in the 1857 Fort Tejon, California, earthquake are considered to be the most likely sites for 

the next M ~ 8 earthquakes (fig. 4). More recently, the 

essential features of the earthquake-cycle concept have been 

formalized in the time-predictable model of earthquake 

occurrence. The essence of this model is the same as the Reid 

hypothesis: the time to the next large earthquake can be 

estimated from the strain drop (or faulting displacement) of 

the preceding large earthquake and the rate at which strain (or 

displacement) is accumulating. For six situations in which 

three or more earthquakes are known in the historical record, 

predicted repeat times are in agreement with the observed 

repeat times to an accuracy of 2 to 50 percent. 

0 

\j 4SS 

901 180 

An important ingredient of the earthquake-cycle 

model, which in fact is an assumption of the Reid hypothesis, 

is that all of the strain accumulated during the interevent 

period will be released seismically-that is to say, all of the 

strain accumulation is elastic. For many of the major fault 

systems of the world, this seems to be the case: elastic-slip 

rates obtained from seismic-moment sums are in reasonable 

agreement with long-term (millions of years) fault-slip rates, 

as obtained from geologic or paleomagnetic data-a result in 

keeping with the tenets of rigid-plate tectonics. Although 

some uncertainty is attendant to this coincidence owing to the 

less reliable seismic data of the early 1900's and before, and 

although the significance of measurable permanent (nonelastic) tectonic deformation has only recently been appreciated, the importance of continually monitoring strain 

accumulation in seismically active areas can hardly be overestimated. 

In fact, a variety of studies can help to determine what 

stage of the earthquake cycle some particular fault segment is 

in, at least for continental areas. Studies of paleoseismicity, 

especially those involving detailed investigations of the 

EXPLANATION 

1 ~:~~gT GSRE~ :~~~~~~~~~~:c 2 100 YEARS AGO 

2 ~~TTE ,o:O Gy~EA"fi E:~~HQ AKE <100 

l ~~:~~~E R: ~~T~~C°F~~L~!~GE EARTHQUAKE 

4 PLATE MOTION SUBPARALLEL TO ARC 

5 ~~YH~6~~~~V~E~g~~N~~A~RFE ART ~NAtTHQUAKE 

6 ~ii! ~~ ~~~sGJ ~i~Z¥fA1E s 30 YEAR S AGO 

SUCCESSFUL FORECAST 

REPRESENTATIVE 

FOCAL MECHAN ISM 

ACT IVE VOLCANO 

TSUNAMI 

90 w 

• 

• 

SOUTH 

SANOWICH 

I 

<V "' ) :: 

Figure 4. Seismic potential of the major circum-Pacific plate boundaries, as reckoned in 1980. Updated from W.R. McCann, 

S.P. Nishenko, L.R. Sykes , and j. Krause, "Seismic Gaps and Plate Tectonics : Seismic Potential for Major Plate Boundaries" 

(Pure and Applied Geophysics , v. 117, p. 1082-1147), 1979. 

8 The National Earthquake Hazards Reduction Program-Scientific Status 
Holocene history of faulting, are of great value in estimating 

average fault-slip rates and recurrence intervals between 

major earthquakes. Seismicity studies can reveal any temporal variation in earthquake occurrence related to an impending large earthquake. Advances in these specific study 

areas, as well in crustal-deformation studies, are detailed in 

later sections of this report. 

The results of such investigations have recently been 

synthesized to estimate the time of occurrence of the next 

great San Francisco earthquake, reckoned as a "repeat" of the 

1906 earthquake. On the basis of average slip rates and strain 

accumulation since 1906, the repeat of such an earthquake 

appears to be 50 years or more into the future. On the other 

hand, the recent increase of seismicity at the M ;:::::: 5 level in 

and around the San Francisco Bay region (fig. 5) suggests that 

the 6 :::; M :::; 7 earthquakes that preceded the 1906 earthquake may return to the bay region at any time. 

These are promising developments with respect to the 

long-term prediction of major earthquakes. Continuing 

development of these ideas, however, can only proceed at the 

rate at which new observations are collected-which is slow 

in the case of major plate-boundary earthquakes. With no less 

concern for the major gaps of highest seismic potential (fig. 

4), much interest is now directed to smaller fault segments, 

where M ~ 6 earthquakes occur more frequently. Near Parkfield, California, at the northwest extremity of the 1857 rupture of the San Andreas fault, comparable-size (5 V2 :S; 

M :::; 6) earthquakes have occurred with a striking regularity 

(1881, 1901,1922,1934, and 1966). Itseemsquitelikelythat 

another such event will occur within the next 5 years. Some 

concern exists that such an event might trigger a repeat of the 

great 1857 earthquake, because the 1857 earthquake is inferred to have initiated near Parkfield and because the time 

elapsed from the 1857 event is now close to the average repeat 

time for such events as revealed at Pallett Creek (see fig. 13 

and attendant discussion). 

1855- 79 

EXPLANATION 

M=5 o 

60 

0 

0 0 

100 KILOMETERS 

1880-1904 1905-29 

0 \ 

0 

Structural and Tectonic Setting 

Underlying all considerations of the structural and 

tectonic setting of earthquakes is the unifying theme of plate 

tectonics. Most of the world's earthquakes and near! y all great 

(M ;:::::: 8) earthquakes occur along the major (nonridge) plate 

boundaries (fig. 6). Most of the results of the preceding 

section, in fact, can be regarded as natural corollaries of rigidplate tectonics. These ideas do not, however, explain all 

aspects of crustal earthquakes; in fact, rigid-plate tectonics 

fails to explain most crustal earthquakes in continental 

areas- just those earthquakes of greatest social concern. In 

the United States, the distribution of seismicity and faults in 

southern California is left unexplained by rigid-plate 

tectonics, as is the seismicity and faulting throughout the 

Basin and Range province and in the eastern and central 

United States as well. Neither do the recent 1983 earthquakes 

at Coalinga, California, and Borah Peak, Idaho, have much to 

do with these ideas. Volcanic and induced seismicity, of 

course, are also separate issues. 

In all of the situations described immediately above, an 

understanding of the structural and tectonic setting beyond 

that provided by rigid plate tectonics is necessary to explain 

why these earthquakes occurred, when they might occur 

again, and where else such relatively infrequent events might 

occur. Several of these situations are treated separately in the 

following section, in particular, those advances related to the 

understanding of earthquakes in the eastern and central 

United States, volcanic seismicity, and induced seismicity. 

For the western United States, a scaled-down version of 

plate tectonics, "contemporary block tectonics" has proved 

useful in understanding the structural kinematics and relative 

motions of crustal blocks adjacent to the plate boundary in 

California and Nevada. Figure 7 illustrates details of the 

concept for the southern California region. An important 

result is that the Hosgri fault, a major right-lateral strike-slip 

1930-54 

0 

0 ° 0 0 

0 

1955-80 

0 

0 

0 

0 

Figure 5. Sketch maps showing seismicity of the San Francisco Bay region (M ?: 5) in quarter-century intervals before and 

after the 1906 San Francisco earthquake, which ruptured the San Andreas fault along the shaded zone in the middle frame. 

From W.L. Ellsworth, A. G. Lindh, W.H. Prescott, and D.G. Herd, "The 1906 San Francisco Earthquake and the Seismic Cycle," 

in Simpson, D.W., and Richards, P.G., eds., "Earthquake Prediction: An International Review" (Washington, American 

Geophysical Union, p. 126-140), 1981. 

Trends of the Science 9 
1; .. ,.;. 

SEISMICITY OF THE EARTt 1 960-1980 aYAL I'.A-'11: £Srt.l.TlSA 

rltlll ltlt"·cn.t~T 

...tJIAIUE TII 41U' 

----~ ·-

wo•~o oc:u. .. .-L..,• .. •..o•••" .,.,.,., , ••u~• •• .,,,,,. .• ,.,,,,rt 

Figure 6. Seismicity of the Earth, 1960-80. Compiled by A. F. Espinosa, W. Rinehart, and M. Tharp on the "World Ocean Floor 

Panorama" of B.C. Heezen and M. Tharp, 1982. Reprinted with permission. 

fault subparallel to and offshore of the San Andreas fault in 

central California, must undergo a component of east-west 

compression, and this prediction is borne out by focal mechanism studies. Just as in rigid-plate tectonics, block tectonics 

holds that seismicity should be concentrated at block boundaries, with little or no deformation internal to the block. 

A parallel to these ideas exists in the finding that much 

of the seismicity in the eastern United States is apparently 

controlled by ancient zones of weakness, reactivated by the 

modern stress regime. The possibility that these weak zones 

define modern block boundaries could be of much importance in defining "seismic source zones" in the eastern 

United States, even if this is not the only reason for earthquakes in the region. 

Yet a higher level of detail in how the structural and 

tectonic setting conditions earthquake occurrence may be 

sought in the synthesis of various kinds of geologic and 

geophysical data. In a later section, we describe how such 

information has led to a greatly improved understanding of 

the workings of the New Madrid, Missouri , seismic zone. 

Just below, earthquake occurrence in the larger scale Basin 

and Range province is considered in the light of such developments. 

For the past 30 million years or so, the region between 

the Wasatch front in central Utah and the Sierra front in 

eastern California has opened by as much as several hundred 

kilometers. The extensional tectonics of the Basin and Range 

province, which may accommodate as much as 25 percent of 

the relative motion between the North American and Pacific 

plates in these latitudes, is evidenced by all manner of geologic , geophysical, and seismologic phenomena, including a 

thin crust, high heat flow, extensive magmatism and hydrothermal mineralization, low seismic velocities, high attenuation of seismic waves, and local compensation of individual 

basin-and-range pairs. This extensional deformation continues to the present, as evidenced by modern geodetic measurements (see fig. 11) and the normal faulting associated with 

historical earthquakes. Hydrofracture measurements have, in 

addition, revealed that the Basin and Range province is only 

marginally stable with respect to normal faulting, according 

to frictional-failure models. 

In the brittle, seismogenic upper crust, the WNW-ESE 

extension of the Basin and Range province is accommodated 

by normal faulting, which produces northerly striking, alternating sequences of uplifted and backtilted mountain ranges 

and downdropped, sediment-filled basins, from which the 

1 0 The National Earthquake Hazards Reduction Program-Scientific Status 
.... co 0 . ~ 

> "0 z (/) E 

..9:! co (/) 0 > co > 

cti > u a_ c Q.) 0 co 

~-<? > z a_ 

.... 

co ~ Q.) Cii G 

~/-· t5 en 1o ~/.$> 0 -<1. "'0 

" ,()~~,.,. 

-s- Mojave ~ /(' 

"'~ Ql" 

San Bernardino 

Mountains 

/ 

Figure 7. Idealized tectonic blocks of southern California. 

Top, initial configuration. Faults: B, Brawley; DV, Death 

Valley; E, Elsinore; G, Garlock; H, Hosgri; I, Imperial; M, 

Malibu; Nl, Newport-Inglewood; PV, Panamint Valley, SA, 

San Andreas; SJ, San Jacinto; SN, Sierra Nevada; SY-BP, 

Santa Ynez-Big Pine. Bottom, block motion following incremental displacement in response to north-south compression and east-west extension. From D.P. Hill, "Contemporary Block Tectonics: California and Nevada" (Journal of 

Geophysical Research, v. 87, p. 5433-5450), 1982. 

province takes its name. The approximately uniform spacing 

of the basin-and-range pairs (typically, 25-35 km) is clear 

evidence that the extension across the province has been fairly 

uniform, at least on a time scale of millions of years. For 

hundreds of thousands of years and less, however, this deformation has been decidedly nonuniform (fig. 8), and this poses 

important problems not only for the mechanics of this extensional deformation but also for the realistic assessment of 

earthquake hazards in this region. The major earthquakes of 

the historical record, for example, have occurred entirely 

within a narrow corridor in the west-central part of the 

province (fig. 8). Nevertheless, there is clear evidence that the 

Wasatch front has been faulted numerous times by M ~ 7 

earthquakes in the past several thousand years (fig. 9), and a 

scarp along the North Creek section of the Wasatch fault zone 

is so fresh that it is unlikely to be older than several hundred 

years. 

Recent studies have focused on the subsurface geometry of faults and faulting and the mechanisms of deep-level 

extension. Analysis of a growing collection of seismic-reflection data and borehole logs has revealed a complex hierarchy 

of faults responsible for the modem deformation. In some 

areas, low-angle normal faults have been detected at only 4- to 

6-km depth, and these apparently serve as detachment surfaces, because high-angle normal faults observed at the 

Earth's surface merge with (but do not cut) these detachment 

surfaces. In other places, however, these high-angle normal 

faults evidently penetrate to depths of 10 to 15 km. These 

results point to varying mechanisms and sizes of earthquakes, 

depending on the nature and extent of the detachment surfaces and the mechanism by which they accommodate extensional displacements. 

Inhomogeneous faulting 

Heterogeneous structures along active crustal fault 

zones apparently exist on all length scales, as do the earthquakes that occur along them, from wavelengths of a thousand kilometers or so down to microscopic grain-size 

dimensions. It has been shown many times that the size of an 

earthquake is controlled by offsets or discontinuities separating otherwise-coherent fault segments, again across a wide 

range in length scales. Many of the major gap-filling earthquakes, for example, have filled the entire preexisting seismic 

gap, the end points of which are commonly associated with 

clear evidence for structural discontinuity. On the San 

Andreas fault, on a somewhat smaller scale, the southern 

extent of the 1906 earthquake and the northern extent of the 

1857 earthquake (and future recurrences of either) are probably controlled-and isolated from each other-by the 180 

km of the "creeping" section of the fault that lies in between. 

On yet a smaller scale, the Wasatch fault in central Utah and 

southern Idaho apparently breaks in earthquakes the size of 

which are controlled by structural discontinuities along strike 

(fig. 9). On scales of just several kilometers or so, normal 

faults in the Albuquerque-Belen and La Jencia Basins within 

Trends of the Science 11 
the Rio Grande rift valley of central New Mexico demonstrate 

discontinuous behavior spatially and highly sporadic movement temporally. On an even smaller scale, a Y2-km offset of 

the trace of the San Andreas fault near Parkfield is associated 

with clear mechanical complexity at depth, which includes 

an offset of the plane of seismicity at depth and fault-plane 

solutions significantly rotated from the trend of the fault. This 

offset is now thought to be the southeast terminus of the 1966 

Parkfield earthquake. Seismicity studies in the 1980's now 

allow us to examine such heterogeneous structures 

throughout the seismogenic depth on even smaller length 

scales. 

From such observations over such wide ranges in scale, 

it is apparent that inhomogeneous fault structures play two 

important roles in generating earthquakes. In the first place, 

one may regard the faults themselves as inhomogeneities, 

separating the stronger, less deformable plates or blocks on 

either side. In this role, the inhomogeneity concentrates 

deformation to the faults themselves; the ideas of plate or 

block tectonics follow naturally from this mechanism. Second, inhomogeneous structures along or within fault zones 

control the size or length of earthquakes along any particular 

fault. This has led to the recent development of the characteristic earthquake model, the idea that any particular fault 

segment will tend to rupture in a certain size event, the length 

of that event being controlled by binding structures along the 

fault. The rupture of these high-strength structures, in tum, 

can heavily condition the spectral composition of the resulting ground motion. 

The significance of the characteristic-earthquake model 

is that it allows an estimate of the maximum size of an 

earthquake for any given fault segment from geologic considerations alone. Such estimates are especially important in 

regions for which the maximum-magnitude earthquake may 

not be known from the available historical record. Moreover, 

the characteristic-earthquake model admits the interpretation 

that regional earthquake frequency-of-occurrence statistics 

are fundamentally controlled by the length distribution of 

coherent fault segments within that region, a proposition that 

has recently been verified for the island of Honshu, Japan. 

These developments indicate that much is still to be learned 

from the detailed mapping of active crustal fault zones to 

obtain coherent segment lengths and slip rates. This information, in tum, can be used to calculate much improved earthquake-risk estimates. 

Equally plain are the manifestations that the earthquake 

rupture process itself is highly inhomogeneous. It has been 

known for some time that M ~ 8 earthquakes, with faulting 

durations of 100 seconds or more, can be arbitrarily complex 

on 1-second time scales; so, too, M ~ 6Y2 earthquakes can be 

arbitrarily complicated on 0 .1-second time scales, and so on 

for even smaller earthquakes. It is natural to suspect that 

material inhomogeneities, as may be seen from geologic or 

seismicity evidence, play important roles in causing dynamically heterogeneous faulting. The rupture of high-strength 

asperities is now thought to play an important role in initiating crustal earthquakes-often dramatically--and barriers 

to faulting can produce large bursts of acceleration. Various 

types of earthquake series-for example, foreshock/mainshock/aftershock sequences or earthquake swarms-are now 

being modeled in terms of a distribution of inhomogeneous 

faulting elements. 

It is difficult to overstate the importance of inhomogeneities in the Earth's crust with respect to the faulting 

process. Crustal earthquakes start and stop because of these 

material and geometric inhomogeneities, and what happens 

dynamically in between is also affected by them. All manner 

of geologic and seismologic phenomena are controlled by 

these inhomogeneities and their size/frequency-of-occurrence distribution. It is not too much to say that earthquakes 

would not occur as we presently know them, except for these 

inhomogeneous structures; indeed, but for them, earthquakes 

might not occur at all. Recent developments in the mapping 

and stochastic representation of these phenomena, both statically and dynamically, have led to much progress in understanding the causes and effects of earthquakes; it is certain 

that continuing research in these areas of multidisciplinary 

concerns will result in further advances. 

SPECIFIC ACCOMPLISHMENTS 

Crustal deformation 

The Earth is a deformable body, and the ultimate cause 

of earthquakes is material deformation driven by relative 

displacements arising from large-scale plate motions. 

Beyond some critical strain level, this deformation can result 

in dynamically unstable shear failure, that is, an earthquake, 

although aseismic faulting and folding can and do occur as 

well. Geologic investigations of the long-term patterns and 

rates of crustal deformation have long held a central position 

in the earth sciences, for it is this deformation that shapes the 

face of the Earth in its great variety of geologic structures. 

Many of these structures, of course, are associated with 

economic, political, or some interest other than those important here, namely, geologic, tectonic, and seismologic structures associated with the causes and effects of crustal 

earthquakes. 

In general, we need to know the sense of deformation 

(horizontal and vertical components, with their magnitudes), 

the patterns of deformation (both along strike and away from 

some fault of interest, for example), and the history of deformation. Recent developments in deciphering and dating geologic events in the late Quaternary (the past several hundred 

thousand years) have provided an important context for 

instrumental measurements. Studies of uplifted marine terraces in Washington, Oregon, and California, for example, 

have revealed that the Pacific coast of the coterminous 48 

States has been uplifted substantially during the Quaternary. 

These uplift rates can exceed 10 mrnlyear in regions of crustal 

12 The National Earthquake Hazards Reduction Program-Scientific Status 
120' 

I 

( 

I 

EXPLANATION 

• Historical faulting- 3 x 1 o- 3 > event 

> 1 X 1 0- 4 per year per 1 ,000 km2 

I_ 

1·-~:~·.:>:·:l Holocene faulting (past 1 2,000 ±years)- 5 X 1 o- 5 

> event > 5 X 1 o-s per year per 1 ,000 km 2 

D No late Quaternary events (Past 5 X 1 o- 5 ± years) 

UTAH 

ARIZONA 

100 MILES 

0 100 KILOMETERS 

Figure 8. Generalized rates of surface faulting in the Great Basin province. From R.E. Wallace, "Active Faults, Paleoseismology, and Earthquake Hazards in the Western United States," in D.W. Simpson and P.G. Richards, eds., "Earthquake Prediction: An International Review (Washington, American Geophysical Union, p. 209-216), 1981. 

Specific Accomplishments 13 
~-1~1~3~----------~1~1~2° __________ ~1~1~1" __________ ~110° 

Salt~ Lake Little Cottonwood Canyon 

City 0.8 (+0.6; -0.3) 

- -t I 09,000 ± 2ooo; 

WASATCH \ 

FAULT ZONE - '\ Hobble Creek .Y 0.85- 1.0 -tJ-(13,500) 

b North Creek 

< 1.3 (± 0.1) 

--- (4580 t4C yr B.P.) 

1-

P. Deep Creek 

\ ~0.35 (±0.05) 

(7300 14C yr B.P.) 

39°~~~----------~------------~----------~ 

A 

r--111r3_o __________ ~1~1~2o __________ ~1~1~1" ___________ 1~10o 

) i-EASTCACHE 

420 _____ !_Q:~.I::!.Q__ FAULT ZONE 

UTAH - ----l 

Logan 1 

1.4 l 

Kaysville 

1.6 

1.7-3.4 

I 

I 

I 

I 

L._ __ ~Y_9~1NG 

[.,;;; 3.3-.,;;; 3.7(3)1 

~:~\ Uttle Cottonwood Canyon 

-o'-T [2.o12!1 

WASATCH \ 

FAULT ZONE - '\ Hobbie Creek 

~ 3.0 

- -yL [1.6- 2.3,6 -7ll 

b North Creek 

< 2.0- 2.2 ___ 2.0- 2.5 

I 2.6? 

\ ]2.31311 

50 KILOMETERS 

c 

113° 112° 110° 

) i-EASTCACHE 

420 ______ !_Q:~.I::!.Q__ FAULT ZONE 

UTAH - L~gan --l 

I (after 13,500) 

MAE (13,500 t 8,000?) 

I 

I 

I . I 

Kaysvtlle- 1 WYOMING 

41 o 2 (after 1580 14C yr B.P.) ---

MAE..;;; 500 

Salt~ ~ke Little Cottonwood Canyon 

lty [2500] (19 000) 

I- ' 

WASATCH - c 

FAULT ZONE - '\ Hobble Creek 

l

6- 7 (after 13,500) 

[1700 to 2700] --r MAE~ 1000? 

b North Creek 

< 2 (between 4580 and 

-- 3640 14C yr B.P.) 1 -I (after 1100 14C yr B.P.) 

Deep Creek P. MAE (300 to 500) 

1 (after 7300 14C yr B.P.) \ 

MAE (<1750 14C yr B.P.) 

39°~--L-------------~----------~L-----------~ 

B 

14 The National Earthquake Hazards Reduction Program-Scientific Status 
shortening, such as the Ventura-Santa Barbara area on the 

southern margin of the Transverse Ranges, but even in 

regions dominated by strike-slip faulting, uplift rates are 

several tenths of a millimeter per year. In view of these 

observations, the Palmdale uplift in southern California is, 

geologically speaking, no surprise, although charting very 

short period (years), episodic behavior requires instrumental 

measurements (fig. 10). Further examples of the use of the 

recent geologic record to derive patterns and rates of deformation are given in the next section. 

Regional geodetic measurements can determine modem rates of crustal deformation driven by longer term geologic processes. Figure 11 shows the principal strain rates at 

selected trilateration networks of the western United States. 

The modem seismic and tectonic activity of this region exists 

115' 

0 

0 

0 

largely because the relative displacement between the North 

American and Pacific plates is not entirely accommodated at 

the principal plate boundaries along the Pacific coast. The 

observations in figure 11 yield estimates of this ''missing" 

component of plate motion, thereby providing a quantitative 

basis for assessing the earthquake potential of this large 

intraplate region. 

The most complete geodetic measurements in 

tectonically active areas come from California and Japan, 

where areal coverage is extensive, the history of measurements is long, and present-day measurements are made 

annually or more frequently. Along the San Andreas fault, for 

example, strain accumulation now occurs along the two 

locked segments, those which broke in the great earthquakes 

of 1857 and 1906; little if any strain accumulation is taking 

oe og 36 

(mml (fLGal )(ppm) 1977 1978 1979 1980 1981 1982 100--20- -I 0-

50- -10- -0 5- ,l 0- o- o- }. lffi-.. ~ I 

I 

I l- ~ -50- 10- 0.5- I 

-100- 20- 1.0- 't Tejon Pass f 't -150- 30- 1.5-

0 

0 ,:_ -:~ 0:_ p-t- t i ~/+ f! I 0 

-5o - IO- o 5- fCajon Pass , !I+---/ ! 

EXPLANATION 

o Gravity stat1on 

e Reference gravity station 

~- Leveling lme 

f\".... Trilaterat1on network 

L)--1 (local network shaded) 

--Fault 

50 100 KILOMETERS 

32'~----------------------------------------~ 

-100- 20- I o- r EXPLANATION -150- 30- I 5- e Gravity 

100- -20- -10-

50- -10--05-

0- 0 - 0-

-50- 10- 0 5-

-1oo- 20- 1 o-

-150- 30- I 5-

-200- 40- 2.0-

Palmdale 

D Elevation 

6 Areal stra1n 

Figure 10. Left, index map showing locations of gravity stations, leveling baselines, and trilateration networks in southern 

California that have been surveyed repeatedly during the past 5 to 10 years. Right, temporal changes in gravity 

(f>g), elevation (8e), and areal strain (8~) in three areas of southern California (shaded on index map). From R.C. jachens, W. 

Thatcher, C.W. Roberts, and R.S. Stein, "Correlation of Changes in Gravity, Elevation, and Strain in Southern California" 

(Science, v. 219, p. 1215-1217), 1983. 

Figure 9. Summary of slip rates, earthquake-recurrence times, and displacements per event for the Wasatch and East Cache 

fault zones in Utah and Idaho. A, Slip rate in millimeters per year; age of displaced datum (in italics) in years before present. 

B, Leading numeral gives number of surface-faulting events (after date, in years before present); bracketed numbers give 

average recurrence time (in years). MRE, most recent event, with elapsed age (in years) in parentheses. C, Displacement per 

event (in meters); brackets contain average displacement per event for number of events, in subscripted parentheses. From 

D.P. Schwartz, K.L. Hanson, and F.H. Swan Ill, "Paleoseismic Investigations along the Wasatch Fault Zone: An Update," in 

A.j. Crone, ed., "Paleoseismicity Along the Wasatch Front and Adjacent Areas, Central Utah" (Utah Geological and Mineral 

Survey Special Studies, v. 62, p. 45-49), 1983. 

Specific Accomplishments 15 
place along the intervening creeping section of the fault 

between Parkfield and San Juan Bautista. Data from highprecision surveys of the past decade can now be used to obtain 

accurate estimates of present-day slip rates along the major 

strands of the San Andreas fault, measurements that are now 

being complemented by long-term estimates extracted from 

the geologic record (see next section). Various satellite and 

astronomic distance-measuring techniques, developed in 

research programs conducted by the U.S. National Aeronautics and Space Administration, are now providing deformation data on the distance scales of the plates themselves. 

Important recent achievements in understanding crustal 

deformation relate to the identification of significant departures from uniformity in the rate of strain accumulation. The 

existence, for example, of postseismic transients that are 

typically 20 to 40 percent of the coseismic strain drop reveals 

a nonconstant rate of strain accumulation; these results, in 

tum, have implications both for the mechanism of strain 

buildup and for the estimation of recurrence times. Evidence 

for these transients from the San Andreas fault is summarized 

in figure 12. These data demonstrate the large magnitude of 

the postseismic strains, but, because of the infrequency of 

postearthquake surveys, their duration is not constrained. 

Figure 11. Trilateration networks (shaded areas) in the western United States and principal strain rates measured in 

each. Beneath each network name are the timespans of 

observations, directions of maximum extension, and principal strain rates (in microstrain per year). Major plate 

boundaries along the Pacific coast are sketched as dashed 

line (transform), double line (ridge), and shaded line 

(trench). J.F., juan de Fuca plate. From j.C. Savage, "Strain 

Accumulation in Western United States" (Annual Review of 

Earth and Planetary Sciences, no. 11, p. 11-43), 1983. 

Observations from great Japanese thrust earthquakes are 

more complete and indicate the existence of both short-term 

(~ 1 year) and long-term (decades) transients, results that 

point to the importance of both aseismic fault slip and transient flow in the asthenosphere for the cyclic accumulation 

and release of stress at major plate boundaries. 

In addition to these systematic postseismic transients in 

strain accumulation, it is now known that strain accumulation 

can be accompanied by remarkable episodicity in deformation rate, as figure 10 clearly indicates for the rise and fall of 

southern California. The mechanisms causing these sporadic 

(but correlated) changes in gravity, elevation, and areal strain 

are not yet known. They do, however, indicate that 

tectonically driven deformation can bump along with disconcerting short-term irregularity, in the absence of significant 

earthquakes. 

a: 

<( 

L.LJ 

>-

a: 

L.LJ 

a.. 

(/) 

z 2 <( 

Ci 

<( 

a: 

0 

a: 

u 

~ 

~ 

L.LJf-

<( 

a: 

z 

<i: 

a: 

f-

(/) 

I 

a: 

<( 

L.LJ 

I 

(/) 

0 

0 30 60 90 

EXPLANATION 

e Northern 

San Andreas 

0 Southern 

San Andreas 

120 150 

TIME SINCE LAST GREAT EARTHQUAKE, IN YEARS 

Figure 12. Shear-strain rate as a function of time since the 

last great earthquake on the San Andreas fault, either the 

1857 Fort Tejon or1906 San Francisco earthquake (see inset). 

Data from local networks: CP, Carrizo Plain; CV, Coachella 

Valley; FR, Fort Ross; M, Mojave Desert; PA, Point Arena; 

PD, Palmdale; PR, Point Reyes; SC, Shelter Cove; SF, San 

Francisco peninsula. Solid curve is a model calculation of 

temporal decay of strain rate for a coupled lithosphereasthenosphere pulsed by earthquake loading; dashed 

curve shows calculation based on a modified elastic-halfspace model. From W. Thatcher, "Nonlinear Strain Buildup 

and the Earthquake Cycle on the San Andreas Fault" (Journal of Geophysical Research, v. 88, p. 5893-5902), 1983. 

16 The National Earthquake Hazards Reduction Program-Scientific Status 
late Quaternary faulting geology 

The greatest difficulty imposed on earthquake studies is 

that of the available historical record of earthquakes. In 

California, for example, the record of felt earthquakes before 

the influx of miners and settlers beginning in 1849 can only 

be regarded as spotty; before the founding of Mission San 

Diego de Alcala in 1769, it is nonexistent. Even in China, 

where the history of recorded earthquakes dates back some 

3,000 years, statistical analysis suggests that the historical 

record is unlikely to be complete before A.D. 1500. As 

prisoners of the historical record, we are not only highly 

uncertain about the average recurrence intervals of large 

earthquakes along major plate boundaries but, in many less 

seismically prone areas, virtually ignorant of whether major 

earthquakes might occur every several hundred years-or 

every several hundred thousand. 

An important development in earthquake studies, then, 

has been the extension of the short instrumental/historical 

record of earthquakes backward in time by as much as several 

tens of thousands of years. The techniques of "paleoseismology" are principally geologic in nature, although 

analytical dating techniques generally play a critical role in 

dating one or more key horizons or structures (below). The 

geologic investigations include analyses of faulting-controlled microstratigraphic relations, both along strike and in 

sections across the fault exposed by trenching; seismically 

induced sediment transport; fault-scarp and fault-trace geomorphology; the attitude of fluvial terraces progressively 

uplifted by faulting; and regional tectonic relations and syntheses. 

Advances in dating late Quaternary materials have also 

progressed rapidly, and more than two dozen techniques are 

now being used. Uranium-series and 14C methods have long 

been the mainstays of late Quaternary dating, when the 

appropriate material is available in sufficient quantity. A new 

technique of accelerator counting of 14C, however, allows one 

to work with very small samples of carbonaceous material. 

Much progress has been made in relative dating techniques, 

which will allow for absolute ages when properly calibrated 

against absolute-age scales. These include 10Be accumulation in soils, the development of soil chronology and identification of paleosols, carbonate accumulation in soils, the 

development of hydration rinds, amino-acid racemization, 

dendrochronology, and lichenometry. Calibration of the 

extent of weathering against the bulk sound speed of exposed 

boulders shows much promise for determining the ages of the 

surfaces on which they lie. 

The locality richest in evidence for repetitive earthquakes is Pallett Creek, California, where ponded sediment 

traversed by a segment of the San Andreas fault that ruptured 

in the 1857 earthquake has now been exposed by downcutting 

of the drainage since its capture in the early 1900's. Here, the 

faulting history of earthquakes dating back to the birth 

of Christ has been left in astonishing detail (fig. 13). 

This and other stratigraphic sections provide evidence of the 

occurrence of 12 earthquakes, comparable in size to the 1857 

earthquake, during the past 1, 700 years, with an average 

recurrence interval of approximately 150 years. 

With observations that are extraordinarily easy to come 

by, the detailed morphology of dip-slip fault scarps is now 

being used to determine the time elapsed since their formation. A significant feature of this approach is that one is dating 

(or at least attempting to date) the earthquake itself, rather 

than some surface or stratigraphic horizon faulted at a later 

and generally unknown time. In essence, the proposition is 

very simple: for two scarps of comparable height in comparable material and climates, the older scarp is the one with the 

more subdued topography, in both slope and curvature. 

Recent applications of diffusion-equation mathematics allow 

quantitative determinations of such relative ages, and absolute ages in cases where the mass diffusivity can be estimated 

independently. Figure 14 illustrates how this technique has 

been used to analyze the faulting-controlled topography on 

which a section of the Great Wall in Ningxia-Hui Autonomous Region, People's Republic of China, was constructed. 

The wall, built here in A.D. 1531, was faulted in 1739; an 

earlier event with somewhat larger faulting displacements 

occurred along this same fault about 12,000 years ago. 

Such investigations, both in the nature and scope of the 

geologic investigations and in improved, redundant age determinations, have contributed a great deal of information about 

average slip rates, number of events, and recurrence intervals 

between events along the fault segments being studied (fig. 9 

represents another such study). Currently, some 150 geologic 

determinations of slip rates are available for California alone. 

These data have obvious and important implications for 

earthquake-risk analysis, as discussed in a later section, but 

these quantitative rates of deformation are also doing much to 

advance the scientific fields of Quaternary geology and 

tectonic geomorphology. 

In thrust-faulting environments, near-surface sedimentary sequences on the upthrown block are commonly folded 

and arched upward, forming anticlines or monoclinal 

upwarps. Ancient faulting events may be recorded in warped 

fluvial terraces adjacent to streams that traverse such structures, as was the case for both the 1980 El Asnam, Algeria, 

and the 1983 Coalinga earthquakes. Recent events, of course, 

are recorded in reverse curvature of the stream profile itself. In 

the case of the El Asnam earthquake, faulting resulted in 

damming of the Chelif River and flooding of several square 

kilometers of the Middle Chelif Basin upstream from the 

upthrown block. Such "earthquake-induced reservoirs" are 

an unusual form of earthquake hazards, one that often can be 

put to good work. 

Indeed, it is worth remembering that although faults, 

faulting, and earthquakes are recognized nowadays principally, if not exclusively, in terms of their hazard potential 

and nuisance value, it would be difficult to recognize many 

terranes, either geologically or economically, in the absence 

Specific Accomplishments 17 
~-N28E94 

Figure 13. San Andreas fault exposed in a trench at Pallett Creek, California. Black horizons are datable peat layers, which are 

progressively offset by greater amounts at greater depths (greater ages), the cumulative effect of earthquake faulting. View southeastward. From K.E. Sieh, "Prehistoric Large Earthquakes Produced by Slip on the San Andreas Fault at Pallett Creek, California" (Journal of 

Geophysical Research, v. 83, p. 3907-3939), 1978. 

of their effects. In California, for example, the Sierra Nevada 

as we presently see it, to which we owe the placer gold and 

much of the State's water supply, exists because of uplift and 

backtilting caused by spectacular normal faulting along its 

east front. The enormous sand-and-gravel operations in the 

Los Angeles Basin would operate at a fraction of their present 

production, if at all, but for the erosional debris delivered 

annually from the faulting-controlled San Gabriel Mountains 

standing just to the north. The Crystal Springs Reservoir, 

which holds Hetch-Hetchy water from the Sierra Nevada for 

use in San Francisco, owes its existence principally to the rift 

zone of the San Andreas fault. And the first Old World child 

born in California opened his eyes to the Coyote Creek fault 

on Christmas Eve of 1775, as the second Anza expedition to 

settle California availed itself of this strand of the San Jacinto 

fault zone on their journey from Yuma to the missions of the 

Los Angeles Basin. 

Seismicity studies 

A fundamental aspect of earthquake studies is simply 

knowing where earthquakes occur (and where they will occur 

in the future), and so systematic location of earthquakes 

has been a central task of seismology since continuously 

recording seismographs were developed a hundred years ago. 

Another point of view is that seismicity studies, in their 

earthquake locations and focal mechanisms, provide us with 

an "instantaneous" picture of the release of crustal strain. In 

this section, we emphasize developments in situations for 

which there is a close association between seismicity and 

crustal fault zones in active tectonic environments. Equally 

18 The National Earthquake Hazards Reduction Program-Scientific Status 
Exposure II Units Meters 

100 y 

-------- -·---- -------------- --------1 90 

80 

99 

70 

t!i~! ~~~~~;~~~~: ~::" ;t%i. " "" / ··~'· ""'/ 

60 

98 

50 

... -- --- 52 

97 

. ··- ~ ...... .....____ .. -.. ------ - --•• ---+-----1 

40 

- C_f ::_2_S_-_-: 

--"""~~~- ~--- ~: 96 

:.~ ~=-~r::<~ i " 

Figure 13. Continued. 

important associations are now known between earthquakes 

and both volcanic processes and human activities; magmatic 

seismicity and induced seismicity are the subjects of the 

following two sections. After a one-section interlude on stress 

estimates, the important problem of seismicity in the eastern 

and central United States is taken up, for which the association between seismicity and geologic structure with surface 

expression is far less clear. 

An important development of the 1960's was the documentation of the close association between earthquake locations and crustal fault zones that could be judged active on 

purely geologic grounds. Since that time, computer-aided 

data-processing techniques have dramatically increased both 

the completeness and accuracy of earthquake catalogues. 

Figure 15 , for example, shows the seismicity of western 

Nevada, California, northern Baja California, and the adjacent offshore areas; depicted here are some 7,000 M ~ 1.5 

earthquakes occurring in 1980. Figure 15 reproduces the 

"close" association between seismicity and active crustal 

fault zones (compare with the block boundaries of fig. 7) but 

also reveals that this association is not exact. The sections of 

the San Andreas fault that broke in the 1857 and 1906 earthquakes, for example, are conspicuously aseismic , as is the 

section of fault that broke in the great 1872 earthquake along 

30 

95 

the east front of the Sierra Nevada. We now recognize all three 

of these fault segments as seismic gaps, in the quiescent stage 

of their earthquake cycles. On the other hand, the boundary 

between the Coast Ranges and the Great Valley east of the 

San Andreas fault is noticeably seismic, in the absence of 

surficial expression of throughgoing faults. The 1983 

Coalinga earthquake occurred along this zone. Significantly, 

no surface faulting has been directly associated with this 

earthquake (although minor surface offsets developed during 

the aftershock sequence )-a rare situation for California 

earthquakes of this magnitude. 

A perplexing question in earthquake studies and one of 

great significance for earthquake prediction involves the processes by which one earthquake may (or may not) condition 

the occurrence of yet another, possibly larger event, either in 

its location in space and time or, possibly, in its source 

parameters. It has been known for a long time, for example, 

that most M ~ 6 crustal earthquakes have aftershock 

sequences easily identifiable as such in terms of their spatial 

and temporal distributions. Detailed studies in recent years, 

especially those of aftershocks of the 1975 Oroville and 1979 

Coyote Lake earthquakes, have further revealed that aftershocks can migrate well away from the surface faulted at the 

time of the main shock. These results are understood in terms 

Specific Accomplishments 19 
of the elastic-stress increases on the fault exterior to the region 

of the main-shock rupture, itself now carrying a reduced 

shear stress. Progressive seismic failure then ensues as the 

more highly stressed exterior regions subsequently fail, at 

least locally, in the form of aftershocks. That is, the main 

shock drives the aftershock sequence with its own stress 

redistribution. 

It has also been known for some time that most crustal 

earthquakes do not have such easily identifiable foreshock 

sequences; if such a "sequence" is identified at all, it need 

involve no more than one or two events. Almost certainly, the 

accumulation of stress leading to some main shock occurs 

(j) 

a: 

w 

1-

w 

~ 

4 

0 

----·-~~--- . 

--------·~-------·--·-·-------.. 

much more slowly than the release and redistribution of stress 

attendant to the main shock, and it seems likely that "foreshock sequences" can only be defined over correspondingly 

longer periods of time. What does not seem likely is that these 

processes of strain accumulation would have no seismic 

expression whatsoever. In fact, several recent studies have 

indicated that localized zones of impending rupture can be 

outlined (in shadow) by the preceding long-term seismicity of 

the surrounding regions, and the 1983 Coalinga earthquake is 

one example. Nevertheless, much work remains to be done 

on what, if any, systematics (both in seismicity patterns and 

in source parameters) are associated with the seismogenic 

EXPLANATION 

--·- Great Wall crest 

---- . ---- Lake Lahontan profile 3 

Model calculation : a = 1.6 m, 

Kt = 25m2 , b = 0.07 

~-~o __ 

·~ ----1 ·-·~<1----..-~.--

4 ·--. No vertical exaggeration 

METERS 

Figure 14. Inset, the Great Wall southwest of Shizuishan, Ningxia-Hui Autonomous Region, People's Republic of China. The 

1739 earthquake faulting cuts obliquely across photograph, from damaged part of the wall to near center of left margin. 

Photograph by R.E. Wallace. Below, profile of crest of the Great Wall shown above, in comparison with a fictitiously offset 

profile of a Lake Lahontan shoreline scarp in Pershing County, Nevada. Shoreline scarp is approximately 12,000 years old. 

This comparison, together with "faulted" diffusion-equation calculations (dots), indicate that the Great Wall here was built 

on faulting-controlled topography approximately 12,000 years old. Faulting displacement of this earlier earthquake is about 

60 percent larger than that for the 1739 earthquake at this locality. From R.C. Bucknam, T.C. Hanks, and R. E. Wallace (unpub. 

data, 1984). 

Figure 15. Seismicity of western Nevada and California in 1980 (M > 1.5). Smallest symbols, M < 2; largest symbols, M?: 7. 

Heavy lines, faults with Holocene activity; shaded areas, Quaternary volcanic centers. Straight-line end points denote 

extents of the 1906 San Francisco earthquake, rupturing the San Andreas fault from Shelter Cove to San Juan Bautista; 

creeping section of the San Andreas fault from San Juan Bautista to Parkfield-Cholame; the 1857 Fort Tejon earthquake, 

rupturing the San Andreas fault from Parkfield-Cholame to Cajon Pass; and the 1872 Owens Valley earthquake, breaking the 

Sierra front from Big Pine to the south end of Owens Lake. Mammoth Lakes and Anza are at end points of straight lines ML 

and A, respectively. Aftershocks of the 1980 Eureka earthquake define northeast-southwest zone of seismicity in upper left 

corner. Modified from D.P. Hill, R.E. Wallace, and R.S. Cockerham, "Review of Evidence on the Potential for Major 

Earthquakes and Volcanism in the Long Valley-Mono Craters-White Mountains Regions of Eastern California" (Earthquake 

Prediction Research, v. 3, p. 551-574), 1985. 

20 The National Earthquake Hazards Reduction Program-Scientific Status 
expression of strain accumulation in regions specifically 

identified as likely candidates forM ;;:: 6 crustal earthquakes. 

In California, such experiments are in place near Parkfield 

and Anza. 

ru 

m 

0.00 

ru 

m 

1906 

+ 

Creeping 

+ 

section 

+ 

+ 

3 1 0. 00 

0 

1857 

+ 

+ 

+ 

~0 

0 

\ 

0 • 

~ . ... 

0 

+ 

Magmatic seismicity 

Nowhere is the association between seismic activity, 

crustal deformation, and natural process so dramatic as in the 

case of the emplacement of partially molten rocks into the 

"<::::> 

0 

0 

!( 

·,IL /1/ 

0 ~~- 0 

r • 

ca o 6«!) 

0 ~ 

+ 

0.00 

+ 

0 + 

...+= 

~ <Sl 0 

<Sl 

<Sl 0 0 at'l~ 0 0 

31 0.00 

Specific Accomplishments 21 
Earth's crust. As has been documented many times at Kilauea 

Volcano, Hawaii, magmatic emplacement at shallow depth 

leads to uplift and extension of the overlying crustal rocks, 

deformation easily monitored by leveling and geodetic techniques. More rapid and localized motion of magma, 

especially as it approaches the Earth's surface, results in 

earthquake swarms (with rates of activity commonly exceeding 1,000 events per day) and harmonic tremor. Many of these 

processes, as revealed by their characteristic seismic and 

geodetic signatures, culminate in volcanic eruptions, numerously at Kilauea and spectacularly at Mount St. Helens, 

Washington, but many others do not, or have not yet, as in the 

cases of Matsushiro, Japan, and Long Valley, California. 

Although such investigations are not written specifically into the mandate of NEHRP, the early recognition of 

Mount St. Helens' awakening from dormancy and the systematic prediction of eruptions subsequent to the May 18, 

1980, explosion, based chiefly on the rapid analysis of data 

from a local seismograph network, are signal seismologic 

accomplishments. The past two decades have witnessed close 

associations between students of volcanic and tectonic earthquakes. The high-gain, short-period telemetered seismograph networks used extensively in California, Nevada, 

Utah, and Washington-indeed, in many parts of the 

world-trace their heritage to instruments developed during 

the 1950's to study microearthquakes on Kilauea; conversely, 

many of the seismologic capabilities that served so well at 

Mount St. Helens were developed in response to NEHRP. In 

any case, real or imagined programmatic distinctions lose 

their meaning in such areas as the Imperial Valley, Coso Hot 

Springs, and Long Valley, where tectonic and magmatic 

processes are intimately related. Around the Pacific margin, 

volcanic studies have also been used to develop stress trajectories, and there is occasional evidence that occurrences of 

major volcanic eruptions along the "Rim of Fire" are related 

to occurrences of adjacent great earthquakes. 

Perhaps more important, however, are the close parallels between earthquake prediction and the prediction of 

volcanic eruptions, both as matters of scientific method and 

as matters eliciting significant economic, political, and social 

responses. With respect to the scientific parallels, the principal techniques in both cases are the same: measurements of 

seismic activity and crustal deformation, and observations of 

other phenomena, such as gas emanation, that relate indirectly to either or both. Anomalous seismic and geodetic 

activity that precedes volcanic activity is of much interest to 

earthquake-prediction studies, both because such anomalous 

premonitory volcanic activity is commonly associated with 

very large signals and because many volcanoes, such as 

Kilauea, have very short interevent times. 

Responding to some level of notice, warning, alert, or 

prediction, however, is another matter, especially in cases 

when the event of interest (an earthquake or an eruption-or 

possibly both) may be long delayed from the onset of premonitory acttvtty. The situation at Long Valley that has 

developed since 1978 is a case in point. Southern Mono 

County, with Mammoth Lakes in its southwest comer, has 

been the locus of volcanic activity for millions of years. Long 

Valley caldera is the result of an enormous volcanic explosion 

700,000 years ago, and smaller surface eruptions have continued almost to the present, the latest being some 500 years 

ago. Against a backdrop of punctuated but more or less 

continuous earthquake-swarm activity since 1978 and 40 em 

of uplift across Long Valley caldera in the period 1979 to the 

present, the USGS issued a Volcanic Hazards Notice for the 

area in May 1982. The intense swarm of January 1983 along 

the southwest caldera boundary, together with increasing 

fumarole activity, did nothing to diminish concern. Scientifically, there is agreement that both the seismic activity and 

the vertical uplift indicate mobilized magma at depths of 4 to 

8 km (fig. 16). None of the scientists concerned would be 

surprised if this activity were to accelerate and result in a 

hazardous eruption next week, but neither would they be 

surprised if it were not to occur for another hundred years. 

While the impending hazard is real, then, its timing is highly 

uncertain. As chronicled by George Alexander of the Los 

Angeles Times, such a situation presents unusual difficulties 

in constructing a measured and prudent response that is both 

scientifically and politically viable. 

Induced seismicity 

The spatial and temporal occurrence of earthquakes has 

been influenced by human works in a number of ways, 

including the filling of large reservoirs, injection of pressurized fluids into deep boreholes for waste-disposal purposes, fluid injection for secondary recovery of hydrocarbons 

at depth, fluid transport related to solution mining and geothermal-energy extraction, and both underground and surface 

mining. The largest induced earthquakes have occurred in 

close spatial association with large reservoirs; earthquakes of 

M ? 5 Y2 have occurred at Koyna, India; Kariba, Zambia/ 

Zimbabwe; Kremasta, Greece; Oroville, California; Hsinfengkiang, China; and As wan, Egypt. The largest earthquake 

to have been associated with mining operations is the M = 

5. 5 earthquake in the Klerksdorp Mining District, South 

Africa, although concern exists there that even larger events 

may occur as gold mining progresses beyond the 4,000-meter 

depth level. 

Although the association between reservoirs and 

enhanced seismic activity had been noted as long ago as the 

filling of Lake Mead behind Hoover Dam, the physical 

mechanism of this process and its potential for generating 

damaging and destructive earthquakes have been appreciated 

only recently, specifically through analyses of earthquakes 

induced by deep-level waste disposal ( 1962-64) at the Rocky 

Mountain Arsenal near Denver, Colorado, and by a controlled fluid-injection experiment (1970-74) at the Rangely 

oil field in western Colorado. It is now understood that these 

22 The National Earthquake Hazards Reduction Program-Scientific Status 
earthquakes were the result of effective stress decreases 

caused by fluid-pressure increases, reducing the in situ frictional resistance to shear failure. Similarly, mining-induced 

earthquakes appear to be directly related to shear-stress 

buildup just ahead of advancing mine faces. These results 

have important implications for the relation between tectonic 

stresses and crustal deformation in general and the frictional 

(stick slip) model of crustal earthquakes in particular, subjects we take up in the next section. 

Since 197 5, field studies of reservoir-induced seismicity in Soviet Central Asia (Nurek and Toktogul Reservoirs) and Egypt (Aswan) have provided detailed case 

histories in different geologic and tectonic settings. These 

studies have shown that the potential for induced earthquakes 

depends strongly on localized geologic and hydrologic conditions. At the Nurek Reservoir, for example, induced earthquakes are concentrated in an area where a plunging anticline 

allows hydraulic connection between the reservoir and 

hypocentral depths. Few earthquakes occur in an immediately adjacent region, beneath an even greater reservoir 

depth; there, a tightly folded, horizontal syncline confines the 

influence of the reservoir to shallow depths. 

The situation at the Monticello Reservoir in South 

Carolina, constructed to provide cooling water for the nearby 

Virgil C. Summer Nuclear Power Station, presented several 

unusual characteristics with respect to "normative" conditions of reservoir-induced seismicity, although the association 

between reservoir filling and earthquake rate of occurrence is 

clear (fig. 17). In the first place, the reservoir itself is small 

and shallow; secondly, the induced seismicity is confined to 

very shallow depth ( :S 1.5 km). Such depths are easily 

reached by drilling, and two boreholes penetrated the seismogenic zones to depths of ~ 1.1 km. In each hole, several 

suites of geophysical measurements were made, including in 

situ stress magnitudes, pore pressure, permeability, and the 

distribution and orientations of faults, fractures, and dikes. 

Several significant conclusions may be inferred from 

these results and those of the companion seismicity and 

source-mechanism studies. First, the hydraulic diffusitivity 

estimates permit the size of the seismically active zone to be 

A Resurgent dome A' 

SE 

0 

SEA 2 

LEVEL 

4 

(J) 6 a: 

UJ 

IUJ 

~ 

0 8 ....J 

:::.::: 

~ 

£ 10 Ia.. 

UJ 

0 

12 

14 

16 

NW 

Crestview 

No vertical exaggeration 

Smokey Bear 

Flat 

Contemporary 

magma 

chamber 

Cas a 

Diablo 

South 

Moat 

Convict 

Lake 

Figure 16. Northwest-southeast cross section through Long Valley caldera, showing principal geologic units. The town of 

Mammoth Lakes, California, is located near A'. Geodetic and seismic data suggest magmatic intrusion beneath south moat 

of the caldera. From D.P. Hill, R.E. Wallace, and R.S. Cockerham, "Review of Evidence on the Potential for Major 

Earthquakes and Volcanism in the Long Valley-Mono Craters-White Mountains Regions of Eastern California" (Earthquake 

Prediction Research, v. 3, p. 551-574), 1985. 

Specific Accomplishments 23 
the result of fluid diffusion alone. Second, the stress magnitudes reveal that the Monticello Reservoir area is only marginally stable to stick-slip (seismogenic) frictional failure. 

Third, the stress magnitudes here are nevertheless concordant 

with those obtained in other areas, otherwise considered to be 

tectonically stable and seismically inactive. All three of these 

conclusions point to a fourth: the seismicity at the Monticello 

Reservoir was, indeed, driven by the perturbing action of the 

reservoir itself. 

Stress measurements 

The forces that drive plate motions, faulting displacements, and earthquakes result not only in deformation of the 

Earth's crust but also in deviatoric (shear) stresses within it. 

From the point of view of earthquake-mechanism studies, 

matters tum on whether or not crustal earthquakes result from 

stress-conditioned failure processes and, if so, what magnitude of deviatoric stress is necessary to drive that failure 

process. The time-predictable model of earthquake recurrence, for example, explicitly supposes a time-invariant level 

of critical shear stress for failure. Over the past two decades, 

laboratory investigations of the failure mechanics of precut 

rock masses have suggested that crustal earthquakes are controlled by frictional processes and their related stress systems. 

An important result of these experiments, formulated in the 

late 1970's as Byerlee's law, is that crustal rocks have nearly 

identical coefficients of friction, 0. 85 at low confining pres-

>-

<( 100 0 

a: 

UJ 

a_ 50 (/) 

Iz 

UJ 

> 0 UJ 

sures (a11 ~ 0.2 GPa) and 0.6 at greater a 11 • (At depths greater 

than ~ 15 km in tectonically active areas other than subduction zones, deformation passes from the brittle-failure regime 

into temperature-dependent, ductile and plastic regimes; the 

deformation at these depths is largely, if not wholly, 

aseismic.) 

Verifying the applicability of the laboratory friction 

results to the real Earth is a different and difficult problem. If 

we knew the crustal deformation field and its history in some 

detail and if we knew the appropriate rheologic laws for the 

long-term stress-strain behavior of crustal rocks, we could 

simply calculate the stress field. Neither supposition holds 

sufficiently well for this to be a fruitful exercise. The results 

from the Rocky Mountain Arsenal and Rangely, however, 

clearly suggest the applicability of the frictional model of 

faulting, at least to shallow depths (~ 5 km); these experiments have not, unfortunately, resolved the problem of shearstress magnitudes. That is, the driving tectonic stresses can be 

arbitrarily small if the local fluid pressure is arbitrarily close 

to the lithostatic pressure of the overlying rock. Geologic 

investigations of remanent frictional heating, expressed in 

anomalous, geometrically controlled metamorphic gradients 

and patterns of argon retention, are promising ways to infer 

shear-stress magnitudes, but such investigations are not 

likely to provide a large areal sampling. On the other hand, 

heat-flow measurements in California argue against significant shear heating (frictional stresses ;;::: 10 MPa) along the 

San Andreas fault or any of its principal branches. 

3000 

(/) 

1-

z 

UJ 

> 

UJ 

2000 0 

a: (/) 

a: Cumulative number of events UJ 

II) 

~ 140 UJ 

~ 

~ 

..... 1 120 UJ 

> 

UJ 

__J 

UJ 

~ 100 __J 

Full pond 

Pumping ....... ·· 

..... ···········started .l ........ / Lak~ le~el.· ..... . 

1 0 20 30 1 0 20 30 10 20 30 

NOVEMBER DECEMBER I 

1977 

JANUARY 

10 20 30 10 20 30 

FEBRUARY MARCH 

1978 

~ 

::J 

z 

LlJ 

1000 2: I10 

APRIL 

<( 

__J 

::J 

~ 

::J 

u 

Figure 17. Number of earthquakes per day, cumulative number of earthquakes, and water level in the Monticello Reservoir, 

South Carolina, as functions of time. Note near-coincidence of onset times of reservoir filling and enhanced rate of seismic 

activity. From J.B. Fletcher, "A Comparison Between the Tectonic Stress Measured in Situ and Stress Parameters from 

Induced Seismicity at Monticello Reservoir, South Carolina" (Journal of Geophysical Research, v. 87, p. 6931--6944), 1982. 

24 The National Earthquake Hazards Reduction Program-Scientific Status 
In these circumstances, the application of hydrofracture techniques to determine, directly and in situ, the principal stress magnitudes and orientations as a function of 

depth in tectonically active areas has been a welcome 

development. Since the inception of NEHRP, stress measurements have been made at Monticello, South Carolina, as 

discussed in the previous section, to illuminate the nature of 

shallow reservoir-induced seismicity at this locale; at 

Auburn, New York, to determine the proximity to shear 

failure and the earthquake potential in -this largely aseismic 

upstate area; and along the Wasatch front, Utah, and at the 

Nevada Test Site to measure stress at depth in these Basin and 

Range settings. The Nevada Test Site data suggest that, at 

least to 1. 3-km depth, this region is metastable with respect to 

normal faulting, if the failure process is that of the frictional 

model. 

The most complete and telling data in this regard come 

from the Mojave Desert, adjacent to a section of the San 

Andreas fault which ruptured in the 1857 earthquake. The 29 

measurements summarized in figure 18 were taken in a series 

of drill holes at distances between 2 and 34 km from the fault, 

to depths of 0. 85 km. The depth coefficient of 7. 86 MPa/km 

of the fitted straight line corresponds to a coefficient of 

0 

1 1 4 

2 5 5 

1 2 H 4 0.2 1 H 2 5 

H X 

Txy = 0.90 MPa+ 7.86 (MPa/km)z (/) 

a: H X w 

..... 

w 0.4 ~ 

0 

...J H ~ 

~ H H 

X I. 0.6 ..... 

~ 

w Symbol Well Distance (km) 0 X 

1 MOJ1 2 

2 MOJ2 4 X 

0.8 X XTLR 4 X 

L L. Hughes 4 

4 MOJ4 22 X 

H HI Vista 32 

5 MOJ5 34 

1 . 0 .____ _ _.___--'-----'----'----'------'-------'-----'-------"--' 

0 2 4 6 8 

Txy, IN MEGA PASCALS 

Figure 18. Horizontal shear stress as a function of depth 

near Palmdale, California; distances are normal to the San 

Andreas fault. From A. McGarr, M.D. Zoback, and T.C. 

Hanks, "Implications of an Elastic Analysis of in Situ Stress 

Measurements near the San Andreas Fault" (Journal of 

Geophysical Research, v. 87, p. 7797-7806), 1982. 

friction of 0.45, with the assumption of a hydrostatic fluid 

pressure. This value is somewhat low in comparison with that 

calculated from Byerlee's law, but it is nevertheless consistent 

with frictional coefficients obtained for gouge taken directly 

from the San Andreas fault zone. 

Extrapolation of the results in figure 18 through the 

seismogenic depth(=: 15 km) yields shear stresses at the 0.1-

GPa level. Even the average stress of 56 MPa for the upper 14 

km exceeds the shear stresses permitted by the heat-flow data, 

by a factor of 3 or more. Either some mechanism acts at depth 

to distribute heat far more efficiently than thermal conduction 

alone (presumably, water circulation), or shear stresses on the 

San Andreas fault must level off quickly at depths below 1 

km. Stress measurements as a whole set, however, are not 

known for such behavior; observations at even the deepest 

levels, in excess of 3 km, fail to show any suggestion of 

decrease in depth gradient. 

Even in the absence of knowledge of stress magnitudes, 

the orientations of the principal stress axes contain much 

useful information on the contemporary forces acting on the 

Earth's crust. Figure 19 shows a recent compilation of such 

data for North America. The observations consist of alignments of geologically young (~ 2 million years) volcanic 

dikes and cinder cones, fault-slip observations, earthquake 

focal mechanisms, and wellbore breakouts. The general coincidence between coherent stress domains and well-defined 

heat-flow provinces suggests an underlying dynamic process 

(convection) beneath the brittle crustal layer. On a larger 

scale, the relatively uniform, northeast-directed compression 

observed in the central and southern United States contrasts 

markedly with the complex pattern of stress directions in the 

western United States, almost certainly the result of the 

Pacific-North American plate interactions in the latter region. 

Figure 19, then, is also the embryo of a tectonic regionalization of the United States, containing especially valuable 

information for earthquake hazards and risk assessments in 

the relatively aseismic eastern and central United States. 

Earthquakes in the "eastern" United States 

Evaluating seismic hazards and earthquake potential in 

the United States east of the Rocky Mountains is complicated 

by the relative infrequency of potentially damaging and 

destructive earthquakes and by the general absence of the 

associations between seismicity and geologic structures with 

surface expression that are so clear in the western United 

States. It is also known that ground-motion attenuation with 

distance is far less in the East than in the West, and there is yet 

concern that earthquake source parameters, at the same M0 or 

M, may differ in the two regions as well. The hazards and 

risks from earthquakes in the eastern United States are nonetheless real for these difficulties: damaging and destructive 

earthquakes have occurred widely through this large region 

(fig. 2); population densities are high, especially along the 

Specific Accomplishments 25 
eastern seaboard; many of the major population centers have 

heavy concentrations of structures that are ancient and dangerous by modem standards; and most of the Nation's nuclear 

reactors are sited here as well. 

The cause-or causes-of the diffuse, low-level seismicity in the eastern regions is not understood. As mentioned 

above in the subsection entitled "Structural and Tectonic 

Setting," an important recent development in the understanding of seismicity of the eastern United States has been the 

accumulating evidence that much of it is associated with 

ancient zones of weakness, reactivated by modem stress 

regimes. It is now thought, for example, that earthquakes 

along the Atlantic Coastal Plain are associated with boundary 

grabens formed upon opening of the Atlantic during Triassic 

time. The problem is that little is known about the underlying 

causes of the "modem stress regimes." It can be said that the 

seismic deformation rates are very low and that, by consequence, elastic-strain energy need be delivered to the eastem region at correspondingly low rates, in comparison with 

the western region say, but this observation almost certainly 

means that the problem will be harder to solve, not easier. 

Nevertheless, if a correlation can be established between sites 

of ancient tectonic activity and modem seismicity, an evaluation of seismic risk in intraplate regions can be made on a 

more fundamental basis than from historical seismicity alone. 

Because nearly all of these ancient structures lie at 

depth beneath more youthful geologic strata, defining their 

nature and geometry relies heavily on expensive and timeconsuming deep-sounding techniques. Perhaps the most 

complete investigation conducted so far in the eastern region 

has been in the New Madrid seismic zone, the site of the 

earthquakes of 1811 and 1812. There, aeromagnetic, gravity, 

seismic-reflection, and seismic-refraction data have outlined 

a northeast-trending crustal rift zone at depths of 5 km and 

greater beneath the modem surface of Arkansas, Missouri, 

Kentucky and Tennessee. This rift, thought to have been first 

Figure 19. Orientations of the maximum principal horizontal stresses for the North American plate. Compiled by M.L. 

Zoback and M.E. Schiltz, from data of M.L. Zoback, M.D. Zoback, and M.E. Schiltz, "Index of Stress Data for the North 

American and Parts of the Pacific Plate" (U.S. Geological Survey Open-File Report 84-157), 1984. 

26 The National Earthquake Hazards Reduction Program-Scientific Status 
activated in late Precambrian and (or) early Paleozoic time, is 

outlined by the heavy dark lines in figure 20, which also 

shows the contemporary seismicity to be closely associated 

with faults within the rift zone; these faults, inferred from 

seismic-reflection data, cut rocks of Paleozoic and Tertiary 

age. Present -day seismicity and faulting are the result of a 

WSW-ENE compressive stress field. Recurrent uplift of the 

Tiptonville dome and Ridgely ridge (northern and southern 

shaded areas, respectively, of figure 20) is apparently driven 

by faulting at greater depths, because basement highs underlie each. Along the Reelfoot scarp, which forms the east 

boundary of the Tiptonville dome, trenching reveals recent 

offset along a set of normal faults that is probably continuous 

with faults at depth. 

Multidisciplinary investigations such as these (another 

is ongoing in the meizoseismal area of the 1886 Charleston, 

South Carolina, earthquake) have had an enormous impact on 

the assessment of the causes, effects, and attendant risks of 

earthquakes in the eastern United States. Mostly out of sheer 

ignorance as to why and where major earthquakes might 

occur in this region, it has ordinarily been necessary to 

assume fairly general and conservative (but almost certainly 

erroneous) propositions-for example, that such events 

could occur anywhere with comparable (albeit low) probability. The studies of the New Madrid seismic zone, however, 

reveal that the earthquakes are there for a reason, namely, the 

tectonic activity of a major, identifiable geologic structure. 

Such investigations, then, which are only beginning in the 

so.5• so• 89.

5o o 

37°r-------------r-----------------~r-----------------~~---------.------~8~9~-----------, 

36° 

• 

MISSOURI 

----------------

1 • 

• 

'-~ I 

) 

• 

• 

• 

•• 

• 

K E N T U C K Y 

EXPLANATION 

.O<M<1 

e1<M<2 

e 2< M < 3 

.3< M <4 

20 KILOMETERS 

Figure 20. Seismicity and faulting in the New Madrid seismic zone. Dots, earthquake locations for the year 1979. Line 

segments are faults, with sense of motion indicated by opposed arrows and (or) U (up) and D (down). Bold lines, 

approximate boundaries of late Precambrian and (or) early Paleozoic rift zone; shading, currently elevated areas. From M.D. 

Zoback and M.L. Zoback, "State of Stress and Intraplate Earthquakes in the United States" (Science, v. 213, p. 96-104), 1981. 

Specific Accomplishments 27 
eastern region, have the potential of yielding vastly more 

accurate assessments of earthquake occurrence, hazards, and 

risk than have heretofore been possible. 

Another important development, of significance to 

strong-ground-motion estimation in the eastern region, is the 

evidence from recently obtained, close-in seismologic 

recordings that eastern earthquakes (Arkansas, New 

Brunswick [Canada], New Hampshire, and South Carolina) 

are indistinguishable from California earthquakes at comparable M0 or M. Figure 21 shows the Franklin Falls Dam 

accelerogram of the largest earthquake instrumentally 

recorded at close distance (R:::::; 10 km) in the United States 

east of the Rocky Mountains, the Gaza, New Hampshire, 

-----~-..y.{o\~'/'11';,·,.,...,,'-l'o.r_~-------

------·--·----·-··-···---------··-

Figure 21. Top, Franklin Falls Dam accelerogram ofthe 1982 

Gaza, New Hampshire, earthquake, with a peak acceleration of approximately 0.3 g. Bottom, Oroville Medical Center accelerogram of the Oroville aftershock of 0350 p.s.t. 

August 6, 1975, with a peak acceleration of approximately 

0.5 g. On both accelerograms, the vertical component of 

motion is the center trace, with two orthogonal components of horizontal motion above and below it; bottom 

trace is a sequence of half-second time marks. Data of U.S. 

Geological Survey. 

earthquake of January 18, 1982 whose magnitude was 4.5. 

Also shown in figure 21 is an accelerogram of an aftershock 

(0350 G.m.t. Aug. 6, 1975; M = 4.8) ofthe Oroville, California, earthquake, written at much the same distance. Allowing 

for the fact that the Oroville aftershock was a slightly larger 

earthquake than the New Hampshire shock and thus wrote a 

slightly larger amplitude accelerogram at a fixed, close distance, these two accelerograms are very similar in overall 

amplitude, frequency content, and duration. This, in tum, 

implies that these two earthquakes share very similar overall 

dynamic properties. 

A more quantitative illustration of this similarity 

between other U.S. and California earthquakes is shown in 

figure 22. Here, seismic moments, source dimensions, and 

stress drops are given for six sets of earthquakes: the Arkansas swarm beginning on January 12, 1982; aftershocks of the 

New Brunswick earthquake of January 9, 1982; and members 

of the set of induced earthquakes at the Monticello Reservoir, 

South Carolina. The California earthquakes were recorded as 

aftershocks of the 197 5 Oroville and 1980 Mammoth Lakes 

earthquakes, and near Anza along the San Jacinto fault zone. 

Above M 0 = 2 X 1019 dyne-em (or M ~ 2.2), all these events 

have essentially constant stress drops (1-10 MPa). More 

importantly, there is no way of distinguishing one set of 

earthquakes from another on the basis of stress drop. This is 

an important result because the constancy of earthquake 

stress differences figures prominently in high-frequency 

strong-ground-motion scaling relations, a subject we take up 

in the next section. 

Strong ground motion 

Strong ground motion is the vehicle by which potential 

earthquake hazards become real ones, irrespective of 

whether the dynamically driven failure affects geologic structures, resulting in faulting or ground failure of a variety of 

types, or structures fabricated by man, resulting in some sort 

of damage, functional loss, or catastrophic collapse. Strongground-motion estimation thus plays a central role in earthquake-hazards reduction in general and is an essential ingredient of the earthquake-resistant design of critical structures 

in particular. 

Although strong-motion seismology "began" 50 years 

ago, with the writing of four strong-motion accelerograms in 

the Los Angeles Basin by the 1933 Long Beach earthquake, 

strong-ground-motion-estimation studies have always been 

plagued by the uncertainties of having no observational constraints on how strong strong ground motion can really be. 

Although close-in records obtained for the 1940 Imperial 

Valley, 1966 Parkfield, 1971 San Fernando, and 1979 Imperial 

Valley earthquakes have been important milestones, they also 

set an ominous trend, as peak accelerations increased with 

each successive earthquake to 1. 75 g for the recent Imperial 

Valley earthquake. Even at the present time, there are very few 

records at close distances for major earthquakes (M ~ 7). 

28 The National Earthquake Hazards Reduction Program-Scientific Status 
Since the San Fernando earthquake, however, and 

especially since 197 5, great progress has been made in the 

field deployment of strong-motion accelerographs, both 

within the United States and abroad, commonly in the form of 

special arrays of closely sited instruments. Equally important 

1000 

1023 

1022 

1021 

1018 

FREQUENCY, IN HERTZ 

100 10 

New Brunswick 

1982 

New Hampshire 

1982 

0.1 0.01 0.001 

EXPLANATION 

.. Arkansas 

• South Carolina 

• New Brunswick 

o Mammoth 

6 Oroville 

o Anza 

SOURCE RADIUS, IN METERS 

Figure 22. Seismic moments (ordinate), source dimensions 

(abscissa), and stress drops (labeled lines of slope + 3) for 

six sets of earthquakes identified in lower right. There is no 

evidence for a population of eastern U.S. earthquakes 

(solid symbols) distinct from California earthquakes (open 

symbols). Modified, with addition of the New Brunswick 

and New Hampshire earthquakes, from L.C. Haar, j.B. 

Fletcher, and C.S. Mueller, "The 1982 Enola, Arkansas, 

Swarm and Scaling of Ground Motion in the Eastern United 

States" (Seismological Society of America Bulletin, v. 74, p. 

2463-2482), 1984. 

have been developments in data-processing schemes, including laser-scanning and photodensitometer digitizing devices, 

whereby the increasingly larger data sets can be rapidly 

processed and made widely available. Finally, much progress 

has been made in the realistic seismologic modeling of strong 

ground motion, through the application of new source-modeling and wave-propagation techniques and better crustalstructure control. 

All these developments came together at the time of the 

1979 Imperial Valley, California, earthquake. By then, the 

seismicity, tectonics, and structure of the Imperial Valley had 

been intensively studied, largely as a result of interest in the 

geothermal potential of the region. An extensive seismicrefraction experiment to refine the crustal structure of the 

Imperial Valley had been undertaken in January-March 1979, 

involving 40 detonations at seven shotpoints and 1,300 different recording locations. Finally, by 1979, the Imperial 

Valley had been densely instrumented with strong-motion 

accelerographs on both sides of the United States-Mexican 

Border, as a result of activities of the California Division of 

Mines and Geology, the California Institute of Technology, 

the U.S. Geological Survey, the Universidad Nacional 

Aut6noma de Mexico, and the University of California, San 

Diego. 

Figure 23 shows transverse components of ground 

velocity obtained from 19 strong-motion accelerographs 

located in the immediate epicentral region of the earthquake. 

A total of 44 strong motion accelerographs recorded this 

earthquake in the United States, and 9 more accelerograms 

were written just south of the border in Baja California, 

Mexico. Of special seismologic interest are the record 

obtained from the 13-station El Centro array; the earthqll 

rupture passed between stations E07 and E06. About Km 

northeast of station EIO, a remarkable set of records (not 

shown here) was obtained from the Imperial County Services 

Building, four reinforced-concrete columns of which partially collapsed during the earthquake. The 13 accelerograms 

written in this structure are of enormous engineering significance, in part because this is the first well-instrumented 

building to sustain significant earthquake damage, and in part 

because the timing and mechanism of structural damage can 

be inferred from the accelerograms. 

Figure 24 (top) compares model calculations with the 

ground velocities recorded at stations E05, E06, E07, and 

E08. The model calculations take into account: (1) the finite 

area of the causative faults, both the Imperial (35 by 13 km) 

and the Brawley (10 by 8 km) faults; (2) the spatial variations 

of slip rate, rise time, rupture velocity, and orientation of the 

propagating rupture; and (3) the crustal structure of the area, 

as determined by refraction surveys, seismicity studies, and 

shallow drill logs. Figure 24 tells us that strong ground 

motion over a fairly wide frequency band can be realistically 

and deterministically calculated in advance for any source/ 

site pair of interest, provided we can be specific in describing 

the source mechanism, crustal structure, and local site condiSpecific Accomplishments 29 
tions. Figure 24 (bottom) illustrates components of displacement on the fault surface, determined from an inversion of the 

data in figure 23. Most of the faulting for this earthquake 

occurred at depths below 5 km. 

At higher frequencies, important advances have been 

realized through the recognition that ground acceleration 

time histories closely approximate finite-duration, band-limited, white Gaussian noise during the strongest motion associated with the direct shear arrivals. The root-mean-square 

ground acceleration, arms, can be associated with a dynamic 

stress drop that is remarkably constant, at least for California 

earthquakes. In figure 25, this approach is used to model a 

large set of M 0 -M L data for California earthquakes. The 

I 

100 

50 

model calculations predict a value of M L for any value of M 0 , 

given the above prescription for the shear waves and an arms 

stress drop of 10 MPa for all events. Although the data 

themselves exhibit considerable scatter, much of which can 

be related to nonsource variables, on average it appears that 

the arms stress drop of 100 bars is a stable and entire feature of 

California earthquakes, from the smallest that can be 

recorded in normal circumstances to the largest that have 

occurred. At present, this approach appears to provide a firm 

theoretical basis for calculating all manner of ground-motion 

amplitudes, including not only Wood-Anderson amplitudes 

(fig. 25) but also peak accelerations, peak velocities, and 

response-spectral ordinates, at least for M = 6\12. For larger 

50 ' 

25 30 

-50 

10 15 20 

TI ME. IN SECONDS 6 6 \ 1\ -100 7~1 ~~ 

40 ' 

IJN \T E D ST ATF.S 

----- -M-EXI-co-

:AI ~ /',"" ~~ 

I /', "" l \ 

/', "'"~~~~- \ ------- 6cxa ~~ -~----

- - - - - ( T\ --- - w 

5 K ILOMETERS 

Figure 23. Ground velocity in the 230° direction, an azimuth approximately perpendicular to the Imperial fault, excited by 

the 1979 Imperial Valley earthquake. Most of the Imperial fault in this area and all of the Brawley fault (north-south fault 

segment in middle of figure) underwent surface rupture during this earthquake. Earthquake rupture initiated at location 

marked by circled cross, at a depth of 8 km, and propagated to the north. From R.j. Archuleta, "Analysis of Near-Source Static 

and Dynamic Measurements from the 1979 Imperial Valley Earthquake" (Seismological Society of America Bulletin, v. 72, p. 

1927-1956), 1982. 

30 The National Earthquake Hazards Reduction Program-Scientific Status 
Vl 

a: 

UJ 

fUJ 

~ 

0 

_J 

~ 

~ 

I . 

fa.. 

UJ 

0 

M, some uncertainty yet remains, partly because the observational constraints are far fewer and partly because of the 

nature of large to great earthquakes, for which one faulting 

dimension (length) becomes progressively greater than its 

width. 

COMPONENTS 

323° 53° DOWN 

0 10 20 30 0 10 20 30 0 10 20 30 r --, 

E05 ~ ~ 

~00 'f ~ * -0.5 

J E06 ~~ l~ ~ 

~ ~ 

o.st E07 ~ 

~0 ~ ~F ~ I -0.5 

l EOB ~ 

~~ ~ 

~ 

~ 

10 20 30 0 10 20 30 0 10 20 30 

T l ME lSI T !ME !5 1 T J ME !51 

STRIKE - SLIP FINAL OFFSET , IN METERS 

0 

0 - M-US 5 30 35 

10 

13 

DIP- SLIP FINAL OFFSET , IN METERS 

0 

10 

13 

0 10 15 20 2 5 30 35 

DISTANCE. IN KILOMETERS 

Figure 24. Top, observed and synthetic ground-velocity records of the 1979 Imperial Valley earthquake for three components of motion at sites E05, E06, E07, and E08 (fig. 23). 

Bottom, di stribution of offset on the Imperial fault, 

obtained from an inversion of all the records in figure 23. 

From R.j. Archuleta, "A Faulting Model for the 19791mperial 

Valley Earthquake" (Journal of Geophysical Research , v. 89, 

p. 4559-4585), 1984. 

Finally, as illustrated in the next section, important 

advances have been realized in the quantitative analysis of 

intensity data, assessments of shaking and damage that are 

vastly more numerous than instrumental measurements. For 

earthquakes antedating the relatively short instrumental era, 

such observations are the only direct information on earthquake effects (or even their occurrence). Moreover, intensity 

30 

28 

119061 + 

• I 

26 I 

• -rv fz 

UJ 

~ + -'1 0 24 ~ I u. +Jjf* 0 

~ 

+ I 

t: 22 a: 

<{ 

(.9 

0 -'~~ ~ +if .....J 

20 

18 

Figure 25. Moment-magnitude (M0-ML) data for central California earthquakes (crosses; box, 1906 San Francisco 

earthquake), and model calculations (dots) of ML given M0 , 

assuming that the maximum amplitude determining ML can 

be calculated on the basis that the shear-wave group at 

close distances is finite-duration 1 band-limited I white 

Gau ss ian noise. Dashed line represents a correction 

accounting for a slight discrepancy between the M L's determined at close and far distances. From T.C. Hanks and D.M. 

Boore, "Moment-Magnitude Relations in Theory and Practice" (Journal of Geophysical Research , v. 891 p. 6229-6235) 1 

1984. 

Specific Accomplishments 31 
scales are closely tied to damage sustained by particular 

classes of structures. This association permits "damage functions" to be calculated for these structures, and so dollar 

losses can be estimated for some impending earthquake of 

specified size and location, given the building inventory 

within the area of strong ground shaking. 

Calculation of risk 

In essence, earthquake-risk analysis is the calculation 

of the probability that something bad will happen as the result 

of one or more earthquakes during a specified time interval. 

In principle, that "something" can be almost anything- for 

example, structural damage, dam failure and flooding, or 

some form of ground failure- but risk calculations are fundamentally keyed to the probability that some level of ground 

motion G-a peak acceleration value, say, or some level of 

modified Mercalli intensity- will be exceeded (or not be 

exceeded) per unit time. For either a specific site or an entire 

region, then, one must integrate over all possible locations 

and sizes of earthquakes that can produce G or greater within 

the time interval of interest. Figure 26 shows such a "risk 

map" for the 48 coterminous United States, here expressed as 

the horizontal ground velocity on hard rock (resulting from 

any earthquake) that, with a probability of 90 percent, will 

ilC.' 

< 2 

not be exceeded in 50 years. Such risk maps play an important 

role in fundamental design decisions, which must reckon the 

difficult compromises between a larger initial investment (for 

a more costly, more highly resistant structure) and the possibility of a larger economic loss some time later (to a less 

resistant structure). Such decisions, of course, are usually 

formalized in general-purpose building codes and in specific 

regulations for the construction of special-purpose structures, such as schools in California and nuclear reactors 

nationwide; risk maps such as figure 26 figure prominently in 

the rational development of these codes and regulations. 

These probability-of-exceedance calculations (of some G, at 

some place, for some time interval), moreover, form the basis 

for anticipated loss estimates, given that some relation 

between G and the amount of damage to a particular structural form (say, wood-frame construction) is known. Such 

estimates of the replacement costs due to structural damage 

for repeats of the 1836 Hayward, 1857 Fort Tejon, 1906 San 

Francisco, and 1933 Long Beach, California, earthquakes 

have recently been calculated. 

The calculations themselves, however, are a complex 

functional of various geophysical, geologic, and seismologic 

relations. These include G expressed as a function of M and 

distance, definitions of seismic-source zones and capable 

~~· 10" 55' 

' 2 

1 DO 200 300 MILES 

1.\1 < 2 \ 

I I I 1

1 I I 

0 1 DO 200 300 400 KILOMETERS 

Figure 26. Horizontal velocities (in centimeters per second) on rock sites that have a 90-percent probability of not being 

exceeded in 50 years for the 48 coterminous United States. Plate 5 of S.T. Algermissen, D.M. Perkins, P.C. Thenhaus, 

S.L.Hanson, and B.L. Bender, "Probabilistic Estimates of Maximum Acceleration and Velocity in Rock in the Contiguous 

United States" (U.S. Geological Survey Open-File Report 82-1033), 1982. 

32 The National Earthquake Hazards Reduction Program-Scientific Status 
faults, ground-motion-amplification factors for various geologic materials, and the local or regional earthquake-size/ 

frequency-of-occurrence relations. 

Any developments, several of which have been presented earlier, that lead to refinement of these relations 

plainly allow more accurate risk and loss estimates. The 

development of the characteristic-earthquake model has had 

especially important implications for the calculation of risk. 

The size-distribution/frequency-of-occurrence relation for 

earthquakes is empirically known to be of the form log 

N =a- bM, where N is the number of earthquakes of magnitude ?: M within a given region and time interval. This 

relation permits an arbitrarily large earthquake to occur 

arbitrarily infrequently, a possibility that is implausible geologically but one that has been difficult to rule out in view of 

the long period of time involved. As discussed in the subsection above entitled "Inhomogeneous Faulting," however, 

there is a growing body of evidence that M or M 0 of the 

maximum-size earthquake can be defined in terms of the 

coherent length of any potential causative fault. With the 

growing body of slip-rate data that are now becoming available for these same faults, as discussed above in the subsections entitled "Crustal Deformation" and "Late Quaternary 

Faulting Geology," we can estimate the times of future earthquakes, if the date of the last maximum-magnitude event is 

known. Figure 27 summarizes such predictions for California 

in terms of their 30-year cumulative probability of occurrence. 

It is straightforward to translate the information in 

figure 27 into ground-shaking probabilities for a fixed period 

of time, say, 30 years. We might calculate intensities directly 

and, from them, expectable losses; or we could calculate the 

distribution of ground-motion parameters about these source 

regions. Figure 28 presents such calculations for the islands 

of Honshu and Shikoku, Japan, here expressed as the probabilities of being shaken with JMA (Japan Meteorological 

Agency) intensity V for 20-, 50-, 100-, and 200-year intervals. The intensity distributions in figure 28 were obtained 

from recently developed relations between areal distributions 

of specific intensity levels and M0 , summed over all possible 

source regions on and near Honshu and Shikoku. 

Although figures 26 through 28 reflect very recent 

developments and can be expected to be improved upon 

significantly in the coming years as better and more complete 

scientific information becomes available, they are, as statements of potential earthquake hazards at regional scales, 

essentially end products of the scientific enterprise. It is 

virtually certain that, in at least some areas of figures 26 

through 28, potential hazards will become real during the 

next few decades. Indeed, the damaging earthquake of September 13, 1984, on the island of Honshu occurred almost at 

the center of the 90- to I 00- percent probability-of-occurrence 

distribution in figure 28(a). The mitigation of these impending threats, however, must be achieved in other arenas, in 

engineering-design decisions, land-use planning, and earthquake preparedness at several political levels. Such decisions, planning, and preparedness nevertheless depend 

heavily on realistic assessments of earthquake hazards on 

smaller spatial dimensions. 

Seismic zonation 

Seismic zonation is the identification and evaluation of 

potential earthquake hazards on local scales. Primary hazards 

are those related to fault rupture at the Earth's surface and to 

strong ground motion. These phenomena, themselves known 

to vary spatially, can also effect a variety of secondary hazards-those related to ground failure (landslides and liquefaction) and inundation potential (tsunami run-up and flooding due to dam failures), for example. The potential 

occurrence and severity of these hazards, as well, can vary 

rapidly from one place to another. 

In all such studies, the primary consideration is accurate mapping and determination of the rates of activity of the 

faults themselves, because these matters determine where the 

earthquakes will occur, how big they will be when they occur, 

how frequently such events will occur, and the distances to 

which any particular effect-at some fixed source strengthcan occur. In addition, it is obvious enough-now-that 

manmade structures astride active crustal fault zones constitute especially dangerous situations. In California, this 

danger is recognized in the Alquist-Priolo Special Studies 

Zones Act of 1972, which requires special scientific investigations to be undertaken before building structures for 

human occupancy in the immediate vicinity of potentially 

active faults. These Special Studies Zones are detailed on 

official maps, with USGS 7 '12-minute topographic quadrangle maps as the base. 

Comprehensive seismic-zonation studies in the United 

States began with a study of potential earthquake hazards in 

the 7 ,400-square-mile, nine-county San Francisco Bay 

region. These studies were first brought together in a single 

volume in 1975, which included contributions on faults and 

future earthquakes, bedrock-ground-motion estimation, discrimination of surficial sedimentary deposits, seismic 

response of local geologic units, liquefaction potential, landslide potential, and the predicted geologic effects of an 

M = 6.5 earthquake on the San Andreas fault, centered 

behind Stanford University. From these results and those of 

more recent studies, figure 29 is drawn from digital data 

banks available to all local governments in the nine-county 

bay region. It shows the severity of ground shaking, measured 

in terms of the intensity scale describing the 1906 San Francisco earthquake, that may be expected as the result of major 

earthquakes along the principal faults in the Bay region. 

Just as in the case of earthquake-risk estimation, seismic-zonation maps are only as good as the geologic, 

geophysical, and seismologic observations and techniques on 

which they are based. Since 1975, such investigations have 

become much more sophisticated, and their scope has broadSpecific Accomplishments 33 
J70 

ened considerably, almost entirely as a result of advances 

since the inception of NEHRP. Many of these advances have 

been recounted earlier, but in addition to them, considerable 

progress has been made in the identification of liquefiable 

deposits and in the estimation of local ground-motion-amplification factors as a function of the material properties of nearsurface sedimentary deposits. Thus, a nearly completed seisSan Francisco 

Peninsula, M= 7 

North } 

South 

120° 

Hayward, M=6.5-7 

mic-zonation study of the Los Angeles metropolitan area 

includes 14 contributions and some 1,200 manuscript pages, 

with very detailed predictions of various geologic effects and 

estimates of attendant economic losses. Essential studies for 

similarly comprehensive volumes are now being conducted 

in the regions in and around Anchorage, Alaska; Reno, 

Nevada; Salt Lake City, Utah; and Seattle, Washington. 

EXPLANATION 

CUMULATIVE 30-YEAR PROBABILITY 

•••• 2: 60 percent 

E=::=3 2: 40 percent 

~== 2: 20 percent 

2: 1 0 percent 

---- 2: 5 percent 

All other faults < 5 percent 

USGS hazard watch 

Mammoth-Mono Lakes 

San Juan Bautista, M =6.5 

100 200 KILOMETERS 

' ' ' ', Creeping, M= 7(?) 

' ' ' ' ' \ 

Carrizo, M=B 

\ 

\ 

\ 

\ 

I 

Figure 27. 30-year cumulative probabilities of occurrence of earthquakes along selected fault segments of the San Andreas fau 

system. Reconstructed from A.G. Lindh, "Preliminary Assessment of Long-Term Probabilities for Large Earthquakes Alon 

Selected Fault Segments of the San Andreas Fault System in California" (U.S. Geological Survey Open-File Report 83--63), 198

34 The National Earthquake Hazards Reduction Program-Scientific Status 
EXPLANATION 

PROBABILITY 

1111 0.9-1.00 

++$+ 0.8-0.89 

~~~~ 0.5 - 0. 79 

xxxx 

~~~~ 0.1 -0.49 xxxx 

++++ 

++++ <0. 1 ++++ 0 <0.01 

20 years 

(b) 50 years 

(c) 1 00 years 

Figure 28. Probabilities that either interplate or intraplate earthquakes will give rise to ground shaking of JMA intensity ~ V 

on the islands of Honshu and Shikoku , Japan , during the next 20 (a), 50 (b), 100 (c), and 200 (d) years. From S.C. Wesnousky, 

C. H. Scholz, K. Shimizaki, and T. Matsuda, " Integration of Geological and Seismological Data for the Analysis of Seismic 

Hazard: A Case Study of Japan'' (Seismological Society of America Bulletin, v. 74, p. 687-708), 1984. 

Specific Accomplishments 35 
Although seismic-zonation techniques are most generally applicable to the development of land-use policies, they 

are most intensively used in the siting of nuclear reactors. At 

the site itself, considerable effort is directed to searching for 

recently active faults and to determining local groundmotion-amplification factors. Consideration of the regional 

geology, seismology, and tectonics is used to define two 

levels of earthquake excitation, the Operating-Basis earthquake and the Safe-Shutdown earthquake, to which the plant 

must be designed. Inadequate or incomplete investigations in 

advance of construction can be far more costly than the 

studies themselves, as has been the case for the Diablo 

Figure 29. Maximum intensity of ground shaking expected for any earthquake in the nine-county San Francisco Bay region, 

according to the Bay Area Special Information System (BASIS). 

36 The National Earthquake Hazards Reduction Program-Scientific Status 
Canyon, California, nuclear reactors. In this situation, the 

discovery of the Hosgri fault just offshore of Diablo Canyon, 

as a result of commercial oil-exploration studies, led to 

enormous expenditures and delays, in rationalizing the construction as designed previous to this discovery and in retrofitting the structure to higher levels of earthquake resistance. 

SCIENTIFIC PROGRESS AND PROGRAM 

PERSPECTIVES 

The application of the scientific results discussed 

herein, as well as the many others that were not, to the 

meaningful reduction of earthquake hazards is a subject 

beyond the scope of this report. As intimated in the introduction, such an enterprise is much larger than the business of 

research science alone, with or without the equally important 

advances achieved in engineering disciplines not considered 

here. Even so, it would be by and large an empty exercise to 

conclude that NEHRP enjoys excellent scientific health without connecting, however briefly, the scientific accomplishments of the past 7 years to the objectives of the program. 

Sure evidence that the Nation is progressing steadily 

toward the objectives of NEHRP is the widespread popular 

interest in matters earthquake, and much of this is attributable 

to the visibility of NEHRP itself. At least by the standards of 

pre-San Fernando earthquake times, this evidence is almost 

overwhelming. It exists in the form of how-the-Earth-works 

television movies, almost inevitably containing some footage 

on earthquakes; easy-to-read primers on earthquakes and 

their effects; homeowner's guides to do-it-yourself earthquake resistance; regular newspaper reporting of earthquake 

phenomena; and increasingly more readable accounts of ever 

more sophisticated concerns. Figure 30, for example, is a 

detail of the results portrayed in figure 29 for the San Francisco peninsula, a region including the subscription area of 

the Peninsula Times Tribune. Although such matters as these 

are rarely discussed in newspapers as they would be in scientific communications, in most cases this can be regarded as a 

positive development, not a negative one. There can be little 

doubt that the most certain protection against the dangers of 

earthquakes is an informed citizenry well prepared for them. 

Just as importantly, this popular interest and concern 

have made themselves felt at city, county, State, and Federal 

political and public-policy levels. In California, for example, 

two major projects in earthquake-preparedness planning are 

now underway-the Southern California Earthquake Preparedness Project (SCEPP) and the Bay Area Earthquake 

Study (BAES). These efforts are funded by FEMA and State 

monies, and the Seismic Safety Commission, Office of Emergency Services, and Division of Mines and Geology of the 

State of California all participate in them. These projects also 

avail themselves of research expertise in university and government laboratories. In both cases, the project staffs work 

with representatives of cities, counties, businesses, schools, 

and various volunteer groups to stimulate and review earthquake-preparedness procedures. Specific model plans call for 

the development of a prediction warning system, response to 

earthquake predictions, response to unpredicted earthquakes, 

and recovery guidance. 

But within the scientific context that is the basis of this 

document, how have the accomplishments recounted earlier 

led to progress toward the scientific objectives of NEHRP? 

We take these scientific objectives to be a full physical 

understanding of the cause and effect of earthquakes so as to 

( 1) provide a complete identification and evaluation of potential earthquake hazards and (2) predict earthquakes, in whatever study areas are of concern. 

It is commonly held, by citizens and scientists alike, 

that these matters of earthquake hazards and earthquake 

prediction are separate entities. Indeed, until quite recently, 

that part of the program administered by the USGS has 

operated on this basis, both internally and externally, since 

the inception of NEHRP. In the past several years, however, 

and as a direct result of scientific research conducted under 

NEHRP, many scientists now see this distinction in grayer 

tones. By example, the commonly used (nowadays) phrases 

"long-term prediction," "intermediate-term prediction," 

and "short-term prediction" plainly carry some implications 

about impending earthquake hazards on progressively shorter 

time scales. If we regard earthquake prediction (on whatever 

time scale) as simply a statement of impending earthquake 

hazards, we may also regard earthquake prediction as nothing 

more than one of many mitigation strategies. As such, the 

scientific essence of NEHRP is the accurate and complete 

evaluation of potential earthquake hazards and the mechanisms by which they become real. Here is a safe harbor for the 

scientists and engineers of NEHRP: it is widely agreed that 

the scientific and engineering advances achieved under 

NEHRP are now leading to-and will continue to lead tomeaningful reduction of the dangers attendant to earthquakes. 

Although this logic properly underscores both the scientific significance and practical importance of long-term 

statements of impending earthquakes and the mitigation 

strategies timed to them, it misses the special nature of 

earthquake prediction, both as a unique mitigation strategy 

and as an endlessly fascinating research area. Even in such 

modern, industrialized, and affluent societies as the United 

States, for which it may be hoped that major loss of life can be 

averted through prudent land-use policies and earthquakeresistant design and construction, it may take a century or 

more for all of the hazardous structures in seismically prone 

areas to be razed or upgraded. When allowance is made for 

any structural degradation as a function of time, it is reasonable to anticipate that there will be a finite inventory of unsafe 

structures essentially forever. Earthquake prediction is the 

last line of defense against the building that might collapse in 

the event of a nearby earthquake; an accurate short-term 

Scientific Progress and Program Perspectives 37 
prediction of that earthquake simply allows for the structure 

to be evacuated. As a special mitigation strategy, however, 

earthquake prediction only makes sense if the prediction is 

accurate within a narrow time window, measured in weeks or 

less. It is simply unrealistic to expect that all the unsafe 

structures in a major metropolitan area could be evacuated for 

a longer period of time. 

Scientifically, short-term prediction has been an elusive 

goal. Although important and exciting results have been 

reported for a few earthquakes (and incompletely documented reports exist for a number of others), earthquakes 

with definitive precursory activity on time scales of months or 

less are very much the exception, not the rule. An impressive 

string of M ~ 6 earthquakes have occurred in California 

Earthquake country 

Map predicts damage from severe temblor By Dave Fa rrell to have one like it .. 

T'mes Tribune staff The map was devised by three USGS 

A un ique map 1s available that can rei\ scient ists. Roger Borcherdt. James Gibbs and 

,-ou what kind of damage vou can expect Kenneth LaJOie. The three men took damage 

to vour house if the Pemnsula is rocked reports from the 1906 quake and projected 

b); an earthquake the same size as th e what kind of damage a similar quake would 

1906 quake mar shook ~:an Francisco. cause if it hit today. The '06 quake struck on 

The map is 50 detailed that it even the San Andreas fault. wh1ch bisects the 

will tell you what kind of c'mage your Peninsula 

neighbors across the street can expect. "We used the data from observations 

It is map MF-709. the most popular made after the (1906) quake," Borcherdt 

map ever sold by the u.s. Geological said. "We only used damage reports from 

Survey office in Men\·) Park observations that were reliable. We relied 

The map. which predicts on observations that were made of damage 

earthquake intensllies for the entire to several buildings in an area. not just one. 

Bay Area. was fiN printed in 1975 for example." 

lt has been out of print for several Using that data. the scientists developed 

,·ears. but the USGS recentl v a clear picture of the scope and magnitude 

• repnnted 11 by oopular demand. It of the 1996 quake. The quake has been 

is av ail ~ble from the USGS for esllmated at 8.25 on the R1chter scale. 

$3.75 . The amount of damage ca used in any 

g1ven area by any earthquake IS determined 

b)' three factors: the size of the quake. the 

diStance from the earthquake fault and the 

local geologic conditions 

"Boy am l glad we got 11 

back." a spokeswoman at the 

map sales Cllunter at th e USGS 

office on Middlefield Road 

>aid. "It is the most popular 

map we hrH·e. Ir is th e only 

one of its kind in rne 

countrv. The people down 

in L.A. would 101·e 

In drawing th etr map. the scientists 

assumed a "great" quake of the same size 

as the ·oo San Francisco temblor. The\· then 

calculated the distance from the major fault 

llnes lac ing the Bay Area to any place in the 

reg10n 

The third factor in the equation. the local 

geologic conditions. was provided by the 

extremely detailed maps already publ ish,•d 

bv the GSGS 

·"Once we had all the data. II onlv rook a 

cou ple of months to draw the map_-.. 

Borcherdt said 

While the map gives an tnd1carion of !he 

effects of an earthquake on a 

neighborhood-by-neighborhood basis 

throughout the Bay Area. it does not define 

what kind of damage might result. 

For example. the mud flats in the 

Peninsula's low-lying bay-shore area will be 

particularly vu lnerable to liquefaction 

during a strong quake. Liquefaction. in rough 

terms. occurs when water-saturated ground 

liqui fies and collapses during a quake. 

Other areas might be prone to just as 

much damage, but for different reasons. 

"Essentially. the map gives you an overa\1 

picture of where you'd expect the effects of 

an earthquake to be most severe," Borcherdt 

said 

Many residents have bought the map to 

g1ve tl1em an idea of what to expect during 

a big quake. he said. Using information from 

tile map. they can get more detailed 

Information about the soil conditions 10 their 

yard and around their house. They then can 

make structural changes to their hom< s to 

make them more earthquake-proof, 

Borcherdt said 

Public agencies also have used the map 

to help devise pubilc sa fety plans. San Mateo 

and Santa Clara counties have incorporated 

th e map into their seismic safety plans. 

Other c1t1es and public agencies have used 

the map to help them plan where to locate 

dumps and where to limit development. 

Borcherdt said. 

The California Bureau of Mines and 

Geology used the map to help develop a 

detailed earthquake-planning scenan o that 

predicts which road> Will be ruined in a 

quake and whiCh water and g<Js mains most 

likely w1ll erupt. 

Figure 30. Detail of figure 29, with accompanying text, as printed in the Peninsula Times Tribune, September 18,1983 (Palo 

Alto, California). 

38 The National Earthquake Hazards Reduction Program-Scientific Status 
since 1979, for example, including the 1979 Coyote Lake, 

1979 Imperial Valley, 1980 Livermore, 1980 Mammoth 

Lakes, 1983 Coalinga, and 1984 Morgan Hill earthquakes, 

but none of these have been associated with definitive shortterm precursors, even in an after-the-fact mode of operation. 

From this, we might conclude that short-term earthquake prediction is now and forever beyond our reach. More 

rationally, we may conclude that we have simply performed 

the wrong experiments in the wrong places and at the wrong 

times. And, to be critical about matters, we might say that we 

have hardly conducted any experiments at all, to the extent we 

have allowed the occurrence of the earthquake itself to drive 

subsequent analysis on observations more often than not 

recorded from afar. 

That is to say, the earthquakes we would most like to 

predict-and are most likely to predict-have not yet 

occurred, but we now know where to look, perhaps the single 

most significant accomplishment of NEHRP. In fact, an M ~ 

6 earthquake has already been predicted to occur near Parkfield within the next 5 years, and it is appropriate to execute 

experiments here with a very sharp focus. Also as a result of 

the scientific accomplishments of NEHRP, we now have a far 

better idea of what experiments to perform and how to perform them at a high degree of resolution. We know that we 

can obtain high-resolution seismologic and geodetic data 

when the appropriate instrumentation is clustered in dense 

arrays, and we also know that instruments in deep boreholes 

offer enormous advantages, both in signal-to-noise enhancement and in proximity to the source region. 

With less certainty as to timing, many important 

developments also point to impending great earthquakes 

beneath the Shumagin Islands, Alaska, and along the Fort 

Tejon segment of the San Andreas fault that broke in 1857. 

The section of the San Jacinto fault near Anza has also been 

identified as a likely candidate for a 6 ~ M ~ 7 earthquake. 

And quite apart from the possible volcanic hazards, the 

Mammoth Lakes area is the most active region in California 

with respect to the occurrence of M ~ 6 earthquakes. 

The many accomplishments of NEHRP, then, have 

brought us to the edge of earthquake prediction on short time 

scales. They clearly point to the next step of intensively 

focusing on areas specifically identified as candidates for 

significant earthquakes, with experiments requiring substantial investments in intellect and funding. Until such experiments progress to their logical conclusion, we will not know 

whether short-term earthquake prediction is feasible or not. 

In the meantime, there is much to do. 

Scientific Progress and Program Perspectives 39 
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