National Seismic System 

Science Plan 
AUTHORS' ADDRESSES 

Thomas H. Heaton 

U.S. Geological Survey 

525 S. Wilson Ave. 

Pasadena, CA 91106 

Don L. Anderson 

Division of Geological and Planetary Sciences 

California Institute of Technology 

Pasadena, CA 91125 

Walter J. Arabasz 

Dept. of Geology and Geophysics 

University of Utah 

Salt Lake City, UT 84112 

Ray Buland 

U.S. Geological Survey 

Denver Federal Center MS-967 

Denver, CO 80225 

William L. Ellsworth 

U.S. Geological Survey 

345 Middlefield Road MS-977 

Menlo Park, CA 94025 

Stephen H. Hartzell 

U.S. Geological Survey 

525 S. Wilson Ave. 

Pasadena, CA 91106 

Thome Lay 

Dept. of Geological Sciences 

The University of Michigan 

Ann Arbor, MI 48109-1063 

Paul Spudich 

U.S. Geological Survey 

345 Middlefield Road MS-977 

Menlo Park, CA 94025 
National Seismic System 

Science Plan 

By THOMAS H. HEATON, DON L. ANDERSON, 

WALTER j. ARABASZ, RAY BULAND, 

WILLIAM L. ELLSWORTH, STEPHEN H. HARTZELL, 

THORNE LAY, and PAUL SPUDICH 

U.S. GEOLOGICAL SURVEY CIRCULAR 1031 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

First printing 1989 

Second printing 1993 

Any use of trade, product, or firm names 

in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Government 

library of Congress Catalog Card Number 89-600729 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1989 

Free on application to the 

Book and Open-File Reports Sales 

U.S. Geological Survey 

Federal Center, Box 25286 

Denver, CO 80225 
CONTENTS 

Executive summary 1 

Introduction 1 

Configuration and uses of existing networks 2 

Current regional networks 2 

Uses of current regional networks 3 

Earthquake physics and hazard analysis 4 

Structure of the Earth 8 

Current strong-motion instrumentation 10 

Limitations of existing networks 15 

Configuration of proposed digital National Seismic System 17 

U.S. National Seismic Network 17 

Relationship between regional and national seismic networks 21 

Need to develop digital regional networks 22 

Applications of a National Seismic System 24 

Estimation of current earthquake risk 24 

Short-term warning of imminent ground shaking 25 

Rapid estimation of shaking intensity 27 

Tsunami warning 28 

Volcano monitoring 29 

Prediction of site effects in strong ground motions 29 

Research possibilities of three-component digital regional networks 32 

Earthquake studies 32 

Seismicity patterns 32 

Source characteristics 34 

Strong-motion attenuation 36 

Strong-motion simulation 36 

Earth structure and wave propagation 37 

Shear waves 37 

Surface waves 37 

Understanding the coda 38 

Deep structure of basins and mountains 38 

Structure of the Earth's interior 39 

Nuclear discrimination 40 

Conclusions 40 

References cited 40 

FIGURES 

1. Seismicity map of the contiguous United States 3 

2. Map of seismograph stations operated by regional networks in the contiguous United 

States 5 

3. Map of seismograph stations operated by regional networks in California 6 

4. Map of seismic activity in California and Nevada from 1980 through 1986 7 

5. Diagram showing seismic hazard analysis methodology 8 

6. Probabilities of the occurrence of major earthquakes on the San Andreas, Imperial, San 

Jacinto, and Hayward fault systems in California 9 

7. Mislocation vectors from locating distant earthquakes using regional networks 11 

8. Map of southern California crustal P-wave velocities derived from tomographic inversion 

of travel times of local earthquakes 12 · 

Contents Ill 
IV Contents 

9. Map of southern California upper-mantle P-wave velocities derived from tomographic inversion of travel times of local earthquakes 13 

10. Three-dimensional upper-mantle P-wave velocity structure beneath southern California derived from tomographic inversion of travel times from distant earthquakes 14 

11. P-wave travel times for earthquakes located at regional distances and recorded on the 

Southern California Seismic Network 15 

12. Waveforms of P-waves traveling through the upper mantle and recorded on the Southem California Seismic Network 16 

13. Distribution of strong-motion accelerographs in the contiguous United States (excluding California) 18 

14. Distribution of strong-motion accelerographs in California 19 

15. Dynamic range plotted against frequency for typical regional network seismograph 

and strong-motion accelerograph 20 

16. Map of proposed National Seismic Network stations in the contiguous United States 21 

17. Schematic representation of National Seismic Network 22 

18. Map showing seismicity of the intermountain region 23 

19. Probability per hour of a large (~7 .5) earthquake following aM 6.5 earthquake on 

the San Andreas fault 24 

20. Probability of a characteristic Parkfield earthquake following the occurrence of a potential foreshock 25 

21. Conceptual design of a Seismic Computerized Alert Network (SCAN) to provide 

short-term warning of imminent strong shaking 27 

22. Warning times expected in southern California from SCAN 28 

23. Distribution of Holocene volcanoes in North America 30 

24. Strong-motion waveforms together with geologic structure in the Los Angeles region 

from the 1971 San Fernando earthquake 31 

25. Variation of strong-motion response spectra from M 6.5 earthquakes recorded at 50 

km distance 32 

26. Stereoscopic views of aftershocks of the 1984 Morgan Hill, California, earthquake 33 

27. Map of seismic activity in the central United States 35 

28. Cross section of seismic activity beneath Kilauea volcano, Hawaii 36 

29. Aftershocks of the 1984 Morgan Hill, California, earthquake coplotted with slip distribution inferred from modeling strong-motion waveforms 37 

TABLES 

1. Attendees of July 1987 meeting to discuss National Seismic System Science Plan 2 

2. Summary of U.S. regional seismic networks 4 

3. Summary of strong-motion accelerographs in the United States 17 

4. Problems requiring high densities of seismographic stations 24 

5. Holocene volcanism of North America and Hawaii 30 
National Seismic System Science Plan 

By Thomas H. Heaton, Don L. Anderson, Walter j. Arabasz, Ray Buland, William L. Ellsworth, 

Stephen H. Hartzell, Thorne Lay, and Paul Spudich 

EXECUTIVE SUMMARY • Seismicity maps of active fault systems. 

• Recognition of seismic gaps. 

Recent developments in digital communication and seismometry are allowing seismologists to propose revolutionary 

new ways to reduce vulnerability from earthquakes, volcanoes, and tsunamis, and to better understand these 

phenomena as well as the basic structure and dynamics of the 

Earth. This document provides a brief description of some of 

the critical new problems that can be addressed using modem 

digital seismic networks. It also provides an overview of existing seismic networks and suggests ways to integrate these 

together into a National Seismic System. 

A National Seismic System will consist of a number of 

interconnected regional networks (such as southern California, central and northern California, northeastern United 

States, northwestern United States, and so on) that are jointly 

operated by Federal, State, and private seismological research 

institutions. Regional networks will provide vital information 

concerning the hazards of specific regions. Parts of these networks will be linked to provide. uniform rapid response on a 

national level (the National Seismic Network). 

A National Seismic System promises to significantly 

reduce societal risk to earthquake losses and to open new areas 

of fundamental basic research. The following is a list of some 

of the uses of a National Seismic System. 

Emergency Information Management: 

• Near real-time estimation of damage patterns after significant earthquakes. 

• Very short term (less than several minutes) warning of 

imminent strong shaking during significant earthquakes. 

• Real-time probabilistic estimation of seismic risk by 

monitoring of potential foreshock sequences. 

• Short-term warning of imminent danger from tsunamis. 

• Monitoring of volcanic activity. 

Estimation of Long-Term Risk: 

• Accurate prediction of ground motions during future 

earthquakes. 

Manuscript approved for publication, February 23, 1989. 

Basic Research: 

• Uniform catalog of earthquake activity. 

• Systematic mapping of crustal stress. 

• Better understanding of U.S. earthquakes. 

• Better understanding of worldwide earthquakes. 

• Systematic mapping of crustal and upper mantle structure 

beneath the United States. 

• Mapping of whole-Earth velocity structure. 

• Recognition of magma bodies. 

• Nuclear-test treaty verification research. 

INTRODUCTION 

In this document we describe ways that seismic information can be used to significantly reduce the hazards from 

earthquakes, tsunamis, and volcanoes. We also describe some 

of the fundamental problems about the structure and dynamics 

of the Earth that can be addressed. 

Elastic waves in the Earth are generated by a number of 

sources that range from earthquakes to weather, machinery, 

and explosions. The nature of seismic waves varies tremendously with time and space. Ground motions may have accelerations of about 10-8 

g during relatively quiet times and 

they may exceed 2 gat distances close to large earthquakes. 

Similarly, the frequency of these waves varies from less than 

one cycle per hour to hundreds of cycles per second. Seismometer systems have been constructed to record these motions, but because of practical mechanical limitations, the 

range of amplitudes (dynamic range) and frequencies 

(bandwidth) that can be recorded by traditional systems is 

severely limited. Dynamic range and bandwidth have generally been less than three orders of magnitude and two orders of 

magnitude, respectively. Furthermore, the analysis of data 

from these systems has been time consuming. 

These instrumental limitations have profoundly affected 

the nature of problems that seismologists could address. The 

application of modem digital technology to seismic recording 

systems has dramatically expanded their capabilities. It is 

now practical to build systems that have dynamic ranges of ten 

orders of magnitude and bandwidths that range from one cycle 

Introduction 
per hundreds of seconds to one cycle per hundredth of a second 

(four orders of magnitude). Furthermore, data can be rapidly 

collected and analyzed using computer systems. These new 

systems dramatically expand the types of problems that can 

solved. 

We now describe some of the events that led to the writing of this document. The development of regional seismic 

networks and of modem digital seismometry both have long 

and complex histories that will not be covered. This document 

is an immediate result of a plan for a United States National 

Seismic Network developed by the National Earthquake Information Center (NEIC) of the U.S. Geological Survey 

(Masse and Boland, 1987). The network is to consist of approximately 150 modem digital stations that are distributed 

throughout the United States. Data from these stations are to 

be transmitted via satellite telemetry to a central recording site 

in Golden, Colorado, and the network will provide uniform 

(but rather sparse) national coverage. Funding from the U.S. 

Nuclear Regulatory Commission will permit installation of approximately 60 stations in the Eastern and central United 

States. The U.S. Department of the Interior has yet to make 

funding commitments to install stations in other parts of the 

country. 

Many seismologists who have reviewed the NEIC plan 

have been very excited about capabilities of stations in this 

network. These stations appear to be able to record and rapidly telemeter ground motions that range from the largest motions expected during destructive earthquakes to ambient 

ground noise at quiet sites. They also record ground motion 

over a very large frequency band (approximately 30Hz to0.01 

Hz). Thus stations in the National Seismic Network will 

provide data that can be used to study a very broad range of 

seismic problems. 

At present, there are currently about 1 ,600 seismic stations in locally operated regional networks that are distributed 

throughout the United States, and this new National Seismic 

Network cannot (and is not intended to) perform the many 

functions of existing regional networks. Unfortunately, instrumentation in the existing regional seismic networks has 

very limited dynamic range and bandwidth. This has severely 

limited the applications and types of basic research problems 

that can be addressed with existing regional networks. 

Many seismologists recognized that applying the technology planned for the National Seismic Network to the problem 

of regional seismic networks will greatly expand our abilities 

to reduce our risks from natural phenomena and to better understand the structure and dynamics of the Earth. A small 

group of seismologists (listed in table 1) from universities and 

the Federal government convened to discuss these issues in 

July 1987 at Alta Lodge in Utah. There was a strong consensus that we need to develop an integrated, nationwide approach to the recording, reporting, and exchange of 

seismological data in the United States. In this document we 

present a common vision that arose in the Alta meeting of what 

a National Seismic System might look like. 

2 National Seismic System Science Plan 

Table 1. Attendees of meeting at Alta Lodge, Utah, to discuss a 

National Seismic System July 8-10, 1987 

Prof. Walter Arabasz 

University of Utah 

Salt Lake City, Utah 

Dr. William Bakun 

Chief Scientist for the 

Parkfield Prediction Project 

U.S. Geological Survey 

Menlo Park, California 

Prof. James Brune 

University of Nevada 

Reno, Nevada 

Prof. Robert Clayton 

California Institute of Technology 

Pasadena, California 

Prof. Robert Crosson 

University of Washington 

Seattle, Washington 

Dr. William Ellsworth 

Chief of Branch of Seismology 

U.S. Geological Survey 

Menlo Park, California 

Dr. John Filson 

Chief of Office of Earthquakes 

Volcanoes, and Engineering 

U.S. Geological Survey 

Reston Virginia 

Dr. Thomas Hanks 

Chief of Branch of 

Seismology and Geology 

U.S. Geological Survey 

Menlo Park, California 

Dr. Thomas Heaton 

Scientist in Charge of the 

Pasadena Field Office 

U.S. Geological Survey 

Pasadena, California 

Prof. Robert Hemnann 

SL Louis University 

St. Louis, Missouri 

Prof. Arch Johnston 

Memphis State University 

Memphis, Tennessee 

Prof. Hiroo Kanamori 

California Institute of Technology 

Pasadena, California 

Dr. Robert Masse 

Chief of Branch of Global Seismology 

U.S. Geological Survey 

Denver, Colorado 

Dr. Elaine Padovani 

Manager, External Research Program 

U.S. Geological Survey 

Reston, Virginia 

Dr. David Simpson 

Lamont Doherty Geological 

Observatory 

Columbia University 

Palisades, New York 

Prof. Robert Smith 

University of Utah 

Salt Lake City, Utah 

Dr. Wayne Thatcher 

Chief of Branch of Tectonophysics 

U.S. Geological Survey 

Menlo Park, California 

CONFIGURATION AND USES OF 

EXISTING SEISMIC NETWORKS 

Current Regional Networks 

From maps of earthquake activity in the contiguous 

United States (fig. 1), it is clear that seismic activity is distributed throughout the Nation. Alaska, Hawaii, and Puerto 

Rico also have high rates of seismic activity (the magnitude 

9.2 1964 Alaskan earthquake is the largest known U.S. 

earthquake and the second largest in the world in this century). 

In order to understand this widespread earthquake activity, approximately 1,600 permanent seismographic stations are 

maintained throughout the United States by regional networks. Table 2 and figure 2 summarize the geographic loca-
Figure 1. Seismicity within contiguous United States. All historical earthquakes with shaking intensity of at least VII (M approx 

5.5), all earthquakes of magnitude of at least 5.0 since 1925, all earthquakes of at least magnitude 4.0 since 1962, and all 

earthquakes of at least magnitude 3.5 since 1975 are plotted (largest to smallest circles, respectively) (courtesy of E.R. Engdahl). 

tion and operating organization for the largest of these regional 

networks (compiled from Simpson and Ellsworth, 1985). 

There is great diversity in the size of these networks, in the 

volume of data that is processed, in the nature of the operating facility, and in the funding sources. Although there are 

some notable exceptions, most of the stations consist of shortperiod vertical seismometers whose analog signals are continuously telemetered via voice-grade fr~uency-modulated 

(FM) telephone or radio links to a central recording site. In 

most instances, the incoming signals are digitized (typically at 

100 samples per second) and processed on minicomputers. 

Although processing hardware and software varies considerably, all of the processing systems are designed to record 

only when several stations simultaneously detect signals 

above a threshold. Detected events are then analyzed to pick 

the times of seismic arrivals, locate the source of the seismic 

energy, and then catalog and archive the data on magnetic 

tape. The number of earthquakes recorded by these networks 

varies from less than 100 per year in much of the Eastern 

United States to more than 15,000 per year for the large 

California networks. Since the primary mission of regional 

networks is to monitor regional earthquake activity, many 

regional networks do not attempt to consistently record signals from distant earthquakes (teleseisms), although P-waves 

from larger teleseisms often trigger event detectors and are 

hence well recorded. Stations in the regional networks of 

California are shown in figure 3, and a compilation of 

earthquakes located with these networks for the period 1980 

through 1986 is shown in figure 4 (D.P. Hill, written commun., 

1987). 

Uses of Current Regional Networks 

Regional seismic networks are a fundamental multipurpose tool of observational seismology. Although commonly 

perceived as simply a tool for earthquake "surveillance" or 

"monitoring," existing seismic networks provide data and information for a host of uses: 

-Public safety and emergency management 

-Quantification of hazards and risk associated with both 

natural and human-triggered earthquakes 

-Surveillance of underground nuclear explosion 

-Investigation of earthquake mechanics and dynamics 

-Investigation of seismic wave propagation 

-Investigation of seismotectonic processes 

-Earthquake forecasting and prediction research 

-Probing the internal structure of the Earth 

Importantly, seismic networks are also key facilities for 

the graduate education and training of this country's professional seismologists, and they provide direct outlets for public 

information and for expert assistance to public policy makers, 

planners, designers, and safety officials. 

Configuration and Uses of Existing Networks 3 
Table 2. Existing U.S. regional seismic networks 

Network 

Northeastern U.S. seismic networks 

Lamont Doherty (Columbia University) 

Weston Observatory (Boston College) 

Woodward Oyde Consultants 

Others 

Southeastern U.S. seismic networks 

Center for Earthquake Research 

Total 

Number of 

Stations 

27 

29 

42 

25 

123 

(Memphis State University) 30 

Georgia Institute of Technology 17 

University of South Carolina (also USGS) 22 

Virginia Tech 17 

Tennessee Valley Authority 18 

Others 40 

Central U.S. seismic networks 

SL Louis University 

Oklahoma Geophysical Observatory 

University of Michigan 

Others 

Total 144 

Total 

37 

12 

15 

14 

78 

Great Basin, Intermountain, and Rocky Mountain networks 

University of Utah 75 

University of Nevada, Reno 65 

U.S. Geological Survey Southern Nevada (Denver, CO) 55 

Others 65 

Total 260 

California seismic networks 

USGS Central and Northern Calif. 360 

USGS - California Institute of Tech. Southern Calif. 240 

University of California, Berkeley 20 

University of Southern California 30 

Others 75 

Total 125 

Northwestern U.S. seismic networks 

University of Washington-USGS Network 110 

North American Rockwell 30 

Total 140 

Alaskan seismic networks 

U.S. Geological Survey (Menlo Park, CA) 64 

University of Alaska 36 

Lamont Doherty (Columbia University) 20 

Alaska Tsunami Warning Center 16 

CIRIES (Boulder, CO) 11 

Total 147 

Hawaii seismic networks 

Hawaiian Volcano Observatory (USGS) 60 

Pacific Tsunami Warning Center (NOAA) 10 

Total 10 

Approximate total number of stations in U.S. regional networks 1,600 

Hanks (1985, 1987) published two informative summaries that reflect, in considerable part, the broad scope of current network seismology (see also Simpson and Ellsworth, 

4 National Seismic System Science Plan 

1985). They deal respectively with (1) the current scientific 

status of earthquake-related studies under the National 

Earthquake Hazards Reduction Program, and (2) seismology 

in the period 1983-1986. We elaborate on some of the uses 

of current seismic networks in the following two subsections, 

broadly labeled "Earthquake physics and hazard analysis" and 

"Structure of the Earth." 

Earthquake Physics and Hazard Analysis 

The majority of existing seismic networks in the United 

States relate fundamentally to understanding and mitigating 

the danger of damaging earthquakes. This involves anything 

and everything amenable to seismological observation-from 

the mechanics and dynamics of an earthquake source to the 

ground motions produced at any site from transmitted seismic 

waves. Describing the "anything and everythi.Jig" that network seismologists investigate is beyond our scope. However, 

one way to outline much of that body of effort is to consider 

the aspects of a modern earthquake hazard analysis. Numerous 

interrelated pieces of information from network seismology 

become involved in the process. 

To begin with, an earthquake hazard is a physical 

phenomenon with potentially adverse effects associated with 

an earthquake-for example, ground shaking, ground failure, 

surface faulting, tectonic deformation, inundation. In a 

rigorous seismic hazard analysis (for example, Electric Power 

Research Institute, 1987; Savy and others, 1986), quantitative 

models of earthquake behavior and effects are specified so that 

the level of hazard, such as the likely non-exceedance value 

of ground motion, can be computed for one or more sites for 

some exposure time, usually using a probabilistic approach. 

The flowchart shown in figure 5 outlines the basic elements of a seismic hazard analysis for the hazard of ground 

motion. The sequence of necessary procedures is shown by 

steps 1 through 5 in the left-hand column; interrelated aspects 

of observational seismology are shown in the right-hand 

column. 

Earthquake catalogs (step 1, fig. 5) are a basic starting 

point of earthquake seismology. Although the historical 

earthquake record is approximately 400 and 150 years in the 

Eastern and Western United States, respectively, good instrumental monitoring in many regions dates from only the 

1970's (Simpson and Ellsworth, 1985). At a time when 

studies of prehistoric earthquakes attract attention (for example Allen, 1986), it seems worth emphasizing that 

earthquake catalogs are essential for estimating seismic hazard 

for earthquakes below the threshold of surface faulting (for example, the 2 May 1983 M 6.5 Coalinga earthquake or the I 

October 1987 M 5.9 Whittier Narrows earthquake). Along the 

Wasatch Front, Utah, such earthquakes (M 6.0-6.5) are the 

largest contributor to probabilistic ground shaking hazard for 

exposure periods of 50 years or less (Arabasz and others, 

1987). In the central and Eastern United States, historical surface ruptures are virtually absent and information on prehis-
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30° 

Figure 2. Seismograph stations operated by regional networks in the contiguous United States (map prepared by the National 

Earthquake Information Center). This compilation reflects station locations in the early 1980's, and although the current configuration of regional networks is similar, there are discrepancies between this map and the configuration of regional networks in 1988. 

A listing of the operators of regional networks is given in table 2. 

toric earthquakes is sparse. Therefore, the historical and instrumental earthquake record is of great importance in assessing the potential sources of future earthquakes. In regions 

away from the active plate boundaries of western North 

America, the numbers of total earthquakes for any time period 

are significantly fewer than near the boundary. In the seismic 

regions interior to the plate, modem instrumental data become 

particularly important in the statistical processing of 

earthquake catalogs to estimate reliable seismicity parameters 

(step 4, fig. 5; see Veneziano and VanDyck, 1986). 

The characterization of seismotectonic framework (step 

2, fig. 5) encompasses extensive efforts of network seismologists and gets to the heart of understanding earthquake 

behavior in diverse tectonic regions. The definition and 

geometric depiction of seismic source zones (step 3, fig. 5) is 

intimately related. Precise mechanisms and associated source 

parameters, stress state and strain rate, models for crustal 

structure, the location and geometry of active faults, and the 

fault mechanics and operative tectonic processes within a 

given region must all be investigated. We refer the reader to 

Allen (1986) and Hill (1987) for more comprehensive review 

papers. 

Increasingly elegant techniques have become available to 

network seismologists for seismotectonic studies. Four examples (and representative citations) are: (1) cross-spectral 

analysis of waveforms for high-resolution earthquake locations (Pechmann and Kanamori, 1982; Ito, 1985); (2) inversion of focal mechanisms to obtain the stress field (Angelier, 

1987; Michael, 1987); (3) determination of rupture charac-

... teristics of earthquakes from ground-motion data using the 

wavefonns of adjacent small earthquakes as empirical Green's 

functions (O'Neill, 1984; Frankel and others, 1986), and (4) 

the mapping of seismic slip distributions on a single fault plane 

to investigate details of the earthquake generation process 

(Bakun and others, 1986). Despite such advances, there emphatically remain first -order problems throughout much of the 

United States in associating observed seismicity with specific 

geologic structures-and in confidently identifying the sources of future moderate-to-large earthquakes. Examples in the 

Pacific Northwest, the intennountain west, and eastern North 

America (including the problematic source of the 1886 Charleston, South Carolina, earthquake) were reviewed by Hill 

(1987). 

Earthquake physics, based on network observations, becomes an important part of the modeling of ground-shaking 

hazard (step 5, fig. 5) in the specification of the source 

spectrum, its scaling with earthquake size, and effects on wave 

propagation and attenuation. Earthquake physics also governs 

Configuration and Uses of Existing Networks 5 
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Figure 3. Major California regional network stations. 

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the assessment of maximum magnitude (step 4, fig. 5), although empirical approaches are more generally adopted. An 

important part of the hazard analysis is the recurrence modeling (step 4, fig. 5), which involves specifying time and size 

distributions of earthquakes for the identified seismic source 

zones. The so-called Poisson-exponential model is common6 National Seismic System Science Plan 

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ly assumed in which earthquake occurrences are postulated to 

follow a memory less Poisson process in time and a truncated 

exponential distribution in size. The associated parameters 

depend fundamentally upon earthquake catalogs. If the place 

and time of an earthquake can be predicted, however, probabilistic estimates of hazard become drastically altered. 
... ,. 

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MAGNITUDES 

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200 KILOMETERS 

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Figure 4. Seismicity throughout California and western Nevada for the period 1980-1986 (D.P. Hill, written commun., 1987). 

Bakun ( 1987) provided a perspective on the current status 

of progress toward earthquake prediction, including description of a specific prediction by the U.S. Geological Survey 

(USGS) for the occurrence of a characteristic magnitude 6 

earthquake on the Parkfield section of the San Andreas fault 

in 1988±5 years. Patterns of earthquake occurrence documented from global, regional, and local earthquake monitoring provide viable approaches (with different degrees of 

general acceptance) for a probabilistic approach to earthquake 

prediction on different time scales. These include (1) recognition of seismic gaps along plate boundaries, (2) the seismic 

quiescence hypothesis that proposes a decrease in seismicity 

before some larger earthquakes, (3) repetition of similar or 

characteristic earthquakes along definable fault segments, and 

(4) recognizable slip deficits along parts of well-monitored, 

seismically active faults. 

Recently, an integrated assessment of the probability of 

occurrence of major earthquakes along the San Andreas fault 

during the next 30 years (fig. 6) was released by the USGSsponsored Working Group on California Earthquake Probabilities (1988). The potential for future damaging 

earthquakes on each segment of the fault was derived through 

Configuration and Uses of Existing Networks 7 
~ 

6. Studies of pre-instrumental, historical 

eanhquakes 

1. CompilatioD of eanbquab catalog (earthquake -- times. locarioal, IDd mea> -

~ 7. Instrumental seismic monitoring and 

routine data analysis (earthquake locations, 

sizes, focal depths) 

w 

2. CharactaizalioD of seismorectonic: framework 

(crustal structure, conremporm y deformation 8. Network and ponable-an:ay seismology 

[stress stare IDd Sll'liD rate], location and - (special studies of bypocenttal resolution. 

geometry of active faults, conelation of seis- - focal mechanisms. seismic momenl, stress 

micity with geoloaic suucture, and fault drop, and other source parameters; also 

mecbanics) crustal structure) 

'r 

3. Defillicioa IDd pometric depiction of seismic 

source zoaea 

'f 

4. Estimation of seismicity parame1er1 (pretesting of catalog for uniformity of size 9. Analysis and in~tation of space-time 

estimates. dependency of events. and sample -- patterns of earthquake behavior for recurrence 

completeness; assessment of maximum mapi- modeling and earthquake forecasting 

tude; recurrence modeling and computations) 

', 

5. Modeling and estimation of ground motion 

for engineering applications-including 10. Analysis and interpretation of ean.hquakc 

description of source specuum (and its scaling - source mechanics and wave propagauon - with earthquake size), wave propaptioa, and effects, including attenuation 

attenuation 

Figure 5. Steps in a formalized hazard analysis (left column) and interrelated aspects of observational seismology (right column). 

This example (from Arabasz and others, 1987) is for the Wasatch Front, Utah, region; a similar flowchart for California would involve more elaborate elements in the right column. 

the synthesis of instrumental and historical seismic records, 

geologic studies of prehistoric events, and geodesy through the 

application of the same probabilistic approach employed at 

Parkfield and supported by worldwide observations 

(Nishenko and Buland, 1987). The report identifies the 

southern third of the San Andreas fault, including segments 

posing the highest risk to the major metropolitan regions of 

southern California, as having an aggregate probability of 60 

percent for aM 1.5 to 8 earthquake by the year 2018. Just as 

the regional seismic data base has already played a critical role 

in defining the hazard, it has the potential to significantly 

8 National Seismic System Science Plan 

reduce the risk posed by the anticipated earthquakes (see later 

discussion on applications of seismic networks). 

Structure of the Earth 

Since the 1960's there have been many applications of 

seismic arrays for analysis of detailed Earth structure. The 

principal advantages offered by an array are (1) that dense station distribution allows for direct measurement of the 

azimuths of approaching wave fronts and slopes of travel time 

curves, dt/M, which are directly used in earthquake locations 
LEVEL OF RELIABILITY 

(with A being most reliable) 

CJE 

CONDITIONAL PROBABILITY OF MAJOR 

EARTHQUAKES ALONG SEGMENTS OF THE 

SAN ANDREAS FAULT 

1988-2018 

CONDITIONAL PROBABILITY OF MAJOR 

EARTHQUAKES ALONG SEGMENTS OF THE 

HAYWARD, SAN JACINTO AND IMPERIAL FAULTS 

1988-2018 

LOS 

ANGELES 

;t~ z- o:! 

E!:2 

om 

zo 

oa: 

(.)C.. 

--'>-

~!::: o:! -CD !:::< 

om 

zo 

oa: 

(.)C.. 

Figure 6. Conditional probabilities of the occurrence of major earthquakes on the San Andreas, Imperial, San jacinto, and Hayward fault systems for the 30-year period from 1988 to 2018 (Working 

Group on California Earthquake Probabilities, 1988) 

Configuration and Uses of Existing Networks 9 
and earth structure inversions, and (2) that signal-to-noise enhancement can be achieved by stacking closely spaced observations. Many specialized arrays such as LASA and 

NORSAR have been deployed with regular geometries and 

standardized instrumentation; however, regional networks 

have proved equally useful as large "arrays, despite their irregular geometries and variable instrumentation. 

Array diagrams for regional networks and arrays (fig. 7) 

are a way to indicate earthquake location capabilities of each 

array relative to a global network of stations (Powell, 1976). 

The CIT regional network (Southern California Seismic Network) shows remarkably small mislocation vectors, which is 

a consequence of its large aperture and high station density. 

Systematic analysis of the regional mislocation vectors has 

been adapted to a variety of studies, including analysis of 

mantle velocity heterogeneity near the earthquake source, 

deep in the mantle, and in the upper mantle and crust beneath 

the regional network. The data are acquired in a passive mode 

as the regional networks are operating to accomplish their 

regional monitoring functions, but many additional array 

processing capabilities can potentially be implemented for 

regional networks. Examples would include routine beam 

forming for enhanced detection of seismicity in global seismic 

gaps, aftershock zones, or nuclear test sites. Such applications 

have been sparse largely due to limitations imposed by cumbersome data management systems and by the limited frequency bandwidth and dynamic range of existing regional 

networks. 

The dense spatial coverage provided by regional networks 

has been directly exploited in many studies of crustal velocity 

structure to map the crust at geologically meaningful scales. 

In one example of detailed imaging of lateral variations in 

shallow crustal velocity structure (Hearn and Clayton, 1986a), 

obtained by use of stations and local seismicity in southern 

California (fig. 8), the crustal velocity variations are strongly 

associated with surface tectonic features such as the San 

Andreas fault. Similar analysis of lateral variations in uppermost mantle structure based on the P n phase also reveals striking spatial heterogeneity, as shown in figure 9 (Hearn and 

Clayton, 1986b ). Given that regional seismic networks are intrinsically located in interesting tectonic environments (for example, similar models have been obtained using regional 

networks near New Madrid, Missouri, and Puget Sound, 

Washington), it is always of interest to obtain such detailed 

crustal models in order to understand the underlying physical 

processes causing the seismic activity. The data for such 

analysis are a direct by-product of the seismic monitoring 

function of networks. 

In addition to using local earthquakes to investigate shallow crustal structure, regional networks accumulate teleseismic travel times and waveforms that allow deeper mantle 

structure to be studied. A spectacular example is seen in figure 

10 (Humphreys and others, 1984), showing a high-velocity 

tabular root that extends several hundred kilometers into the 

mantle beneath the Transverse Ranges of southern California. 

1 0 National Seismic System Science Plan 

New sophisticated tomographic inversion techniques have 

been developed to reliably resolve such structure from 

regional network data, and practical experience has shown that 

upper mantle heterogeneity is ubiquitous. This has many implications for the dynamics and chemical evolution of the crust 

and upper mantle. Regional network data have been used to 

produce three-dimensional images of velocity structure 

beneath southern California, northern California, New 

England, Washington, and the New Madrid seismic zone. 

Accurate measurements of travel times and wave slowness by regional networks have played a major role in improving our detailed knowledge of upper and lower mantle radial 

and lateral variations. Figure 11 (Walck, 1984) shows the high 

resolution of travel time and ray-parameter measurements that 

can be attained using regional network data (in this case from 

the Southern California Seismic Network). The critical identification of secondary arrivals is abetted by the dense station 

distribution, as shown in figure 12, where data from several 

events are combined to develop a profile for the ray paths 

beneath the Gulf of California (Walck, 1984 ). 

Current Strong-Motion Instrumentation 

The term "strong motion, is used in the engineering and 

seismological communities to mean ground motions that are 

sufficiently large to be capable of causing damage, and 

"strong-motion instruments, are seismographs (usually accelerometers) that can record these large motions without 

overdriving the seismograph. Presently, a large number of 

Federal, State, and local governmental agencies, cotporations, 

public utilities, and universities operate and maintain about 

3,000 strong-motion instruments in the United States (National Research Council, 1987; table 3, figures 13 and 14). Each 

organization installs instruments to satisfy its own particular 

needs, and about 40 percent of the instruments are private and 

yield data not available for research. In general, the primary 

goal of the instrumentation is to gather data for engineering 

purposes. The majority of these instruments are installed 

within or on structures ranging in size from one-story fire stations, schools, and post offices to high-rise buildings, dams, 

bridges, storage tanks, and power plants. Few of these instruments are installed in free-field or ground sites, which typically consist of an accelerograph housed within a small fiberglass 

hut resting upon a concrete pad 1 meter square. 

Although some digital accelerographs have been installed 

within the last few years, the vast majority of strong-motion 

instruments are analog and record their signals on moving 

photographic film. For a large structure in which many acceleration sensors are deployed throughout the structure, the 

signals from all sensors are conveyed to a central multichannel recorder that records all channels simultaneously on 

photographic film. For smaller structures and for free-field 

sites, the most commonly used accelerograph is a self-contained unit that records three orthogonal components of ac-
w dt!d!J. 

w 

~ '~ ·. ~, 

., 1\ ·~·''' ,' ·)'· . . ·~,.. 

s s 

CIT N 

CORRECTED 

s 

Figure 7. Mislocation vectors from Large Aperture Seismic Array (LASA) in Montana (discontinued in the 1970's), regional 

network in eastern Washington (Hanford), and southern California (CIT) regional network. Tail of each vector represents 

azimuth and incidence angle of a planar P wave from a teleseismic earthquake as observed at these networks, and the head 

represents azimuth and incidence angles expected from known locations of earthquakes and standard earth model (Powell, 

1976). Apparent velocity, dt/d!J., in seconds per degree. 

celeration on a 70-mm photographic film strip. All of these 

accelerograph systems are "triggered" units, which sit dormant until detecting a ground acceleration that exceeds a 

preset threshold (usually 0.01 g on the vertical component). 

Once the threshold is exceeded, there is a short interval (about 

0.1 s) during which the instrument's film transport accelerates 

to its desired operating speed. Because of the triggering and 

the delay of the film transport, these accelerographs cannot 

record the initialP-wave motions of the earthquake or any preevent ground noise. In addition, many of these instruments 

have no external time reference, so that absolute wave arrival 

times cannot be determined, and in cases of multiple 

Configuration and Uses of Existing Networks 11 
35° 

34• f---!---l!!l~~~o 

I 

I 

·-------

~~§¢¢1 -.6 -.4 .0 .4 .6 

DELAYS, IN SECONDS 

-. 

Figure 8. Pg velocity variation (top map) in southern California as inferred from tomographic 

inversion ol Pg-wave travel times from local earthquakes recorded on the Southern 

California Seismic Network. Pg velocities are representative of the average crustal P-wave 

velocity. Hachured region indicates relatively low velocities and shaded regions are 

relatively fast. A strong correlation between Pg velocities and major geologic discontinuities 

such as the San Andreas fault (SAF) and Garlock fault (GF) can be seen. Station delays 

(bottom map) for corresponding study. Hachured areas show regions of early arrivals. late 

arrivals (shaded areas) are associated with los Angeles and Ventura basins and Salton 

trough (Hearn and Clayton, 1986a). 

earthquakes (such as aftershocks) the lack of an external time 

reference prevents unambiguous identification of a recorded 

event. Because a rather strong signal (0.01 g) is required to 

trigger these accelerographs, these instruments detect relative12 National Seismic System Science Plan 

ly few events; they are usually only triggered by very local 

events of about M 4.5 or larger, or by larger more distant 

events. In many cases a strong-motion instrument will trigger 

on only the main event of a sequence. 
Figure 9. Pn velocity variation (top map) in southern California as inferred from tomographic 

inversion of Pn -wave travel times from earthquakes recorded on Southern California 

Seismic Network. Pn velocities representative of uppermost mantle beneath southern 

California. Hachured regions indicate relatively low velocities; shaded regions are relatively 

fast. Pn velocities are generally higher on North American plate, east of San Andreas fault 

(SAF), than on Pacific plate, west of San Andreas fault. Contour plot (bottom map) shows 

station delays. Hachured areas indicate regions of relatively early arrivals. Largest delays 

(shaded areas) are associated with sedimentary basins, and early arrivals are associated 

with thin crust (Hearn and Clayton, 1986b). GF, Garlock fault. 

Most accelerograms recorded on photographic film are 

processed using a fairly standard procedure. The first step is 

the de:cision of which recordings are sufficiently interesting 

from 1m engineering standpoint to merit digitization; generally only those recordings having accelerations larger than 0.05 

g are digitized. Thus, the detection threshold for digitized data 

is around M 5. The digitization is perfonned by an automated 

trace-following system supplemented by hand digitization of 

Configuration and Uses of Existing Networb 13 
121° 

37° 

120° 119° 118° 117• 11 s• 115° 114° 

c Salton trough C' o~r-r-r-~~~~~~~~ 

30 

··:.~~---·-! 

200 .. ~ .... ··· 

• 

300 

31°L-------~------~------~------~--------~------._------~ 

A Transverse Ranges A ' o~-r~~~WP~V7.~~.-.-~~~~~ 

Ul 

a: 

~ 

30 

~ 200 

g 

>2 

~ 

:i 300 t: 

uJ 

Cl 

400 

~ 

·<.> 

4 

t 

t .> 

Figure 10. Three-dimensional upper-mantle P-wave 

velocity structure beneath southern California as inferred 

from tomographic inversion of teleseismic P delays derived 

from Southern California Seismic Network. In upper left 

panel, horizontal section at depth of 100 km is 

superimposed on location map of southern California. 

Locations are shown for three cross sections (A-A 1

, 8-81

, 

C-C~ displayed in other panels. Vertical and horizontal 

scales are identical. Contour interval is 1.5 percent velocity 

variations, with high-velocity regions indicated by dotted 

portions of the record where accelerations are large and the 

automated system cannot follow the traces properly. The digital acceleration data are then corrected for the instrument 

response and integrated to yield ground velocity and ground 

14 National Seismic System Science Plan 

0 

30 

100 

B 

.·: 

Transverse 

Ranges 

.. ./\.. '· 

,.. .... ·.\\:. / l' 

.. ·· · ... · 

300 • ....... 

·.· 

400 ··. 

B' 

..... · ....... ~ 

......... 

areas, and low-velocity regions indicated by chevron 

pattern. Zero contour is dashed. East-west-trending, 

tabular, high-velocity region beneath Transverse Ranges 

can be seen in cross sections A-A 1 and 8-81

• This slab of 

high-velocty material may represent downwelling in mantle 

beneath Transverse Ranges. Cross section c-c I 

perpendicular to Salton trough, reveals region of velocities 

that are 2 to 4 percent slow at depths extending to 1 00 km 

beneath this region of crustal extension (Humphreys and 

others, 1984). 

displacement records. The effective bandwidth of the data is 

from about 0.1 or 0.2 Hz at low frequencies to about 25 to 50 

Hz at high frequencies. The bandwidth is much less broad for 

small events due to the large amount of noise introduced into 
the data during digitization. The effective dynamic range of 

these systems is about 60 dB. 

Limitations of Existing Networks 

The development of the current generation of regional 

networks within the United States began in the 1960's in 

response to the need to learn more about the distribution of 

seismicity within regions of recognized earthquake hazards. 

The most basic questions (such as, "Do earthquakes occur 

along recognizable fault planes?") had no answers at that time. 

Consequently, observational seismology was in a position to 

make rapid progress by adapting well-established seismological techniques of earthquake location and magnitude determination to local and regional scale problems. By the early 

1970's the design characteristics of the network systems were 

largely established. These now-antiquated technologies have 

continued to operate into the 1980's with only modest upgrading of their data analysis capabilities and without any improvements in the resulting data. 

The characteristics of existing regional seismic and 

strong-motion networks have been very strongly influenced 

by the objectives attainable with then available technology. In 

the case of regional seismic networks, the primary objective 

has been the construction of a high spatial resolution catalog 

of earthquake activity within each network. It has thus been 

imperative to obtain numerous P-wave arrival times for as 

many earthquakes as can practically be observed. Economic 

considerations dictated the recording of high-frequency, vertical-component ground motions from many sites. 

The actual ground motion history has largely been 

sacrificed in this mission. Because of the need for high sample 

rates, the only practical solution in the past has been to continuously telemeter analog data streams. Furthermore, therequired high sample rates have made it difficult to store digital 

records from long-duration records such as those expected 

from teleseisms. The use of analog FM data telemetry has 

severely restricted the dynamic range (typically 40 dB) of the 

seismic systems. Because the mission calls for the monitoring of small earthquake activity, gains are typically set high 

enough to resolve earth noise. Consequently, the signals are 

off scale for most of the significant earthquakes. Furthermore, 

small earthquakes are best detected and timed using high-frequency ground motions. Since there is typically high ground 

noise at periods near 6 seconds, there has been a conscious effort to record only frequencies higher than about 1 Hz. The 

effective dynamic range of typical existing networks compared with expected seismic signals is shown in figure 15. 

Clearly, much important ground-motion information is not 

currently recorded by the existing regional networks. 

The need for continuous telemetry has also made the cost 

of telemetry a major consideration for the design of networks. 

In the present situation, the cost of telemetry increases linearly with the number of channels that are sent. The cost of 

telemetry, together with the limited dynamic range of the sysw 

:;; 

i= 40 ..J 

w 

> 

<( 

a: 32 1-

0 

w 

u 

::> 24 0 

w 

a: 

16 

A 

8 

0 

10 20 30 40 

DISTANCE, IN DEGREES 

.. 14 

w "' w 

a: 

I!J 

w 

0 

a: 

w 

0.. 

IJl 12 eJ 0 

z 

0 

u 

w eJ IJl 

~ 

a:" 

w 

1-

w 

:;; 10 

~ <( 

- GC A 0.. 

> <( 

a: 6 F1rst om val 

~ Lat er om va l 

8 

10 20 30 40 

DISTANCE, IN DEGREES 

Figure 11. Reduced travel times (top plot) of P waves traveling in upper mantle from earthquakes located at regional distances and recorded on Southern California Seismic Network. Triplications caused by velocity discontinuities in 

upper mantle are easily identified (travel-time branches AB, 

CD, EF). Ray parameters (bottom) measured from corresponding data together with those expected for a model of uppermantle P-wave velocities. See Walck (1 984) for further discussion of Gulf of California region models GCA and GCA'. 

tern, have largely contributed to the decision not to record 

horizontal components of ground motion at most sites. Unfortunately, this has led to very uncertain interpretation of 

shear-wave arrivals. 

Configuration and Uses of Existing Networks 15 
TIME- tJ. x 10, IN SECONDS 

40 60 80 (0 60 80 

(/) 

ttl 

a: 

Cl 

w 

0 

~ 25 

ui 

0 

z 

~ (/) 

0 

30 

35 

(0 

10 

15 

20 

25 

30 

35 

(0 

-/\J\1' 

~ 

~ 

~ 

~ 

~ ~ 

~ ~ 

~ 

~ r= -"! 

_;r;,v. 

--./ IV\",.. ~ ,v, 

~t 

~X 

_J 

~ 

~ . 

~ 

#== 

~ 

~ 

~ 

~ 1Y 

----IV- 1{ 

1Y 1Y 1Y jj 

:jj 

Figure 12. Data record section (left) for upper-mantle P-waves from 10 earthquakes spanning the distance from 9° to 40° and 

recorded on the Southern California Seismic Network. Mantle triplication phases are clearly visible. Synthetic record section 

(right) on the same scale (Walck, 1984). 

Serious noise problems, including noise "glitches" in 

telephone and radio systems, are also associated with analog 

FM telemetry. Furthermore, in most networks the analog signals from many stations are transmitted on twisted pairs of 

16 National Seismic System Science Plan 

wires to a single analog to digital converter (ADC). Inductive 

crossfeed between stations can sometimes be a serious 

problem that is difficult to recognize and which can lead to 

very serious errors in interpretation. These noise problems are 
Table 3. Summary of strong-motion instrumentation efforts in 

the United States 

Organization Number 

of instrwnents 

California Division of Mines and Geology 500 

U.S. Army Corps of Engineers 350 

U.S. Geological Survey 275 

Pacific Gas and Electric Company 91 

State of Washington 90 

University of Southern California 81 

U.S. Department of Energy 80 

U.S. Bureau of Reclamation 70 

U.S. Veterans Administration 65 

Nuclear power plants 62 

California Department of Water Resources 70 

University of California, Los Angeles 36 

Los Angeles Department of Water and Power 35 

U.S.Navy 35 

Federal Highway Administration 30 

Metropolitan Water District of Southern California 30 

Southern California Edison Company 26 

Los Angeles Flood Control District 25 

University of California, San Diego 21 

International Business Machines Company 20 

Columbia University 18 

Stanford University 15 

California Institute of Technology 15 

Washington Department of Transportation 15 

Idaho National Engineering Laboratory 15 

Lawrence Livermore National Laboratory 15 

Buildings instrumented in cities using Uniform Building Code 321 

Instruments installed by various organizations 325 

The City of Los Angeles requires owners of large buildings to 

install and maintain strong-motion instruments. This is the 

largest uncoordinated collection of instruments 500 

not present in systems in which data are digitized at the station and then transmitted via error-detecting telemetry. 

The large volume of data that must be managed in order 

to record many events at many stations and at high sample 

rates has necessitated the development of specialized computer hardware and software. Actual seismcgrams are stored 

on magnetic tapes, and it is usually an arduous task to retrieve 

subsets of the data for research. These high-rate data streams 

also make it difficult to stay current with data analysis during 

seismic crises, just when such analysis is most needed. 

Although the monitoring oflocal seismic activity is a crucial one, the attainment of that goal has severely limited the 

usefulness of the data for many other areas of seismology. 

This has caused the study of regional network seismology to 

become intellectually isolated from other fields of seismology. 

Strong-motion networks also have a relatively narrow, 

but very different, mission. Their primary function is to record 

three-component earthquake ground motions that could cause 

damage to facilities. They must operate on scale for shaking 

from relatively rare earthquakes that are large or close enough 

to cause damage. Continuous telemetry of these signals has 

been a very low priority since there are rarely any data to 

telemeter. Hence, it usually requires many days for strong -motion records to become available. Furthermore, since the 

knowledge of absolute time is of little interest to the response 

of an engineered structure, there is usually little information 

about the absolute time of seismic arrivals during strong shaking. This often considerably complicates any fundamental 

physical interpretation of the cause of the ground shaking. 

Because of the difference in their primary missions, highgain seismometers and strong-motion seismometers are very 

rarely collocated. As will be discussed later, this has several 

important implications: (1) Ground motions from small 

earthquakes are dominated by the effects of propagation 

through complex geologic structure. If these effects are understood from the study of recording of weak motions from 

small earthquakes, then they can be removed from the strong 

motions that occur during large earthquakes and the detailed 

nature of the seismic source can be ascertained. Therefore, it 

is difficult to separate the effects of rupture and wave propagation. (2) High-gain seismometers are rarely located in regions 

of intrinsically high noise, such as cities, or even basins. 

However, these are the areas having most inhabited structures. 

Important propagation effects (such as that which happened 

on 15 October 1985 in Mexico City) are usually not recognized until after a tragedy has occurred. (3) Perhaps the largest 

disadvantage of the configuration of present networks is the 

lack of interaction between earthquake engineers and 

earthquake seismologists. 

CONFIGURATION OF PROPOSED 

DIGITAL NATIONAL SEISMIC SYSTEM 

The U.S. National Seismic Network 

The U.S. National Seismic Network (USNSN) is a new 

program being undertaken by the National Earthquake Information Center (NEIC) of the U.S. Geological Survey. Although a tentative plan has been developed to instrument the 

entire United States (see figure 16 for a preliminary distribution of stations), funding has only been obtained for the portion of the continental United States that is east of the Rocky 

Mountains as part of a joint project with the U.S. Nuclear 

Regulatory Commission (USNRC). The USNSN program is 

large and complex. The major elements are (1) the field system, (2) the telemetry system, (3) the central processing system, and (4) a data archival and distribution center. 

The USNSN design goals reflect an attempt to satisfy a 

number of diverse requirements including national and global 

monitoring and research on a regional scale within the United 

States. However, the design goals have also been strongly influenced by known and suspected financial constraints in an 

attempt to ensure that the network can be completed and 

operated over the coming decades. Further, the design has 

been affected by the conscious management strategy of attempting to maximize functionality and minimize cost by the 

use of state-of-the-art technology without taking undue risks 

with emerging technologies. 

Configuration of Proposed System 17 
goo 

• • 

:~···········~-\. •• •• 

•• • • • '; . 

.... .. . _:. • 

. : 

: . 

. -... , .. _: --{.'!.. ··: 

.•. 

• 

. . 

···•.J:_· 

..... ... . ~-- • i• • .. ~ · ..... · . . . . 

( ~ ~.~-·. . ......... ··· ... · .-·· 

. · ... • : . • • • • 

3()• 

•·· ·-~ 

.. : ... 

-,· • 

IJ 

• 

• 

~-- •. • j l \ . --- .: __ .-- !: .. 

110° 

• 

100" 

• ;J!.~ ~ • ... ~<--·· -:· ...... -.-

•. ·.yi .. --~ ~· _; .t·:·.~--- ·• · ... . • .. ·: · . . ... ,. . . . ...... . /•. · ....... . . ·.. . •. • Ground site 

• Dam 

• Bridge 

0 200 400 600 KILOMETERS 

ao· 

Figure 13. Known accelerographs in the United States outside of California as of April30, 1981. Excludes commercial nuclearpowered electrical generating plants. Numbers indicate where more than one station is located. 

The seismic monitoring design goals of the USNSN arise 

from the requirements of the National Earthquake Information 

Service {NElS) Earthquake Early Alerting Service. These 

design goals tend to complement the capabilities of existing 

USGS and USNRC funded regional seismic networks that 

were described in previous sections. In fact, the NElS currently depends on real-time data telemetered from a small subset 

of regional seismic networks for their Alert Service. The 

monitoring goals of the USNSN are (1) uniform coverage of 

the United States and (2) on-scale recording of all seismic 

phases of interest from all earthquakes of interest. Uniform 

coverage is defined as the ability to record any event of magnitude 2.5 or larger by at least five stations anywhere in the 

continental United States and any event of magnitude 3.5 or 

larger anywhere in Alaska, Hawaii, and Puerto Rico. Phases 

of interest for seismic monitoring are various compressional 

and shear wave groups within the frequency band 0.5 to 15.0 

Hz and surface waves of 15 to 30 s. To be of use in Alert Service monitoring, all data must be available within several 

minutes of real time. 

The research design goals for the USNSN can be summarized as a requirement for three-component, broad-band, 

high-dynamic-range data. The network should record both 

body waves and surface waves from local, regional, and 

18 National Seismic System Science Plan 

teleseismic sources. Although data streams that are triggered 

and record only during the arrival of significant phases are 

considered to provide a practical solution to data management 

problems, it is important to develop sophisticated triggers to 

allow flexibility in the types of research problems that can be 

investigated with this network. 

In order to meet these monitoring and research design 

goals with available technology, the USNSN will consist of 

the following components (shown diagrammatically in figure 

17). The required dynamic range, linearity, and bandwidth of 

the seismometers dictates the use of force balance sensors. 

Even using state-of-the-art seismometry, the desired dynamic 

range will require the use of two sets of seismometers (a highgain and a low-gain sensor for each component). In order to 

preserve this dynamic range, the seismometer outputs must be 

digitized onsite. This will be accomplished by means of stateof-the-art 24-bit (144 dB) analog to digital converters. 

A station processor is required to perform the following 

functions: (1) acquire six channels of seismic data and up to 

eight channels of state-of-health data, (2) low-pass filter and 

decimate the six high-frequency (HF) channels to derive six 

broad-band (BB) channels, six long-period (LP) channels, and 

one short-period (SP) channel, (3) manage rotating buffers of 

pre-event data, ( 4) perform signal detection on the BB high-
SAN FRANCISCO AREA 

28 Ground sites 

24 Buildings 

3 Bridges 

2 Dams 

1 Tunnel 

• 

•• • 

•• ••• 

• 

/~· 

I •• 

.. ··1 I II TO ~{ I 

LOS ANGELES AREA (A; .. N5I.Ltf 

122 Ground sites 

20 Buildings 

8 Dams 

EXPlANATION 

• 

• 

• Ground site 

• Dam 

• Bridge 

• Building 

* Nuclear plant 

N 

• 

• 

• • 

• 

• • 

• • 

··: 

. 

. 

,... . . 

• U4Pfi/IIIA.l 

• • •• • • • 

• •• 

Figure 14. Known accelerographs in California as of April 30, 1981. Excludes instruments required by building codes. 

Numbers indicate where more than one station is located. 

Configuration of Proposed System 19 
1o1r------------r------------~----------~----------~ 

1o-1 

0 

w 

a: 

<( 

::l 

0 ········~···················· \ \ (/) TYPICAL ~ · Mw 5.5 •••• \ 1o-3 

a 

z 

0 

0 

w 

(/) 

NETWORK ~·~ -:-:-:-:-:-:-:-:-:-:-:--:-:-:.STATION/. -...•!» ......... ,.,_ ·······.:,:.,.-· ., ...... .,.,.. ... .,.. a: 

w 

G.. 1o-s 

/~~ _M.,a.o _,.,.. . .._ 

(/) - --;?"'------.... , a: 

w 

1-

w 

~ 

0 

.-

§ 

z 

0 

~ <( 

a: 

w 

_, 

w 

0 

0 

<( 

1o-1 

1o-9 

,o-11 

EXPLANATION 

- Earth noise -- Surface wave at 3,000 km 

------ P-wave at 3,000 km 

-·-·- Shear wave at 10 km 

··········- Shear wave at 100 km 

10-13~~---------L~----------~----------~----------~ 1o-2 10 100 

FREQUENCY, IN HERTZ 

Figure 15. Dynamic range plotted against frequency for typical regional network station 

and typical strong-motion station as well as expected levels of ground motion for different 

seismic arrivals from earthquakes of different sizes and recorded at different distances. 

gain vertical channel based on an integer fast-Fourier transform, (5) format and manage prioritized output queues of continuous and event data, (6) communicate with the satellite 

telemetry system, (7) acquire and maintain absolute time, (8) 

interpret and execute remote commands, and (9) provide 

calibration and control signals initiated by remote command. 

Ku band (14-16 GHz), time division multiple access 

(TDMA), very small aperture telecommunications (VSA n 

satellite telemetry technology has been chosen since no other 

system has been found to be nearly as cost effective. This approach requires a master station with a 4.5-7.0 m antenna at 

the NEIC to control the multiplexing of 56-96 kilobyte-persecond satellite channels and VSAT's with 1.2-3 m antennas 

and associated electronics at each field site. The system will 

have sufficient capacity to telemeter all data simultaneously 

in the event of a great earthquake in North America. Furthermore, the system will be capable of two-way communications, 

20 National Seismic System Science Plan 

thereby greatly increasing the flexibility of future trigger algorithms and station maintenance. A modified VSAT X.25 

protocol will provide error detection and correction, thereby 

providing a very low bit error rate. This will greatly simplify 

the processing at the station and the central recording site. In 

addition, the VSA T system will provide absolute time (broadcast periodically by the master station). 

Two different scenarios are being considered for the 

physical installation of the sites. In either case, the seismometers will be mounted on a concrete pad in a shallow pit 

and covered by a partially buried fiberglass dome. The seismometers will be adequately coupled to the pad and adequately thermally insulated. In the first scenario, commercial 

electric power will be available at the field site. This power 

will be filtered through an uninterruptable power supply 

(UPS) and distributed by a custom DC regulation system to 

the seismometer filter and control electronics, the station 
Figure 16. Proposed National Seismic: Network stations in the contiguous United States. Additional stations are proposed for Alaska, Hawaii, and Puerto Rico. Only stations east of the Rocky Mountains in the contiguous United States are currently funded (from 

Masse and Buland, 1987). 

processor, and the VSAT electronics which will all be housed 

in a separate, partially buried, vented enclosure. The VSA T 

antenna will be mounted on a standard kingpost set into a concrete pad. In order to minimize noise from cooling fans and 

wind coupled through the antenna, the seismometer will be 

removed from the VSAT hardware (using a hardwire cable) 

by as great a distance as is practical. 

The second installation scenario is considered to be more 

desirable, but also more costly. In this case, commercial 

electric power will be available within a few kilometers of the 

field site, but not at the field site itself. The seismometer, seismometer electronics, and the station processor will be located 

at the field site and operated by batteries recharged by solar 

panels. As no fans will be required for the electronics, all 

equipment could be housed in a single vented enclosure. The 

VSAT electronics and antenna will be placed where commercial electric power and reasonable security are available. A 

2,400-baud telemetry link will connect the seismometer and 

the VSAT sites. Although the latter scenario is somewhat more 

expensive, it provides the possibility of lower seismic noise, 

greater physical security, and greater lightning protection. If 

the VSAT electronics become available in a low-power configuration, it may be possible to eliminate the fans and commercial power at all sites. At sites where an adequate 

pre-existing borehole is available, provision is being made to 

mount the seismometer package in the borehole. 

Relationship Between Regional and National 

Seismic Networks 

Although the proposed 150-station National Seismic Network will provide exciting new waveform data on a national 

scale (only 60 stations are currently funded), it cannot perform 

the functions of the 1,600 stations currently in regional networks. In particular, the primary function of detecting and 

locating earthquakes cannot be accomplished at an acceptable 

level with only 150 stations nationwide. As an example, we 

show earthquakes located by the 75-station regional network 

operated by the University of Utah together with proposed 

sites for the National Seismic Network in figure 18. It is clear 

that the relatively low station density for the National Seismic 

Network would be inadequate to resolve the detailed patterns 

of seismicity seen with the existing regional network. As we 

discuss later, study of these seismicity patterns is vital for a 

better understanding of a wide variety of basic problems. 

The relatively high station densities of existing regional 

networks are also vital for a wide range of other important seismological problems. These problems, listed in table 4, are discussed in detail in the sections on applications and research 

possibilities for a National Seismic System. 

Even if sufficient station density were available in the National Seismic Network, regional networks would remain a 

focal point for research on important, but localized, 

Configuration of Proposed System 21 
R Ku Band U Transponder 

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Satellite 

Electronics 

Digitizer __ ....___....,/ event detector 

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Processor 

3-component 

seismometer 

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strong motion 

\ 

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w.s. 

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Earthquake identification 

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Master 1 ==========f---l==:::::Jl Station t= 

Seismic 

Data Base 

Network 

Processor 

Figure 17. Schematic representation of National Seismic Network. Three-component, force-balance, broad-band seismometers 

(both high and low sensitivity) are digitally recorded at the station for a total dynamic range of approximately 10 decades (200 

dB). Significant seismic signals detected at the station processor are telemetered via satellite to the Master Station in Golden, 

Colorado, for analysis and archival. 

earthquake and tectonic problems. Although the National 

Seismic Network is not designed to replace regional seismic 

networks, it provides an opportunity to dramatically improve 

the capabilities of regional networks. That is, the National 

Seismic Network provides regional networks with the technology for recording broad-band, high-dynamic-range, threecomponent seismic data in real time and with low telemetry 

costs. Furthermore, the National Seismic Network provides a 

communications network that will interconnect regional networks. Standardized data manipulation procedures will allow 

better access to these important data sets by all researchers. 

Need to Develop Digital Regional Networks 

We have demonstrated that the existing regional networks 

provide a vital function in the observation of seismic waves, 

and their continuing operation should have a high national 

priority. We have also demonstrated that the existing regional 

22 National Seismic System Science Plan 

networks are severely limited by the outdated technology on 

which they are based. Therefore, the upgrading of existing 

networks to digitally telemeter high-dynamic-range, broadband seismic data is a long-range goal of high priority. Unfortunately, a coordinated plan to ensure that such a goal is 

met has not yet been formulated. 

In the beginning of this decade, the seismological community recognized similar shortcomings in global seismic networks (principally the World-Wide Standardized Seismic 

Network, WWSSN) and in the field of dense portable networks. As a result, approximately 57 research institutions 

formed a nonprofit corporation, the Incorporated Research Institutes for Seismology (IRIS). IRIS has three principal goals: 

(1) develop a Global Seismic Network (GSN) of approximately 100 high-quality digital stations, (2) develop a portable network of approximately 1,000 portable digital seismic stations 

(Program for Array Seismic Studies of the Continental Lithosphere, P ASSCAL), and (3) develop a Data Management Cen-
0 

0 

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0 100 

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00 

00 

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000 0 

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0 

0 

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Figure 18. Seismicity of the intermountain region located using the 75-station regional network operated by 

the University of Utah. Earthquakes since 1962 and larger than magnitude 2.0 are plotted. Proposed station 

locations for the National Seismic Network (triangles) illustrate that the National Seismic Network is far too 

sparse to study detailed features of regional seismic activity (courtesy of R. Smith, R. Engdahl, and j. Dewey). 

Configuration of Proposed System 23 
Table 4. Problems requiring high station densities 

• Detection of microearthquakes (magnitudes less than 2.5) 

• Accurate determination of hypocentrallocations. 

• Accurate determination of earthquake rupture characteristics. 

• Rapid determination of shaking distribution for significant 

earthquakes. 

• Short-term warning of imminent strong shaking during significant earthquakes. 

• Real-time tsunami warning. 

• Volcano monitoring. 

• Determination of path effects to better predict strong-rnation characteristics at individual sites. 

• Determination of velocity distributions of the crust and the 

Earth's interior. 

ter (DMC) to store and disseminate the data from these networks. IRIS helps to coordinate activities within the United 

States to see that these important goals are met However, it is 

important to recognize that these programs do not address the 

vital problems of developing regional networks. A national 

plan to deal with the issues of regional networks should be 

developed. It is not within the scope of this document to 

specify the way in which such a plan should be developed. 

However, it is clear that the formulation of such a plan should 

be a high priority of the U.S. seismological community. 

APPLICATIONS OF A NATIONAL 

SEISMIC SYSTEM 

Estimation of Current Earthquake Risk 

When an earthquake occurs, the risk of a major 

earthquake occurring within a short period of time increases 

significantly because of the possibility that the first event is a 

foreshock. Seismologists have always become more wary in 

the first few days after an earthquake, but have not usually issued public statements because the chance of a false alarm in 

this situation is also high. In the last few years, however, more 

accurate estimates have been made of how the probability of 

a major earthquake increases following other seismic activity. 

In addition, both seismologists and emergency management 

personnel have recognized that an earthquake prediction 

should include not only time, place, and magnitude of the 

event but also the probability that the prediction is correct. 

These developments have allowed predictions of earthquake 

risk that are well above background but well below 50 percent 

probability to be issued and used. This has also increased the 

need for accurate information immediately after an 

earthquake. An example of the way that seismic risk increases 

dramatically after a potential foreshock is shown in figure 19. 

In this example, Jones (1985) shows that the probability per 

24 National Seismic System Science Plan 

0.0-r---------------------., 

-1.0 

it) 

,....: 

I\ I 

:e 

u.. -2.0 

M 6.5 at Cajon Pass 

0 

a: 

:J 

0 

:I: -3.0 a: 

w 

Q.. 

~ ~ -4.0 

m 

<{ 

CD 

0 

~ -5.0 

(!) 

0 

_J 

-6.0 

-7.0_2 -1 0 1 2 3 4 5 

TIME, IN DAYS 

Figure 19. Probability per hour of a large (~7.5) earthquake 

occurring on the Palmdale section of the southern San Andreas 

fault as a function of time when a M 6.5 earthquake occurs at 

Cajon Pass Oones, 1985). 

hour of a large (M?.7 .5) earthquake occurring on the Palmdale 

section of the San Andreas fault is a function of time when a 

M 6.5 earthquake occurs on the segment. 

OnMay27, 1980,theDirectoroftheU.S.GeologicalSurvey issued a Hazards Watch for potentially damaging 

earthquake activity in the Long Valley region of eastern 

California in the wake of the occurrence of three magnitude 6 

earthquakes two days earlier. This first public statement in the 

United States was followed by a fourth magnitude 6 event (Hill 

and others, 1985). 

The next statement from the U.S. Geological Survey 

about an increase in the probability of a damaging earthquake 

was made in June 1985 (Golz, 1985). Three M 4 earthquakes 

in San Diego increased the probability of a damaging 

earthquake to 5 percent within five days. Limitations of the 

old regional network in southern California led to delays in 

determining the location and magnitudes of these smaller 

earthquakes. However, because of the location of these 

earthquakes directly under a city of 1 million people, the 

California State Office of Emergency Services was notified of 

the increased probability as soon as it was recognized, four 

hours after the start of the sequence. San Diego responded by 

putting disaster management personnel on alert, checking 

water supplies and moving frre engines outdoors, appropriate 

for a 5 percent chance of having an earthquake. 

Plans are being made to issue a similar short-term warning if foreshocks precede the Parkfield earthquake (Bakun and 

others, 1986). Parkfield is a site on the San Andreas fault 

where moderate earthquakes (M approx 6) have occurred on 
the average once every 22 years. Because of this apparent 

repeatability, an intermediate-term prediction has been issued 

for another M 6 event by 1993. In preparing for this 

earthquake, the U.S. Geological Survey is working with the 

California Office of Emergency Services to develop response 

scenarios for possible changes in the Earth that might precede 

the Parkfield earthquake. In particular, USGS seismologists 

have determined the probabilities of the Parkfield earthquake 

occurring within three days after earthquake activity on the 

San Andreas fault (figure 20). These probabilities range from 

1 to 2 percent for aM 2 earthquake to over 35 percent for aM 

4.5 event These probabilities have been assigned to alert 

levels, such that during a level A alert, the chance of the 

Parkfield earthquake occurring is greater than 35 percent; 

during a level B alert, the chance is 10 percent to 35 percent, 

and so forth. The Office of Emergency Services has 

developed appropriate response plans for each alert level 

(State of California, 1988). Thus when an alert is actually 

called, information can be quickly and efficiently exchanged 

and plans activated because all of the decisions for that alert 

level have already been made. 

A crucial element of the Parkfield plans is real-time location of earthquakes. Studies have shown that the increase in 

probability after a potential foreshock is concentrated in the 

frrst few hours after the event; one quarter of all foreshocks 

occur within one hour of their mainshock. The foreshock to 

the last Parkfield earthquake occurred only 17 minutes before 

the mainshock. Thus an extensive network has been installed 

in the Parkfield area and new computer systems developed to 

produce locations and magnitude estimates for earthquakes in 

real time. 

New computer systems would allow real-time assessments similar to those at Parkfield to be made in other regions 

as well. For instance, a moderate earthquake on the southern 

San Andreas fault, like the North Palm Springs earthquake of 

July 1986, has been estimated to have a 10 percent chance of 

being followed by aM 8 great earthquake. However, six hours 

after such a moderate earthquake, the probability of a M 8 

earthquake occurring is down to 5 percent; thus, quick 

response is essential. Although the chance of a false alarmthat the earthquake will not occur -is 90 percent, disaster 

planners have stated that a warning issued on this basis would 

be useful to them. Responses to such a warning could involve 

canceling vacations for emergency response personnel, 

moving fire engines outdoors, delaying toxic waste disposal 

operations, and many other steps. 

Seismicity patterns on longer time scales than immediate 

foreshocks may also reflect changes in the earthquake hazard. 

As these are better understood, it will be possible to have 

earthquake risk maps that change with time, reflecting the 

probability of earthquakes over time scales of weeks and 

months. Significant seismic sequences that are potential 

foreshocks to hazardous earthquakes will continue to occur, 

and seismologists must be prepared to provide useful, timely 

information to mitigate potential hazards. 

' en Level a a: -t- :::::> 

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m 

0 

a: 

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0.001 ~-....___._ __ ._....;,___ __ __._ ___ .___....._~ 

0 1 2 3 4 

MAGNITUDE (M) OF POTENTIAL FORESHOCK 

Figure 20. Probability of a characteristic Parkfield, California, 

earthquake in 24 hours following occurrence of potential 

foreshock of magnitude M. Levels a through e refer to alert 

levels as defined in Bakun and others (1986). 

Short-Term Warning of Imminent Ground Shaking 

In earthquakes of great fault length, substantial damage 

often occurs at great distances from the earthquake's 

epicenter. Because of the relatively slow speed of seismic 

waves, it is possible to electronically warn a region of imminent strong shaking as much as several tens of seconds 

before the onset of very strong shaking. Automated safety 

responses could be triggered by users after receiving estimates 

of the arrival time and strength of shaking expected at an individual site. 

The great earthquake of 1857 that ruptured a 300-km-long 

segment of the San Andreas fault in southern California is an 

example of how a Seismic Computerized Alert Network 

(SCAN) could provide more than a minute of warning time 

before the occurrence of strong shaking in a heavily populated 

area. There is evidence that the rupture initiated in the vicinity 

of Parkfield, a small town 275 km northwest of metropolitan 

Los Angeles. It seems likely that the rupture propagated south 

toward the Los Angeles region at a velocity of about 3 km/s 

or less, and the strongest shaking in the Los Angeles region 

probably occurred at least 100 seconds after the ground began 

to shake at Parkfield. 

Applications of a National System 25 
In similar earthquakes, a SCAN could provide users with 

information during this time so that they could initiate certain 

safety precautions. The most suitable applications are those 

operations that come under computer control and can be 

safeguarded quickly. For example, a SCAN could initiate 

electrical isolation and protection of delicate computer systems, isolation of electric power grids to avoid widespread 

blackouts, protection of hazardous chemical systems and offshore oil facilities, closing of natural gas valves to minimize 

fire hazards, warning of nuclear power plants and national 

defense facilities, protection of emergency facilities such as 

hospitals and fire stations, and protection of fixed rail transportation systems. It may even be possible to provide protection 

to individuals in hazardous structures. For instance, structurally strong areas could be built in schoolrooms (such as a 

heavily reinforced table) to which students could rapidly 

evacuate. 

Heaton (1985) discussed the basic principles and expected performance of a SCAN of the type shown schematically in figure 21. Ground motions recorded by a dense array 

of broad-band, high dynamic range seismometers are digitally telemetered to a central processing site. The occurrence of 

a large earthquake is detected and the location, time of origin, 

amplitude, and reliability estimates are transmitted instantly 

to microcomputers operated by individual users. The user's 

computer then combines this information with that about the 

user's site (for example location and geologic conditions of 

the site) to estimate the arrival time and nature of motions expected at the site. Decisions already programmed into the 

user's computer are then made on the basis of this information 

and the appropriate action is taken. 

The problem of false alarms is minimized by continuous 

updates regarding the size of the ground motions at differing 

stations in the seismometer array. If the user is close to the 

epicenter of a developing earthquake, then the user's processor will recognize that little time is available to receive further 

information and immediate action may be appropriate. 

However, if the user is far from the epicenter, then considerable time is available before shaking begins. This time 

may be used to receive further information about the size of 

the earthquake. In this way, users at large epicentral distances take action for only the large earthquakes that present a 

real hazard, and each user adjusts the decision-making process 

to the needs of the site. The use of the seismometer array for 

the routine research study of numerous small local 

earthquakes will help to ensure that the system is maintained 

in good working order. 

The circumstances surrounding the great 1857 

earthquake, a damaging earthquake in Los Angeles with a very 

distant epicenter, are optimal for the operation of a SCAN system. However, there are a number of examples where damage 

is more localized to the epicentral regions such as was the case 

for the 1971 San Fernando earthquake or the 1933 Long Beach 

earthquake. In these cases only several seconds of warning 

could be expected in heavily damaged areas. Heaton (1985) 

26 National Seismic System Science Plan 

provided estimates of the probability distribution that a site 

which experiences a specified strength of shaking will also 

receive a given warning time. The distribution of warning 

time as a function of peak acceleration is shown in figure 22. 

The contours give the probability that a user will receive at 

least a certain warning time in the event of at least a certain 

value of acceleration. The total area of southern California to 

receive a given peak acceleration or greater in a 100-year 

period is shown to the right. This number may exceed the total 

area of southern California (about 3 x 105 km2

) because many 

areas will experience low values of acceleration several times 

in a 100-year period. 

The expected warning time is long at both low ( <0.1 g) 

and high (>0.3 g) values of acceleration, but the expected 

warning time is short for moderate (0.1 to 0.3 g) values. Because small accelerations occur at large distances between site 

and fault, the warning time is large for small accelerations. In 

this model, accelerations of0.2 g are most likely to occur close 

to the numerous moderate-size earthquakes, and hence the expected warning time is short. However, large accelerations 

result from large earthquakes of long rupture length. Thus 

areas that receive large accelerations can also ex pee t to receive 

large warning times. 

The 27 March 1964 Alaskan earthquake (Mw 9.2) is the 

largest earthquake documented in U.S. history and it is the 

second largest earthquake of this century. The fault rupture 

extended more than 600 km in length and had a width of more 

than 200 km. Very strong shaking of unusually long duration 

(several minutes) occurred over a very large region, and devastating local tsunamis were generated (National Academy of 

Sciences, 1972). The strong shaking and giant tsunamis extensively damaged cities and transportation links within the 

regions of Prince William Sound, Kenai Peninsula, Kodiak Islands, and Cook Inlet. Because of the very large rupture 

dimensions, most of the region that was strongly shaken would 

hypothetically have received many tens of seconds of warning if a SCAN system had been operational. This giant 

earthquake was the result of thrusting of the North American 

continental plate over the Pacific Ocean plate, a process 

known as subduction. Similar subduction processes are 

known to occur along most of Alaska's southern coast and also 

along the entire length of the Aleutian Island chain. 

The Cascadia subduction zone is a 1,200-km-long plate 

boundary in the Pacific Northwest along which the North 

American plate overthrusts the Gorda, Juan de Fuca, and Explorer oceanic plates. This plate boundary extends from northern California to Vancouver Island, British Columbia. 

Although there have not been large historic subduction 

earthquakes on this zone, recent studies indicate that very large 

subduction earthquakes (perhaps as large as Mw 9.5) may 

occur there (Heaton and Hartzell, 1987; Atwater, 1987). If 

subduction earthquakes occur on this zone, then relatively 

strong shaking can be expected over a large area of the Pacific 

Northwest, including the Puget Sound and Willamette Valley 

regions (Seattle and Portland). Because of the potential for 
Remote 

seismic 

stations 

Centralprocessing 

station 

Input: ground 

motions 

Processing 

Output: location, 

size, 

reliability User 

processing 

Users 

Power 

(plants and 

grids) 

Emergency 

services 

Computer 

facilities 

Transportation 

systems 

National 

defense 

systems 

Decisionmaking data 

Figure 21. Conceptual design of a Seismic Computerized Alert Network (SCAN). The purpose of this system is to provide very 

short-term prediction of the arrival time and size of imminent strong shaking to areas at some distance from an earthquake's 

epicenter. The system relies on the relatively slow speed of seismic waves (approximately 3 km/s) compared with electronic communications. The system would also provide important emergency information immediately after a damaging earthquake (Heaton, 

1985). 

earthquakes of very large rupture dimensions, a SCAN system 

may provide many tens of seconds of warning in advance of 

very strong shaking for great subduction earthquakes in the 

Pacific Northwest. 

Large historic earthquakes have also occurred in the 

United States that are far from known plate boundaries. 

Specifically, large earthquakes occurred in the central United 

States (New Madrid) in 1811 and 1812 and also in the 

southeastern United States (Charleston) in 1886. Although the 

mechanisms of these events are poorly understood, these 

events probably do not involve large rupture dimensions. 

Nevertheless, the felt areas of these earthquakes were larger 

than those for the largest California earthquakes (Nuttli and 

Zollweg, 1974). It is generally felt that the principal reason 

for this phenomenon is a lesser degree of attenuation of seismic waves east of the Rocky Mountains. Because rupture 

lengths of great earthquakes in the central and Eastern United 

States may be less than 50 km, the regions of strongest shaking that lie adjacent to the rupture zone are not likely to receive 

large warning times. However, Rossi-Forel intensities of IX 

and VIII may have extended to distances of 100 and 200 km, 

respectively, for the 1811 and 1812 New Madrid earthquakes. 

This means that a SCAN system could still provide a significant warning time for areas shaken strongly enough to 

cause great damage during great earthquakes in the central and 

Eastern United States. 

Rapid Estimation of Shaking Intensity 

Past experience has proved that there is always great confusion immediately following damaging earthquakes. Very 

heavy loads are put on communication lines at a time that they 

Applications of a National System 27 
1.2 

1.0 

0.8 

0.6 

0.4 

0.2 

§ 

z 

0 

~ a: 

w 1.2 ...J 

w 

u 

u 

<{ 

1.0 

0.8 

0.6 

0.4 

0.2 

0 

3x102 

1()3 

3x103 

104 0 

w 

a: 

3x104 ~ d 

CJ) 

106 ~ 

w 

~ 

106 ~ ~~~----~--~~--_.----~---- g 30 60 90 120 150 180 S2 

10 20 30 

TIME (t), IN SECONDS 

z 

0 § 

3x102 ct 

w 

a: 1Q3 <{ 

...J 

g 

~ 

3x103 

104 

3x104 

106 

Figure 22. Theoretical calculation of distribution of warning 

times for SCAN system in southern California as a function of 

expected peak ground acceleration. Contours denote percent 

of area having peak acceleration of g or greater that receives 

a warning time oft or greater. Sis total area to receive a peak 

acceleration of g or greater in a 1 00-year period in southern 

California. Warning time is given relative to arrival time of 

maximum shaking. Bottom part of figure is same as top, but 

with an expanded time scale (Heaton, 1985). 

may already be damaged during the earthquake. It is very difficult for emergency management professionals to rapidly assess the nature of the crisis. Unfortunately, seismologists have 

not always provided much assistance because most of their 

available records are completely Qff scale. However with a 

high dynamic range, digitally telemetered seismic network of 

the type that is necessary for a SCAN system, it would be possible for seismologists to provide a very rapid assessment of 

the severity of ground shaking for different regions. 

The California State Office of Emergency Services is4prrently developing plans for a southern California emergency 

28 National Seismic System Science Plan 

response center that will be responsible for coordinating emergency services after a damaging earthquake. Recent 

earthquake disasters, such as those in Mexico City, have emphasized the importance of rapid rescue and relief activities. 

The cost of inadequate response can be many lives. In an area 

as extensive as southern California, it may be very time consuming for officials to assess the true nature of an earthquake 

disaster. Estimates of the strong shaking could be relayed 

directly to the State's emergency response center. These estimates will allow rapid assessment of the overall extent of the 

disaster. It may even be possible to automatically project 

earthquake damage based on this incoming data. For example, 

the Southern California Association of Governments is currently developing a geographic relational data base using census statistics. They plan to use this data base to model 

hypothetical earthquake disasters assuming several different 

earthquake scenarios. Included in the data base are such items 

as hospitals, lifelines, transportation links, hazardous 

facilities, and current distribution of population (how many 

people are away from their homes?). Once such a system is 

developed, it should be possible to input estimates of the actual shaking immediately after an earthquake in order to anticipate the most immediate emergencies. 

Rapid estimation of earthquake damage is also of vital interest to many other organizations. For instance, our national 

defense system may receive severe damage during a large 

earthquake. Proper and timely reallocation of resources will 

depend on accurate estimates of the extent of the earthquake. 

This post -earthquake damage information is also of obvious 

value to the repair and protection of lifeline systems 

(aqueducts, electrical power grids, telecommunications, 

natural gas, and so on). 

Tsunami Warning 

Tsunamis are long -period (usually tens of minutes) ocean 

waves that are commonly generated by large earthquakes 

beneath the ocean. In general, the largest earthquakes are 

responsible for the largest tsunamis. Tsunamis are also sometimes generated by underwater volcanoes and perhaps by underwater landslides. Several devastating tsunamis that 

occurred in Hawaii (1946, 1960, 1964) were actually 

generated by earthquakes several thousands of kilometers distant from Hawaii. In these instances, the tsunami waves, 

which travel less than 1,000 km/hr, took many hours to 

traverse the Pacific Ocean to Hawaii. Much of our present 

tsunami warning system is based on the premise that warning 

will be given for populated coastlines for tsunamis that are 

generated in remote and distant regions of the Pacific. 

However, very large tsunamis with run up heights in excess of 

20m have struck U.S. coastlines from nearby earthquakes in 

1868 (Hawaii), 1946 (Aleutian Islands), 1958 (Alaska), and 

1964 (Alaska). Tsunamis that occur in the region of the 

generating earthquake are referred to as local tsunamis. Local 

tsunamis can be particularly dangerous because they can be 
160° 120° so• 

45° 

1 

• 

15° 

6 

Figure 23. Locations of probable Holocene volcanism (last 10,000 years). Volcanoes with known eruptions since 1880 are shown 

by filled circles. Volcanoes with dated eruptions, but none since 1880, are shown by open circles, and those with undated but 

probable Holocene eruptions are shown by triangles. Volcanoes with uncertain or only solfataric activity are shown by a small x. 

Volcanic belts that are numbered correspond to table 5 (modified from Simkin and Siebert, 1984) 

type that threaten high-rise buildings was also observed in the 

Los Angeles and San Fernando basins in the 1971 San Fernando earthquake. Figure 24 shows a profile of ground velocity 

records across these basins (Liu and Heaton, 1984). Individual 

sets of surface waves are developed within these basins, and 

these surface waves control the duration and peak amplitude 

of the longer period parts of the ground motion. 

The effects of propagation of strong ground motions in 

complex geologic structures, such as basins, are usually included in earthquake design studies as a simple scalar site 

amplification factor whose value is determined by the local 

site condition (hard rock, intermediate, soil). Such a procedure cannot adequately characterize the phenomena that make 

30 National Seismic System Science Plan 

Table 5. Holocene volcanism of North America and Hawaii 

[from Simkin and Siebert (1984)] 

Length Number of Eruptions 

Belt name (km) Holocene since 1880 

volcanoes 

l. Aleutians 1,457 40 203 

2. Alaska Peninsula 944 33 120 

3. Cascades 1,152 38 141 

4. Mexico 1,043 22 64 

5. Central America 1,254 79 35 

6. West Indies 632 17 59 

7. Hawaii 174 6 114 
exceedingly large and because they may strike within 15 

minutes of the causative earthquake. Although most of the 

coastal areas of the contiguous United States have not experienced historic devastating tsunamis, there is evidence that 

large tsunamis from great subduction earthquakes may present 

a severe problem in the Pacific Northwest (Heaton and 

Hartzell, 1987; Atwater, 1987). Furthermore, it is difficult to 

preclude the possibility of damaging tsunamis along any U.S. 

coastal region. 

Kanamori (1985) presented a methodology for determining tsunami sizes from near-field ground motions that occur 

within the first several minutes of large coastal earthquakes. 

Furthermore, reasonably precise predictions of local tsunami 

runup heights are now feasible using complex models of sea 

waves in detailed models of seafloor bathymetry (Satake, 

1987). However, on-scale measurements of long-period 

ground motions in the near-source region oflarge earthquakes 

must be available in real time in order to provide a working 

local tsunami warning system (Bernard and others, 1988). 

Clearly, regional networks with seismic instrumentation, communication, and real-time analysis systems of the type 

proposed for the National Seismic Network would be able to 

meet these needs. 

Volcano Monitoring 

On 20 March 1980 the regional seismic network operated 

by the University of Washington detected small earthquakes 

beneath usually quiet Mount St. Helens. Over the next two 

months, seismic activity increased dramatically as the volcano 

experienced several small phreati~ (steam-blast) eruptions and 

the flank of the volcano bulged dramatically. Because of this 

precursory activity, thousands of lives were saved from the 

catastrophic eruption of 18 May 1980. Careful monitoring of 

seismicity in the Mount St. Helens region was a key tool for 

the prediction of numerous other eruptions over the next 

several years (Swanson and others, 1983). Seismic monitoring has also been a key tool in the prediction of numerous eruptions in Hawaii (Klein, 1984; Klein and others, 1987). Smith 

and Luedke ( 1984) estimate that there are approximate! y 7 5 

volcanoes distributed in 11 Western States of the conterminous United States that have potential for future eruptions. 

In addition, there are 33 Holocene volcanoes on the Alaskan 

peninsula, 40 in the Aleutian Island chain, and six in the 

Hawaiian Islands (Simkin and Siebert, 1984). Regions of the 

United States that have a potential for future volcanic activity 

are shown on figure 23 and summarized in table 5. 

Not all large explosive eruptions are preceded by significant periods of precursory eruptive activity. Simkin and 

Siebert ( 1984) reported that of 205 of the largest documented 

eruptions, 92 occurred within a day of the onset of eruptive activity. No precursory eruptions were reported for the largest 

volcanic eruption this century, which occurred in 1912 at 

Alaska's Katmai volcano. However, earthquake activity was 

noted for several days before the main eruption (Bullard, 

1962) and most (if not all) great volcanic eruptions are 

preceded by a significant seismicity precursor. "Harmonic 

tremor" or "volcanic tremor," which is characterized by a 

nearly continuous oscillation of the ground, seems to be a 

phenomenon that is particularly diagnostic of magmatic activity. As is the case with earthquake ground motions, the 

range of amplitudes of ground motion during harmonic tremor 

is very large. Because of the unusual nature of the seismic 

source for harmonic tremor, this phenomenon is best studied 

with three-component, broad-band instrumentation. 

Because there are so many potentially active U.S. volcanoes that may have little in the way of precursory eruptions, 

the monitoring of seismic activity is of vital importance for 

U.S. volcano prediction. This monitoring requires relatively 

dense seismic networks throughout the Western United States, 

Alaska, and Hawaii. Furthermore, it is important to monitor 

seismicity from volcanic regions in near-real time. When 

visibility is limited (as it often is on large volcanoes), seismic 

monitoring can also be important for the recognition of significant eruptions that may trigger dangerous lahars 

(mudflows often triggered by the rapid melting of snow and 

ice) as occurred with tragic results in 1985 at Colombia's Ruiz 

volcano, killing over 20,000 people. In addition, even 

moderate-sized eruptions can send ash into the atmosphere 

that can be a serious hazard for aircraft. 

Volcano monitoring is already an important task of existing regional seismic networks. The upgrading of these networks to include three-component, high-dynamic-range 

seismometers, and the development of systems to analyze seismicity in real time will greatly enhance the ability to recognize, understand, and respond to U.S. volcanic hazards. 

Prediction of Site Effects in 

Strong Ground Motions 

One of the most critical factors in the engineering of structures is anticipating the nature of the ground motions that may 

be experienced by the structure. Earthquake ground motions 

are determined by the nature of the seismic energy radiated by 

the source and also by the way in which the seismic energy 

propagates through the medium to the site. A complete physical description of this process is exceedingly complex, and the 

current practice in earthquake engineering is to simplify this 

problem by parameterizing the earthquake source by magnitude and the effects of propagation by distance between the 

earthquake and the site. Unfortunately, this parameterization 

can be tragically inadequate, as was the case for the 1985 

Michoacan earthquake in which thousands lost their lives in 

Mexico City. Although Mexico City was over 350 km from 

this earthquake (the distance between Los Angeles and Las 

Vegas), there was heavy damage to earthquake-engineered 

structures caused by ground motions that were strongly 

amplified by the sedimentary structures beneath the city. Significant amplification of longer period ground motions of the 

Applications of a National System 29 
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earthquake. Records are plotted as a function of epicentral distance along a profile (top) running south across the San 

Fernando Valley and the Los Angeles basin. Corresponding free-surface and basement-surface profiles are shown in 

the center. Dashed lines in records (bottom) indicate probable phase arrival of surface waves. Note that the apparent 

surface waves seen within the basins do not appear to propagate across the Santa Monica Mountains (from Liu and 

Heaton, 1984). 

sites at equal distance from the same earthquake experience 

very different ground motions. Response spectra from M 6.5 

strike-slip earthquakes that were observed at a distance of 50 

km are shown in figure 25. It is clear that simply knowing distance, magnitude, and soil condition still leaves an order of 

magnitude uncertainty in the estimation of ground motions. 

Fortunately, we needn't wait to record destructive ground 

motions at a site before we can anticipate dangerous amplification effects. Since wave propagation in earthquakes is by and 

large a linear process, we can infer the effects of propagation 

by the study of weak ground motions from numerous smaller 

earthquakes. By studying these smaller motions from a 

variety of sources, we can understand which effects are stable 

with respect to the geometry of the source and the site. Ground 

motions recorded by a digital telemetered network will be 

ideally suited for these types of studies. In order to increase 

station coverage to get an even more detailed understanding 

of the variations of ground motion with site location, the array 

would be temporarily supplemented with portable stations that 

will be occupied only long enough to record data from several 

sources which could even be artificial sources (quarry blasts, 

Nevada test site, and so forth). 

Applications of a National System 31 
0.1 10 

PERIOD, IN SECONDS 

Figure 25. Response spectra (3 percent damped) for horizontal 

comp_onents of 15 records from strike-slip earthquakes, scaled 

to a d1stance of 50 km and a magnitude of 6.5 (from Heaton and 

others, 1986). Such scatter is typical of ground motions 

recorded at a given distance and magnitude. 

RESEARCH POSSIBILITIES OF THREECOMPONENT DIGITAL REGIONAL NETWORKS 

The approximate! y 1 ,600 seismic stations in existing U.S. 

regional networks have the potential to form the world's 

largest and densest seismic array. This array could be used for 

highly innovative studies of earthquakes, the structure of our 

continent, and the structure and dynamics of the Earth. Unfortunately, the limited capabilities of the instrumentation in 

these arrays and the lack of standardization have severely 

limited the scope of research problems that have utilized 

regional array data. We now briefly summarize some of the 

research problems that will be analyzed using data from 

regional networks of modem three-component digital instruments. 

Earthquake Studies 

Earthquakes are the result of sudden changes in elastic 

strain within the Earth. Elastic strains within the Earth may 

be caused by several mechanisms that include the overall shifting of the Earth's plates (by far the most important), the migra32 National Seismic System Science Plan 

tion of magma to the Earth's surface, readjustments to load on 

the Earth's surface (rebound of mountains due to erosion or 

glaciers), and injection or withdrawal of fluids. We can infer 

that some of these processes are occurring throughout the 

Earth. The Earth is constantly deforming, and sometimes this 

deformation is accompanied by earthquakes. However, most 

of the deformation within the Earth occurs slowly and steadily without earthquakes. When and where does deformation 

occur through unstable slip (earthquakes)? We wish to be able 

to better predict how large earthquakes will be and how often 

they will occur. We seek a better understanding of the fundamental physics of deformation in the Earth. 

Seismicity Patterns 

Perhaps the key motivation for constructing the existing 

regional seismic networks is the study of patterns of small 

earthquake occurrence. This is a very large subject with many 

volumes of literature, and only a few points can be made here. 

Seismic activity in California for 1980 through 1986 is shown 

in figure 4 (D.P. Hill, written commun., 1987). Data from 

more than 600 stations in three regional networks were combined to construct this figure. A number of major seismic 

lineations can be seen, and the majority of these coincide with 

recognized active faults. Some seismicity lineations occur in 

broad zones for which there is not an identifiable surface expression. One of the most striking features of this seismicity 

is the paucity of small earthquakes along the stretches of the 

San Andreas fault that experienced great earthquakes in 1857 

and 1906. Stretches of the fault that are slipping steadily 

(creep) experienced the highest rate of small earthquake activity. Study of spatial patterns of small earthquakes helps to 

delineate changes in the physical properties of faults. These 

physical properties determine the size and frequency with 

which large earthquakes will occur. Bakun (1980) studied 

spatial seismicity patterns on the Calaveras fault of central 

California and identified a segment of the fault that was ultimately the source of the ML 6.2, 24 April1984, Morgan Hill 

earthquake. An example of the nature of the detailed seismicity patterns that can be seen in the vicinity of the Morgan 

Hill earthquake is shown in figure 26. 

The study of distributions of seismicity is also very important in other parts of the United States. Seismicity defines 

the geometry of subduction zones in the Pacific Northwest, 

Alaska, Aleutian Islands, and the Commonwealth of Puerto 

Fi~ure 26. Stereoscopic views of locations and fault plane orientations for ~ftershocksofthe M 6.2 1984 Morgan Hill earthquake 

(Oppenheimer and others, 1988). Each fault-plane solution is 

represented by a circle centered on a hypocenter and oriented 

in a preferred plane of slip. The line through the diameter is 

parallel to the slip vector. A, View from directly above showing 

s~rface fault traces. 8, Oblique view from south. C, Oblique 

v1ew from southeast along strike of Calaveras fault. Figure can 

be viewed with any standard stereoscope. 
A 

Research Possibilities of Regional Networks 33 
Rico. Seismicity patterns also help to define seismic gaps that 

may experience large earthquakes. Although there have been 

very large historic earthquakes in the Eastern United States, 

surface traces of the causative faults have not yet been identified. Nevertheless, the distribution of small earthquakes in 

the eastern United States allows the identification oflineations 

and zones of activity. Seismic activity in the central United 

States located with the regional network that is operated by St. 

Louis University is shown in figure 27. This was the source 

region of three very large earthquakes in 1811 and 1812 (Nutill, 1973). Although very little has been known about the 

source of these earthquakes, modem seismicity in the region 

reveals a well-defined pattern of lineations that are thought to 

define the extent of the faults that were responsible for these 

important earthquakes. The distribution of seismicity is also 

the key tool for mapping the magma systems in active volcanoes. A cross section of seismicity beneath Hawaii's 

Kilauea Volcano (fig. 28; Klein and others, 1987) reveals a 

striking 40-km-deep pipe of seismicity that is believed to trace 

the magma supply system through the lithosphere. 

Large earthquakes along subduction zones occur deep 

within the Earth and are almost never accompanied by recognizable surface rupture. Furthermore, many important shallow crustal earthquakes, such as those that occur in California, 

do not cause surface rupture. Recent examples are theM 6.5, 

2 May 1983, Coalinga earthquake, theM 6.2, 24 April1984, 

Morgan Hill earthquake, and theM 5.6, 8 July 1986, North 

Palm Springs earthquake. The spatial distribution of aftershocks from such earthquakes is the most important tool for 

defining the geometry and extent of the rupture surface for 

many earthquakes. Data from regional networks are essential 

for the study of aftershock distributions, and the absence of 

this information can result in fundamental misunderstandings 

of the nature of specific earthquakes. 

Important patterns in the temporal distribution of 

earthquakes have also been observed. The occurrence of 

foreshocks and aftershocks are the most obvious examples of 

temporal patterns. The temporal distribution of foreshocks and 

aftershocks appears to vary regionally, and this variation 

seems to correlate with the tectonic environment. For example, swarms and multiple earthquakes (several mainshocks) often occur in extensional environments. In addition, 

Tajima and Kanamori (1985) reported that the temporal expansion of spatial distributions of aftershocks is small for subduction zones that are mostly locked, and the aftershock 

expansion is large for subduction zones that are slipping 

without large earthquakes. There are also indications that the 

average rate of seismicity may vary systematically with time 

for a given region. For instance, Ellsworth and others (1981) 

reported that seismic activity in central California was considerably higher in the half century preceding the great 1906 

earthquake than in the half century following it. There are also 

several examples in which significant southern California 

earthquakes have been preceded by weeks to years of higher 

seismicity (Heaton, 1987). In addition, there is evidence that 

34 National Seismic System Science Plan 

some earthquakes are preceded by months to years of seismic 

quiescence (Wyss and Habermann, 1988). 

The existing U.S. regional seismic networks were 

designed primarily as a tool for the study of seismicity patterns, and in the last 10 years they have enabled a quantum 

leap in the knowledge of seismicity. However, even seismicity studies suffer from the limited capabilities of the existing networks. Limitations of the existing data have severely 

restricted the information available in these studies. 

Earthquake locations and magnitudes are generally well determined, particularly for the numerous small-magnitude events, 

as are focal mechanism solutions for most of the larger magnitude events. However, other essential physical measures of 

the earthquakes, such as moment tensor solutions, stress drop, 

and source dimension, are absent from virtually all catalogs 

and thus severely restrict the types of research that can be performed. 

Most regional network stations record only the vertical 

component of ground motion, and therefore S-wave arrivals 

cannot be reliably recognized in many instances. These Sphases are of great value in the determination of hypocentral 

depth, a parameter of great significance to our understanding 

of earthquake physics. Furthermore, the limited dynamic 

range of the existing networks makes it very difficult to recognize significant events that may occur during crisis periods 

when the network is already off scale from events that have 

just occurred. Determination of earthquake size or magnitude 

is also an important problem that is limited by the capabilities 

of existing networks. Magnitudes are generally determined in 

a relative way with respect to a particular type and configuration of instruments. However, the configuration of any network changes in time, and it is difficult to know if an 

earthquake assigned a particular magnitude decades ago 

would still be given that same magnitude today or in the future. Problems of this type are eliminated by modem digital 

seismometers since they essentially record absolute levels of 

ground motion given in standard units. The use of three-component, high-dynamic-range seismometers in regional seismic 

networks promises to bring another quantum leap in our understanding of the spatial and temporal patterns of earthquake 

occurrence. 

Source Characteristics 

Ideally, the goal of seismic source studies is to achieve a 

sufficient knowledge of the physics of earthquake rupture so 

that the detailed rupture behavior of future earthquakes could 

be predicted in advance given knowledge of the Earth's 

mechanical properties and geological structure. To achieve 

this goal it is necessary to learn the state of stress within the 

Earth and the distribution of strong and weak zones within the 

Earth. In addition, we must be able to model the process of 

spontaneous rupture that occurs in heterogeneous regions. 

While the gross kinematic aspects of earthquake sources 

are fairly well understood, their behavior is poorly known on 
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Figure 27. Seismicity in the central United States, 1974 through 1987 (2,900 events), located by seismic networks operated by St. 

Louis University, Memphis State University, the University of Kentucky, and the University of Michigan. A series of three very large 

earthquakes occurred in this region in 1811 and 1812 (Nuttli, 1973), and a repeat of similar events could result in catastrophic consequences (unpublished map courtesy of Robert Herrmann, St. Louis University). 

time scales short compared to the total source duration and on 

distance scales short compared to the source extent. From an 

observational standpoint it is very important to analyze a large 

number of earthquakes in order to characterize the amount of 

nonuniformity in the distribution of slip on faults and the degree of irregularity with which rupture propagates along faults. 

Once these irregularities are observed, we must attempt to 

determine the irregular stress conditions and fault strength distributions that would result in the observed rupture behavior. 

In particular, recent advances have been made in the field of 

Research Possibilities of Regional Networks 35 
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DISTANCE, IN KILOMETERS 

Figure 28. East-west cross section of seismicity through Kilauea 

Volcano, Hawaii, from the surface to the deepest seismicity at 

60 km. Earthquakes, for the time period 1970 to 1983, appear 

to define the conduit through which magma ascends beneath 

Kilauea Volcano (Klein and others, 1987). 

rock mechanics in the understanding of the mechanical 

properties of faults through state-variable friction laws. Using 

these laws, the material properties of faults can be characterized by two position-dependent material properties and a 

position- and time-dependent state variable. These 

parameters govern the quasistatic and dynamic behavior of 

faults, and are related to other properties such as end-zone 

length. By observing rupture behavior in earthquakes, it may 

be possible to infer the values of these parameters at depth on 

real faults. 

A second poorly understood characteristic of earthquake 

sources is their geometry at depth and its relation to the 

dynamics of rupture and the resulting ground motions. Surface faulting is often observed to consist of a complex group 

of en echelon cracks, often with associated Riedel shears. 

Mapped faults are known to have bends and jogs, which have, 

in some cases, appeared to affect the process of rupture on the 

fault. It is very important to attempt to deduce the geometry 

of faulting at depth from the radiated motions, both to learn 

the geometry of the fault itself and to understand how rupture 

actually propagates when complicated zones of weakness are 

present and offer potential avenues for slip. Such information 

may ultimately be used to predict the characteristics of future 

earthquakes and their associated ground motions given a 

known fault geometry. 

Numerical modeling of ground motion is the primary tool 

for studying seismic sources. Ground-motion data are inverted to discover the slip history as a function of time and 

space during earthquakes. Of course it is essential to under36 National Seismic System Science Plan 

stand and account for the effects of wave propagation on the 

recorded ground motions. These propagation effects are best 

studied using ground motions from small earthquakes that 

have relatively simple rupture histories. By studying the way 

that ground motions vary with source location and fault orientation, we can account for these effects when modeling ground 

motions from significant earthquakes. In order to accomplish 

this, we need three components of ground motion data that are 

spatially well distributed in regions that may produce significant earthquakes. High dynamic range is of very great importance since we require ground motions from very small and 

very large earthquakes that are recorded at the same site. Unfortunately, sensitive seismometers and strong-motion accelerometers have very rarely been collocated. Furthermore, 

the fact that current seismic networks record only the vertical 

component of motion over a very limited frequency band and 

amplitude range has severely limited their usefulness in seismic source studies. 

An example of the way that ground motions recorded on 

dense regional networks can be used to study earthquake rupture is shown in figure 29 (Mendoza and Hartzell, 1988). The 

slip distribution on the Calaveras fault that best models strong 

ground motions observed during the 1984 Morgan Hill 

earthquake (Hartzell and Heaton, 1986) is shown together with 

the aftershock distribution (Cockerham and Eaton, 1987) 

determined using the USGS central California regional seismic network. It appears that the slip is highly heterogeneous 

and that the regions of high slip are characterized by a relative 

paucity of aftershocks. This same feature has been inferred in 

several other instances (Mendoza and Hartzell, 1988). 

Strong-Motion Attenuation 

Engineering design decisions are based partly on expected peak values of ground motion (acceleration, velocity, 

displacement) for the maximum expected earthquake at a 

given distance. The process used to estimate these values 

relies on strong-motion attenuation curves with distance. 

These curves are usually obtained from regression analysis of 

existing measurements of ground motion. Difficulties often 

arise, however, from the limited nature of the strong ground 

motion data set. It seems apparent that there are regional differences in these attenuation laws. For instance, strong ground 

motion in the Eastern and central United States seems to decay 

less rapidly with distance than in the southwestern United 

States. Although there is a fairly extensive strong-motion data 

set for the southwestern United States, there are few records 

for other parts of the country. However, ground-motion data 

from small earthquakes can be used to study the distance attenuation laws that would be expected during large 

earthquakes. Regional networks equipped to record broadband high-dynamic-range three-component ground motions 

could be used to obtain very reliable estimates of distance attenuation laws to predict strong shaking. 
NW Morgan Hill 

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coplotted with slip distribution that Hartzell and Heaton (1986) deduced from modeling strong ground motion 

{contours at 40-cm intervals). In this sequence (and several others) it appears that aftershock activity occurs 

mainly outside area of major coseismic slip {from Mendoza and Hartzell, 1988). 

Strong-Motion Simulation 

In the last section we discussed a commonly used method 

for estimating the ground motion of an earthquake at a given 

distance and magnitude. However, even if the distance and 

magnitude are known, there is large uncertainty in the determination of ground motions that will result. To illustrate, we 

show response spectra from horizontal ground motions 

recorded at distances near 50 km from shallow, crustal, strikeslip earthquakes of about magnitude 6.5 in figure 25. The 

largest ground motion is over 10 times larger than the smallest, and it is obvious that a wide ·variety of ground motions 

have occurred at a distance of about 50 km from M 6.5 strikeslip earthquakes. Wave propagation through geologically 

complex structures is one of the major reasons for the large 

observed scatter. In a previous section, we described how 

records from small earthquakes can be used to remove the effects of wave propagation from records of large earthquakes. 

In a similar manner, the records from small earthquakes can 

be used to simulate the nature of strong shaking to be expected 

when large earthquakes occur. That is, records from small 

earthquakes can be used as empirical Green's functions, and 

these Green's functions can be summed to simulate the shaking in the vicinity of the seismic station (Hartzell, 1978; 

Kanamori, 1979). If stations are located within metropolitan 

areas or near critical facilities, then important effects due to 

wave propagation can be anticipated. 

Earth Structure and Wave Propagation 

In our earlier discussion of the uses of existing networks, 

we pointed to numerous important studies of Earth structure 

that were made possible by the existence of regional networks. 

Most of these studies would have been impossible with sparse 

or temporary networks. In most instances and because of the 

nature of the data, these studies only use P-wave arrival times 

to infer large-scale variations in seismic velocities. However, 

the availability of broad-band three-component data from 

regional networks will greatly expand the nature of studies 

into the structure of the Earth and the manner in which waves 

travel thrpugh complex geologic structure. In particular, it 

will be possible to continuously observe the development in 

long-period waveforms as they sweep across entire regions. It 

will be possible to deterministically study the nature of and 

reasons for scatter in wave amplitudes that have been noted, 

but poorly understood. We now give some examples of the 

types of problems that will be studied with high-quality 

regional networks. 

Shear Waves 

The availability of three-component waveforms will 

dramatically improve the ability to study shear waves. Shear 

waves are typically very poorly recorded on vertical seismometers, and S-wave arrival times can be in serious error 

when only a vertical-component seismogram is used for 

analysis. Furthermore, interactions between P waves and vertically polarized S waves (SV waves) often complicates the interpretation of shear waves, and it is usually best to rotate 

motions into radial and tangential components so that tangential-component shear waves (SH waves) can be studied 

separately. This type of analysis is not possible with existing 

regional networks, but would be routine with high-quality 

three-component data. 

Shear waves provide information about the Earth's interior that is independent from P waves. Since they do not 

propagate through fluids, the search for travel paths along 

which shear waves are missing (or attenuated) is an important 

tool for mapping the subsurface extent of magma bodies. Furthermore, low shear-wave velocities are often inferred for 

zones of high tectonic slip rates, the presence of petroleum 

deposits, and the presence of geothermal resources. The use 

of shear-wave information promises to open a new class of 

problems in the exploration of the Earth's interior. 

Research Possibilities of Regional Networks 37 
Surface Waves 

Surface waves are another class of seismic waves that are 

important for understanding the properties of the crust and uppermost mantle. They are also of interest because they may 

be an important factor in the seismic hazard of longer period 

structures (tall buildings, bridges, and so on). Although surface waves can have any period, they are generally important 

at periods of greater than 1 second They are usually classified 

according to the polarization of the motions they produce, 

either transverse polarization (SH-type Love waves) or radialvertical (P -SV-type Rayleigh waves). At large earthquake distances, these are usually the largest arrivals seen on a 

seismograph, and they contain important information about 

the average properties of the crust. Three-component data are 

essential for the identification and study of surface waves. 

Furthermore, the fact that these are relatively long-period 

waves means that they are usually not well recorded by existing short-period regional networks. 

Understanding the Coda 

The seismic coda has been the focus of considerable interest because it is thought that material properties of the 

Earth's lithosphere can be inferred from the coda, and because 

temporal variations in codas over periods of days to years may 

precede large earthquakes. The seismic coda, which is the part 

of the seismogram following the S wave, is generally thought 

to consist of body and surface waves scattered off material 

heterogeneities in the Earth's structure (Aki and Chouet, 

1975). If the coda consists primarily of single scattered waves, 

then the energy in the coda samples a large volume of the lithosphere, and thus the characteristics of the coda may be sensitive to the large-scale properties of the crust. Many 

researchers have measured the temporal decay rate of the seismic coda in various frequency bands. These observations 

have been widely characterized using a measurement called 

coda-Q (Qc), which has been observed to increase with frequency. Similarly, some researchers (for example, Jin and 

Aki, 1986; Novelo-Casanova and others, 1985) have observed 

temporal variations in coda-Q before large earthquakes. It has 

been hypothesized that variations in average crustal seismic 

velocity would be relevant to earthquake prediction (Poupinet 

and others, 1985). Also, the spectrum of the coda has been 

used to infer the earthquake source spectrum and the site 

response of the seismic station (Phillips and Aki, 1986). 

Despite all these proposed uses of the seismic coda and 

Qc, there is still no general agreement on what property of the 

crust is measured by Qc, on the volume of the lithosphere 

sampled by the coda, on the relative proportion of single- and 

multiple-scattered energy in the coda, and on the relative importance of near-surface heterogeneity on the time domain and 

frequency domain characteristics of the coda. One common 

interpretation of the coda is that it consists of energy that is 

singly back scattered from the lithosphere (Aki and Chouet, 

1975). However, there are alternative models, such as the 

38 National Seismic System Science Plan 

multiple-scattering energy-flux model of Frankel and Wennerberg (1987). In the single-scattering model, Qc represents 

the total transmission Q of the medium, including both the effects of scattering and anelastic attenuation, whereas in the 

multiple-scattering model, Qc is at best a measure of only 

anelastic attenuation. Considerable disagreement exists on 

the portion of the lithosphere that is sampled by the coda. 

Levander and Hill (1985) have shown that a considerable portion of the coda energy may consist of surface waves, converted from body waves, that are trapped within the 

heterogeneous low-velocity region within a few kilometers of 

the Earth's ~urface. Spudich and Bostwick (1987) showed 

that the early part of the coda was dominated by multiply scattered waves reverberating within a few kilometers of the seismic station. If the coda is dominated by waves that sample 

only the near-surface zone, then Earth properties inferred from 

the coda will be biased toward their values near the surface of 

the Earth, and temporal changes in the coda may be related to 

near-surface fluctuations such as water table changes, rather 

than to temporal changes throughout the lithosphere. If the 

coda does not sample the entire lithosphere uniformly, then 

the. widely observed frequency dependence of Qc may not indicate a true frequency dependence of anelastic attenuation in 

the crust, but rather may result from different frequency components of the coda preferentially sampling different depth 

regions of a lithosphere in which anelastic attenuation is a 

function of depth. 

Consequently, it is necessary to determine the type of 

waves composing the coda, the importance of multiple scattering, and the volume of the Earth that these waves sample. 

We need to learn the relative importance of scattering attenuation and anelastic attenuation. Moreover, we need to learn the 

frequency dependence and spatial variation of these Earth 

properties. Careful study of broad-band waveforms recorded 

at a large number of sites from a large variety of seismic sources will be the key to unraveling the physics of seismic coda. 

Deep Structure of Basins and Mountains 

Achieving better resolution of the structural makeup and 

physical properties of the deeper parts of the continental crust 

in the United States has become one of the major goals in the 

U.S. seismological community (Panel on Seismological 

Studies of the Continental Lithosphere, 1984; IRIS, 1984; 

Mooney, 1987). Regions of basins and mountains are key targets whose subsurface study is motivated by their typical association with earthquake processes, their economic potential 

(for instance, hydrocarbon and other mineral deposits), and the 

fundamental need for new, high-quality data to evaluate competing hypotheses about their origin and evolution. 

As outlined in the 10-year program plan for seismic 

studies of the continental lithosphere (IRIS, 1984), outstanding research problems are both topical and geographical. 

Topical studies relating to crustal structure include, for example: the geometry and nature of intracrustal discontinuities 
and lateral heterogeneities in velocity structure; the subsurface 

geometry of structures such as faults, domes, batholiths, and 

volcanoes; and information relevant to understanding processes associated with basin formation and extension, continental terrane accretion, detachment tectonics, magmatic 

intrusion, and volcanism. Mooney (1987) summarized by 

geographical region both ( 1) the nature of important problems 

relating to crustal structure and (2) recent and current investigations. 

Three-component digital seismic networks will significantly enhance imaging of tectonically important 

regions-even though controlled-source studies (near-vertical-incidence reflection and refraction/wide-angle reflection; 

see Mooney, 1987) will continue to be primary investigative 

tools. The analysis of seismic waves from local and distant 

earthquakes complements, and indeed provides well-known 

advantages over, the use of artificial seismic sources for probing crustal structure. Earthquake sources are impulsive, occur 

at depth, generate higher levels of energy over a broader frequency range, and radiate shear-wave energy. Compared to 

short-term experiments, seismic networks provide the advantage of continuous, long-term recording for sampling 

earthquake sources. The broader regional coverage of seismic 

networks may also be advantageous, although some 

tomographic applications require close spatial sampling that 

realistically will only be achieved with dense temporary arrays of digital seismographs. 

The inversion of travel times of earthquake body waves 

is a well-established tool for imaging the three-dimensional 

velocity structure beneath a seismic array. For studying crustal-level structure, station spacing and the availability of 

horizontal-component recordings (for S-wave velocity structure) are important constraints. Some examples of the successful inversion of P-wave travel times for crustal structure 

using local earthquakes recorded by existing vertical-component seismic networks are given by Walk and Clayton 

(1987), Hearn and Clayton (1986a,b), and Kissling and others 

(1984). Networks of three-component seismographs would 

allow similar resolution of S-wave velocity structure. 

Owens and others (1987) demonstrated the power of a 

single three-component digital station for resolving local crustal structure from earthquake sources. Using a teleseismicwaveform-inversion technique, they derived a detailed 

vertical shear-velocity structure for the crust beneath the 

receiver site using converted waves of the P-to-S type. Scherbaum (1987) described another single-station inversion 

method for subsurface impedance structure from locally 

recorded SH waves. Regional earthquake phases that 

propagate in the crust are known to be sensitive to lateral changes in crustal structure (Campillo, 1987) and offer yet another 

potential way of mapping crustal structure with three-component digital networks. 

High-resolution three-dimensional inversion of local 

' crustal structure will unquestionably be pursued with temporary dense arrays of IRIS/P ASSCAL-type instruments, in 

the best cases combining the recording of both earthquakes 

and controlled sources (IRIS, 1984 ). Three-component digital 

seismic networks, with their temporal continuity and regional 

extent, will contribute to resolving subsurface crustal features 

through refined source definition of seismically active structures at depth, through inversion of both travel times and 

waveforms for P- and S-wave velocity structure, and through 

complementary interaction with controlled-source studies. 

Structure of the Earth's Interior 

Uniform, broad-band, high-dynamic-range instrumentation will enable far more complete resolution of deep Earth 

structure. In the future, the capabilities of regional networks 

will become even more important for the analysis of the structure of the Earth's mantle and core. These studies promise to 

provide the underpinnings for detailed models of the dynamics 

of plate tectonics, the evolution of the Earth, and origin of the 

Earth's magnetic field. In particular, density variations 

together with variations in the Earth's viscosity are the basic 

engine of plate tectonics. If these variations in the Earth's interior properties can be mapped, then we will have a much 

deeper understanding of plate tectonics. The use of data from 

regional networks will give an unprecedented look at the 

detailed structure not only beneath North America, but also 

along the travel path between North America and seismic 

sources located throughout the globe. Detailed studies of the 

nature of the Earth's interior discontinuities, such as the coremantle boundary or discontinuities in the upper mantle, will 

be feasible. There are even suggestions that the currents of 

fluid iron within the Earth's core may be observable with seismological studies. If this is true, then very detailed observations will be necessary. Since these currents may also change 

over time scales of years, dense permanent seismic networks 

(such as those in regional networks) are desirable for such 

studies. 

For simplicity, the Earth is usually considered to be elastically isotropic. Detailed studies, however, usually indicate 

the presence of anisotropy, often on the order of 5 percent. 

This effect is comparable to variations caused by changes in 

temperature and mineralogy. Anisotropy shows up as 

azimuthally dependent velocities, S-wave splitting (different 

time shift for different S-wave polarities), and discrepancies 

between Love and Rayleigh wave observations. Although 

anisotropy can be considered as an irritating complication, it 

contains important information about mineralogy, flow, and 

stress. Time-dependent anisotropy has even been proposed as 

a possible earthquake precursor. Since the most important 

diagnoses of anisotropy are S-wave splitting and LoveRayleigh inversion, one requires broad-band, three-component seismic data. Since anisotropy may not be a 

second-order effect, the failure to recognize it may result in 

serious errors in our interpretation of the Earth's interior structure. 

The anelastic properties of the Earth's interior are also of 

great importance for understanding its overall dynamics. 

Research Possibilities of Regional Networks 39 
However, it is difficult to study anelastic attenuation of seismic waves because it is intertwined with geometric spreading, 

focusing, and defocusing. Because of this, studies of the 

Earth's elastic and anelastic properties go hand in hand. The 

study of anelasticity requires spectral studies which, in turn, 

require high dynamic range and broad-band data. Highquality modem three-component instruments are essential. 

Anelasticity sheds light on the physical properties of the crust, 

mantle and core, the temperature, and the state of stress (dislocation density). Anelasticity also causes the elastic properties, including the seismic velocities, to be frequency 

dependent. This information is required in detailed modeling 

of the Earth's structure. 

Nuclear Discrimination 

Artificial explosions are commonly observed on existing 

regional seismic networks. These explosions are commonly 

quarry blasts observed at distances less than several hundred 

kilometers, but many of these explosions are the result of testing of nuclear weapons. There has been considerable study of 

the problem of detecting and recognizing nuclear explosions 

throughout the world. Although earthquakes and explosions 

both generate P, S, and Rayleigh waves, explosive sources are 

fundamentally different from earthquake sources. One of the 

most important differences is the relatively short process time 

of an explosion compared with an earthquake. Furthermore, 

the spatial amplitude pattern (radiation pattern) is completely 

different for explosions and earthquakes. The high station density of regional networks makes them ideal for detecting both 

regional and teleseismic explosions. In particular, stacking of 

signals to detect waves coming from regions of particular interest (such as known test sites) can significantly improve signal-to-noise ratios. Because of their limited bandwidth, 

current regional networks are not particularly well suited for 

explosion discrimination problems. However, regional networks with enhanced instrumentation will be well suited for 

studying explosions. 

CONCLUSIONS 

There are approximately 1,600 permanent stations in existing U.S. regional seismic networks. These networks were 

constructed- with the primary function of locating the 

earthquake activity of different regions of the United States, 

and they are well suited for this task. Because of the now-antiquated telemetry and data-logging technology that was available during their development 20 years ago, existing regional 

networks only record the vertical component of motion over 

a very restricted range of amplitudes and frequencies. This 

severely restricts the role of regional networks. 

Modem seismic and telecommunications systems have 

the capability of measuring all three components of ground 

motion with amplitudes ranging from ambient ground noise to 

the strongest shaking experienced during earthquakes and 

40 National Seismic System Science Plan 

over a very broad band of frequencies. This new generation of 

instrumentation has the potential to revolutionize the role of 

regional networks. Regional networks should play a vital role 

in emergency response activities during and after significant 

earthquakes. Furthermore, enhanced regional networks would 

vastly improve the national capability to study earthquake 

physics, strong ground motions, the structure of the crust, and 

the structure and dynamics of the Earth's interior. The formulation of a national plan to develop new digital highdynamic-range, broad-band regional seismic networks should 

be given a high priority. 

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