THE SEISMIC DESIGN HANDBOOK

Second Edition

THE SEISMIC DESIGN HANDBOOK

Second Edition

edited by

Farzad Naeim, Ph.D., S.E.

Vice President and Director of Research and Development John A. Martin Associates, Inc., U.S.A.

Volume 1

CODECOUNme

SPRINGER SCIENCE+BUSINESS MEDIA, LLC

Additional material to this book can be downloaded from http://extras.springer.com

Library of Congress Cataloging-in-Publication Data

The seismic design handbook I edited by Farzad Naeim.-2"d ed. p.cm.

Includes index. ISBN 978-1-4613-5681-3 ISBN 978-1-4615-1693-4 (eBook) DOI 10.1007/978-1-4615-1693-4

I. Earthquake resistant design-Handbooks, manuals, etc. 2. Buildings-Earthquake effects-Handbooks, manuals, etc. I. Naeim, Farzad.

T A658.44 .S395 200 I 624.1 '762-dc21

Copyright© 2001 by Springer Science+Business Media New York Originally published by Kluwer Academic Publishers in 2001 Softcover reprint of the hardcover 2nd edition 2001 Second Printing 2003

00-054609

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, mechanical, photo­copying, recording, or otherwise, without the prior written permission of the publisher, Springer Science+Business Media, LLC.

Printed on acid-free paper.

"Among mortals second thoughts are wisest. "

Euripides,(480-406 B.c.)

"The change of motion is proportional to themotive force impressed; and is made in thedirection of the right line in which the force isimpressed. "

Isaac Newton,The Principia: Mathematical Principles ofNatural Philosophy (1687 A.D.)

Contents

Contributors IX

Acknowledgements

Preface

1 THE NATURE OF EARTHQUAKE GROUND MOTION 1BRUCE A. BOLT, D.Sc.

2 EARTHQUAKE GROUND MOTION ANDRESPONSE SPECTRA 47BIJAN MOHRAZ, PH.D., P.E. AND FAHIM SADEK, PH.D.

3 GEOTECHNICAL DESIGN CONSIDERATIONS 125MARSHALL LEW, PH.D., G.E.

4 DYNAMIC RESPONSE OF STRUCTURES 183JAMES C. ANDERSON, PH.D.

5 LINEAR STATIC SEISMIC LATERAL FORCEPROCEDURES 247ROGER M. DI JULIO JR., PH.D., P.E.

6 ARCHITECTURAL CONSIDERATIONS 275CHRISTOPHER ARNOLD, FAlA, RillA

7 DESIGN FOR DRIFT AND LATERAL STABILITY 327FARZAD NAEIM, PH. D., S.E.

8 SEISMIC DESIGN OF FLOOR DIAPHRAGMS 373FARZAD NAEIM, PH.D., S.E. AND R. RAO BOPPANA, PH.D., S.E.

Vlll

9 SEISMIC DESIGN OF STEEL STRUCTURES 409CHIA-MING UANG, PH.D., MICHEL BRUNEAU, PH.D., P.ENG.ANDREW S. WHITIAKER, PH.D., S.E., AND

KEY-CHYUANTSAI, PH.D., S.E.

10 SEISMIC DESIGN OF REINFORCED CONCRETESTRUCTURES 463ARNALDO T. DERECHO, PH.D. AND M. REZA KIANOUSH, PH.D.

11 SEISMIC DESIGN OF WOOD AND MASONRYBUILDINGS 563JOHN G. SHIPP, S.E., FASCE AND GARY C. HART, PH.D., P.E.

12 SEISMIC UPGRADING OF EXISTING STRUCTURES 623RONALD O. HAMBURGER, S.E. AND CRAIG A. COLE, S.E.

13 DESIGN OF NONSTRUCTURAL SYSTEMS ANDCOMPONENTS 681JOHN D. GILLENGERTEN, S.E.

14 DESIGN OF STRUCTURES WITH SEISMIC ISOLATION 723RONALD L MAYES, PH.D. AND FARZAD NAEIM, PH.D., S.E.

15 PERFORMANCE BASED SEISMIC ENGINEERING 757FARZAD NAEIM, PH.D., S.E., HUSSAIN BHATIA, PH.D., P.E.AND ROY M. LOBO, PH.D., P.E.

16 COMPUTER APPLICATIONS IN SEISMIC DESIGN 793FARZAD NAEIM, PH.D., S.E., ROY M. LOBO, PH.D., P.E.AND HUSSAIN BHATIA, PH.D., P.E.

APPENDIX 815

INDEX 821

Contributors

James C. Anderson, Ph.D.Professor of Civil Engineering, University of Southern California, Los Angeles, California(Dynamic Response of Structures)

Christopher Arnold, AlAPresident, Building Systems Development, Inc., San Mateo, California(Architectural Considerations)

Hussain Bhatia, Ph.D., P.E.Senior Research Engineer, John A. Martin & Associates, Inc., Los Angeles, California(Performance Based Seismic Engineering; Computer Applications in Seismic Design)

Bruce A. Bolt, Ph.D.Professor Emeritus of Seismology, University of California, Berkeley, California(The Nature ofEanhquake Ground Motions)

Rao Boppana, Ph.D., S.E.President, Sato and Boppana Consulting Engineers, Los Angeles, California(Seismic Design of Floor Diaphragms)

Michel Bruneau, Ph.D., P.Eng.Professor of Civil Engineering, State University of New York at Buffalo, New York(Seismic Design ofSteel Structures)

Craig A. Cole, S.E.Project Manager, EQE International, Inc., Oakland, California(Seismic Upgrading of Existing Structures)

John G. Gillengerten, S.E.Senior Project Manager, John A. Martin & Associates, Inc., Los Angeles, California(Design ofNonstructural Systems and Components)

Arnaldo T. Derecho, Ph.D.Consulting Structural Engineer, Mount Prospect, Illinois(Seismic Design ofReinforced Concrete Structures)

Roger M. Dijulio, Jr., Ph.D., P.E.Professor of Engineering, California State University, Northridge, California(Linear Static Lateral Force Procedures)

Ronald O. Hamburger, S.E.Senior Vice President, EQE International, Inc., Oakland, California(Seismic Upgrading of Existing Structures)

Gary C. Hart, Ph.D., P.E.Professor of Civil Engineering, University of California, Los Angeles, California(Seismic Design of Wood and Masonry Structures)

x

Contributors (continued)

M. Reza Kianoush, Ph.D.Professor, Ryerson Polytechnic University, Toronto, Ontario, Canada(Seismic Design ofReinforced Concrete Structures)

Marshall Lew, Ph.D., G.E.Corporate Consultant, Law/Crandall, Inc., Los Angeles, California(Geotechnical Considerations)

Roy F. Lobo, Ph.D., P.E.Senior Research Engineer, John A. Martin & Associates, Inc., Los Angeles, California(Performance Based Seismic Engineering; Computer Applications in Seismic Design)

Ronald M. Mayes, Ph.D.Engineering Consultant, Berkeley, California(Design ofStructures with Seismic Isolation)

Bijan Mohraz, Ph.D.Professor of Civil Engineering, Southern Methodist University, Dallas, Texas(Earthquake Ground Motion and Response Spectra)

Farzad Naeim, Ph.D., S.E.Vice PresidentIDirector of Research and Development, John A. Martin & Associates, Inc., Los Angeles, California(Design for Drift and Lateral Stability; Seismic Design of Floor Diaphragms; Design of Structures with SeismicIsolation; Performance Based Seismic Engineering; Computer Applications in Seismic Design)

John G. Shipp, S.E., FASCEManager Design Services and Senior Technical Manager, EQE Engineering and Design, Costa Mesa, California(Seismic Design ofWood and Masonry Structures)

Key-Chyuan Tsai, Ph.D., S.E.Professor of Civil Engineering, National Taiwan University, Taipei, Taiwan(Seismic Design ofSteel Structures)

Chia-Ming Uang, Ph.DProfessor of Structural Engineering, University of California, San Diego, California(Seismic Design ofSteel Structures)

Andrew S. Whittaker, Ph.D., S.E.Associate Professor of Civil Engineering, State University of New York at Buffalo, New York(Seismic Design ofSteel Structures)

Acknowledgements

The editor gratefully acknowledges the efforts of contributors in preparing excellent manuscripts.Thanks are also due to the management and staff at John A. Martin and Associates, Inc., especiallyJack and Trailer Martin, who have always graciously understood and accommodated my frequentshifts of emphasis from everyday office practice to writing textbooks and technical articles. If it hadnot been for their encouragement and support, this project would have not been completed.

The production of this edition of the handbook was made possible by the heroic efforts of twoyoung and very talented persons. Mark Day patiently and diligently managed the digital typesettingand repeated content revisions with his usual grace, smile, and dedication. Hesaam Aslani on hisshort stay with our firm on his way to graduate studies at the Stanford University prepared earlycamera-ready versions of most of the chapters and checked mathematical expressions and numericalexamples.

The International Conference of Building Officials (ICBO) and particularly Mark Johnson of thatorganization were a constant source of encouragement and support. It is a distinct honor to have thishandbook endorsed by both ICBO and the National Council of Structural Engineers Associations.

The editor is indebted to the readers of the first edition for their very positive and encouragingfeedback and for their constant reminders of their desire to see a second edition of the handbook.

Last, but not least, the editor is grateful to his life-partner and wife, Fariba, who patientlyunderstood the need for his extended hours of work, and his children Mana and Mahan whoaccommodated a daddy who often could not play because he had a lot of work to do.

Preface

This handbook contains up-to-dateinfonnation on planning, analysis, and designof earthquake-resistant building structures. Itsintention is to provide engineers, architects,developers, and students of structuralengineering and architecture with authoritative,yet practical, design infonnation. It representsan attempt to bridge the persisting gap betweenadvances in the theories and concepts ofearthquake-resistant design and theirimplementation in seismic design practice.

The distinguished panel of contributors iscomposed of 22 experts from industry anduniversities, recognized for their knowledge andextensive practical experience in their fields.They have aimed to present clearly andconcisely the basic principles and procedurespertinent to each subject and to illustrate withpractical examples the application of theseprinciples and procedures in seismic designpractice. Where applicable, the provisions ofvarious seismic design standards such as mc­2000, UBC-97, FEMA-273/274 and ATC-40are explained and their differences arehighlighted.

Most of the chapters have been either totallyre-written or substantially revised to reflect therecent advances in the field. In addition, anumber of new chapters have been added tocover subjects such as perfonnance basedseismic engineering, seismic upgrading of

existing structures, computer applications, andseismic design of wood structures.

A new and very useful feature of this editionis the inclusion of a companion CD-ROM disccontaining the complete digital version of thehandbook itself and the following veryimportant publications:l. UBC-IBC (1997-2000) Structural

Comparisons and Cross References, ICBO,2000.

2. NEHRP Guidelines for the SeismicRehabilitation ofBuildings, FEMA-273,Federal Emergency Management Agency,1997.

3. NEHRP Commentary on the Guidelinesforthe Seismic Rehabilitation ofBuildings,FEMA-274, Federal EmergencyManagement Agency, 1997.

4. NEHRP Recommended Provisions forSeismic Regulations for New Buildings andOlder Structures, Part 1 - Provisions,FEMA-302, Federal EmergencyManagement Agency, 1997.

5. NEHRP Recommended ProvisionsforSeismic Regulations for New Buildings andOlder Structures, Part 2 - Commentary,FEMA-303, Federal EmergencyManagement Agency, 1997.

One should realize that seismic design isstill as much an art as it is a science. Therefore,

XIV

no matter how helpful the material in thishandbook might prove to be, it cannot replaceor substitute sound engineering judgment.Furthermore, one must recognize that on someseismic design and detailing issues, a generalconsensus on the appropriate approaches doesnot yet exist. As an eminent engineer once said:"No two design offices completely agree on allaspects of seismic design or proper detailing." Itis the editor's belief, however, that it is throughthe publication of books like this one, andcontinuation of research and development, thata general consensus of these issues will finallybe reached. We have come a long way towardsachieving these objectives during the lastdecade.

The primary purpose of this handbook is toserve practicing engineers and architects.However, its scope and its treatment of boththeory and practice should also make it valuableto both teachers and students of earthquake­resistant design.

Much has been changed in seismic designpractice since the first edition of this handbookwas published in 1989. We have learned manylessons from world-wide damaging earthquakesduring the last decade and these lessons, moreor less, have been implemented in recentseismic design codes and guidelines. This is theprimary reason why the volume of this editionof the handbook is roughly twice that of thefirst edition although its objectives have notchanged.

The first edition of this handbook wasreceived with a degree of enthusiasm that wastotally above and beyond the editor'sexpectations. The book became the de-factostandard textbook for teaching seismic designprinciples at practically all major universities ofthe United States. UC Berkeley, Stanford,UCLA, USC, University at Buffalo, Universityof lllinois, Washington University at SaintLouis, University of Texas at Austin, Georgia­Tech, Cornell, and University of Michigan areamong the schools that have used the firstedition in this country. Overseas, it has beenused at the Imperial College of London, Israel

Institute of Technology, and many other fineinstitutions.

The editor hopes that this second edition ofthe handbook will repeat the success of itspredecessor and will be found as -if not more­useful to the readers. The editor welcomes anyand all comments, criticisms and suggestions.Comments may be sent bye-mail to !arzad@johnmartin.com. Any errata or supplementaryinformation, if and when necessary, will beposted at http://www.johnmartin.com/sdh.

FanadNaeimOctober 2000

Los Angeles, California

Chapter 1

THE NATURE OF EARTHQUAKE GROUND MOTION

Bruee A. Bolt, D.Se.Professor Emeritus, of Seismology, University of California, Berkeley, California

Key words: Attenuation, Causes of Earthquakes, Collapse Earthquakes, Damage Mechanisms, Directivity, Earthquakes,Earthquake Faults, Earthquake Prediction, Elastic Rebound, Fault Types, Intensity Scales, IntraplateEarthquakes, Magnitude, Magnitude Scales, Magnitude Saturation, MMI, Near-Fault Effects, PlateTectonics, Reservoir-Induced Earthquakes, Seismicity, Seismic Moment, Seismic Risk, Seismic Waves,Seismology, Source Models, Strong Ground Motion, Surface Rupture, Tectonic Earthquakes, VolcanicEarthquakes

Abstract: The aim of this chapter is to provide a basic understanding about earthquakes, their world-wide distribution,what causes them, their likely damage mechanisms, earthquake measuring scales, and current efforts on theprediction of strong seismic ground motions. This chapter, therefore, furnishes the basic informationnecessary for understanding the more detailed concepts that follow in the subsequent chapters of this book.The basic vocabulary of seismology is defmed. The seismicity of the world is discussed first and itsrelationship with tectonic plates is explained. The general causes of earthquakes are discussed next wheretectonic actions, dilatancy in the crustal rocks, explosions, collapses, volcanic actions, and other likely causesare introduced. Earthquake fault sources are discussed next. Various faulting mechanisms are explainedfollowed by a brief discussion of seismic waves. Earthquake damage mechanisms are introduced and differentmajor damage mechanisms are identified by examples. Quantification of earthquakes is of significant interestto seismic design engineers. Various earthquake intensity and magnitude scales are defmed followed by adescription of earthquake source models. Basic information regarding the concepts of directivity and near­fault effects are presented. Finally, the ideas behind seismic risk evaluation and earthquake prediction arediscussed.

F. Naeim (ed.), The Seismic Design Handbook© Kluwer Academic Publishers 2001

1. THE NATURE OF EARTHQUAKE GROUND MOTION 3

1.1 INTRODUCTION

On the average, 10,000 people die each yearfrom earthquakes (see Figure 1-1). A UNESCOstudy gives damage losses amounting to$10,000,000,000 from 1926 to 1950 fromearthquakes. In Central Asia in this interval twotowns and 200 villages were destroyed. Sincethen several towns including Ashkhabad(1948), Agadir (1960), Skopje (1963), Managua(1972), Gemona (1976), Tangshan (1976),Mexico City (1985), Spitale (1988), Kobe(1995), cities in Turkey and Taiwan (1999) andhundreds of villages have been severelydamaged by ground-shaking. Historical writingstestify to man's long concern about earthquakehazards.

The first modem stimulus for scientificstudy of earthquakes came from the extensivefield work of the Irish engineer, Robert Mallett,after the great Neopolitan earthquake of 1857 insouthern Italy. He set out to explain the "massesof dislocated stone and mortar" in terms of

mechanical principles and m doing soestablished basic vocabulary such asseismology, hypocenter and isoseismal. Suchclose links between engineering and seismologyhave continued ever since(l-I,I-2).

It is part of strong motion seismology toexplain and predict the large amplitude-longduration shaking observed in damagingearthquakes. In the first sixty years of thecentury, however, the great seismologicaladvances occurred in studying waves fromdistant earthquakes using very sensitiveseismographs. Because the wave amplitudes ineven a nearby magnitude 5 earthquake wouldexceed the dynamic range of the usualseismographs, not much fundamental work wasdone by seismologists on the rarer largeearthquakes of engineering importance.

Nowadays, the situation has changed. Mterthe 1971 San Fernando earthquake, hundreds ofstrong-motion records were available for thismagnitude 6.5 earthquake. The 1.2g recorded atPacoima Dam led to questions on topographic

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Figure 1-1. Loss of life caused by major earthquakes [After Hiroo Kanamori(l-IO»).

4

amplification and the construction of realisticmodels of fault-rupture and travel-path thatcould explain the strong motion patterns.Progress on these seismological questionsfollowed rapidly in studies of variation inground motions in the 1989 Loma Prietaearthquake (M 7.0), the 1994 Northridgeearthquake (M 6.8) and the 1999 Chi Chi eventin Taiwan (M 7.6). A harvest of strong motionrecordings was obtained in the latterearthquake, showing numerous horizontal peakaccelerations in the range 0.5g to 1.0g. Digitalrecorders and fast computers mean that bothseismologists and engineers can tackle morefundamental and realistic problems ofearthquake generation and ground shaking.

Knowledge of strong ground shaking is nowadvancing rapidly, largely because of thegrowth of appropriately sited strong-motionaccelerographs in seismic areas of the world.For example, in the Strong MotionInstrumentation Program in California, by theyear 2000 there were 800 instruments in thefree-field and 130 buildings and 45 otherstructures instrumented. Over 500 records hadbeen digitized and were available for use inresearch or practice (see Chapter 16). Inearthquake-prone regions, structural design oflarge or critical engineered structures such ashigh-rise buildings, large dams, and bridgesnow usually involves quantitative dynamicanalysis; engineers ask penetrating questions onthe likely seismic intensity for construction sitesand require input motions or spectra of definingparameters. Predicted seismograms (time­histories) for dynamic modeling in structuraldesign or vulnerability assessments are oftenneeded.

The aim of this chapter is to provide a basicunderstanding about earthquakes, theirworldwide distribution, what causes them, theirlikely damage mechanisms, earthquakemeasuring scales, and current efforts on theprediction of strong seismic ground motions.Additional helpful background on the subjectmay be found in References 1-2 through 1-34.

Chapter 1

1.2 SEISMICITY OF THEWORLD

From the earthquake wave readings atdifferent seismographic observatories, theposition of the center of an earthquake can becalculated(l-l). In this way, a uniform picture ofearthquake distribution around the world hasbeen obtained (see Figure 1-2). Definite belts ofseismic activity separate large oceanic andcontinental regions, themselves mainly, but byno means completely, devoid of earthquakecenters. Other concentrations of earthquakesources can be seen in the oceanic areas, forexample, along the center of the Atlantic andIndian Oceans. These are the sites of giganticsubmarine mountain ranges called mid-oceanicridges. The geological strains that prevailthroughout this global ridge system areevidenced by mountain peaks and deep riftvalleys. Volcanic eruptions are frequent, andearthquakes originating along these ridges oftenoccur in swarms, so that many hundreds ofshocks are concentrated in a small area in ashort time.

Dense concentrations of earthquake centerswith some as much as 680 kilometers beneaththe surface also coincide with island arcs, suchas those of the Pacific and the easternCaribbean.

On the western side of the Pacific Ocean,the whole coast of Central and South America isagitated by many earthquakes, great and small.High death tolls have ensued from the majorones. In marked contrast, the eastern part ofSouth America is almost entirely free fromearthquakes, and can be cited as an example oflow seismic risk country. Other seismicallyquiet continental areas can be seen in Figure 1­2.

In Europe, earthquake activity is quitewidespread. To the south, Turkey, Greece,Yugoslavia, Italy, Spain and Portugal sufferfrom it, and large numbers of people have diedin disasters throughout the years. An earthquakeoff southwest Iberia on November 1, 1755produced a great tsunami, which caused manyof the 50,000 to 70,000 deaths occurring in

J. THE NATURE OF EARTHQUAKE GROUND MOTiON

An l'CticPlat"

•••••••••• Y()lcanne~

5

_ ~I"hon 01 pate

Figure 1-2. Tectonic plates and world-wide distribution of earthquakes. (From Earthquakes, by Bruce A. Bolt. Copyright1978, 1999 W. H. Freeman and Company. Used with permission.)

Lisbon, Portugal, and surrounding areas; theshaking was felt in Germany and theNetherlands. In Alicante, Spain, on March 21,1829, a shock killed about 840 persons andinjured many hundred more. Total or partialdestruction of more than 5,000 houses wasreported in and near Torrevieja and Murcia. OnDecember 28, 1908, a devastating earthquakehit Messina, Italy, causing 120,000 deaths andwidespread damage. The most recent deadlyone to affect that country struck on May 6,1976, in the Friuli region near Gemona; about965 persons were killed and 2280 injured.

On December 27, 1939, in Erzincan,Turkey, 23,000 lives were lost from a majorearthquake. Similar killer earthquakes haveoccurred in Turkey and Iran in recent years. TheErzincan earthquake along the Anatolian faultin Turkey on March 13, 1992 caused manybuilding collapses and the June 21, 1990earthquake (M 7.3) devastated two Iranianprovinces, Gilan and Zanjan. August 17, 1999saw a 50 km rupture of the north Anatoliam

fault along the Marmara Sea south of Izmitproducing a magnitude 7.4 earthquake and over16,000 deaths.

North of the Mediterranean margin, Europeis much more stable. However, destructiveearthquakes do occur from time to time inRomania, Germany, Austria and Switzerland,and even in the North Sea region andScandinavia. For example, on October 8, 1927,an earthquake occurred near Schwadorf inAustria and caused damage in an area southeastof Vienna. This earthquake was felt in Hungary,Germany, and Czechoslovakia at distances of250 kilometers from the center of thedisturbance. The seismicity in the North Sea issufficiently significant to require attention toearthquake resistant desi6n of oil platformsthere.

In Africa, damaging earthquakes haveoccurred in historical times. A notable case wasthe magnitude 5.6 earthquake on November 14,1981 that was felt in Aswan, Egypt. Thisearthquake was probably stimulated by the

6

impounding of water in Lake Nassar behind thehigh Aswan Dam.

An example of infrequent and dispersedseismicity is the occurrence of earthquakes inAustralia. Nevertheless, this country does havesome areas of significant present-day seismicity.Of particular interest is a damaging earthquakeof moderate size that was centered nearNewcastle and causing major damage andkilling fourteen people. It was a surprise from aseismological point of view because no faultmaps were available which showed seismogenicgeological structures near Newcastle.

During an earthquake, seismic waves radiatefrom the earthquake source somewhere belowthe ground surface as opposite sides of aslipping fault rebound in opposite directions inorder to decrease the strain energy in the rocks.Although in natural earthquakes this source isspread out through a volume of rock, it is oftenconvenient to imagine a simplified earthquakesource as a point from which the waves firstemanate. This point is called the earthquakefOCUS. The point on the ground surface directlyabove the focus is called the earthquakeepicenter.

Although many foci are situated at shallowdepths, in some regions they are hundreds ofkilometers deep. Such regions include the SouthAmerican Andes, the Tonga Islands, Samoa, theNew Hebrides chain, the Japan Sea, Indonesia,and the Caribbean Antilles. On the average, thefrequency of occurrence of earthquakes in theseregions declines rapidly below a depth of 200kilometers, but some foci are as deep as 680kilometers. Rather arbitrarily, earthquakes withfoci from 70 to 300 kilometers deep are calledintermediate focus and those below this depthare termed deep focus. Some intermediate anddeep focus earthquakes are located away fromthe Pacific region, in the Hindu Kush, inRomania, in the Aegean Sea and under Spain.

The shallow-focus earthquakes (focus depthless than 70 kilometers) wreak the mostdevastation, and they contribute about threequarters of the total energy released inearthquakes throughout the world. In California,for example, all of the known earthquakes to

Chapter 1

date have been shallow-focus. In fact, it hasbeen shown that the great majority ofearthquakes occurring in central Californiaoriginate from foci in the upper five kilometersof the Earth, and only a few are as deep as even15 kilometers.

Most moderate to large shallow earthquakesare followed, in the ensuing hours and even inthe next several months, by numerous, usuallysmaller earthquakes in the same vicinity. Theseearthquakes are called aftershocks, and largeearthquakes are sometimes followed byincredible numbers of them. The great RatIsland earthquake in the Aleutian Island onFebruary 4, 1965 was, within the next 24 days,followed by more than 750 aftershocks largeenough to be recorded by distant seismographs.Aftershocks are sometimes energetic enough tocause additional damage to already weakenedstructures. This happened, for example, a weekafter the Northridge earthquake of January 17,1994 in the San Fernando Valley when someweakened structures sustained additionalcracking from magnitude 5.5 aftershocks. A fewearthquakes are preceded by smaller foreshocksfrom the source area, and it has been suggestedthat these can be used to predict the main shock.

1.3 CAUSES OFEARTHQUAKES

1.3.1 Tectonic Earthquakes

In the time of the Greeks it was natural tolink the Aegean volcanoes with the earthquakesof the Mediterranean. As time went on itbecame clear that most damaging earthquakeswere in fact not caused by volcanic activity.

A coherent global geological explanation ofthe majority of earthquakes is in terms of whatis called plate tectonics(l-3). The basic idea isthat the Earth's outermost part (called thelithosphere) consists of several large and fairlystable rock slabs called plates. The ten largestplates are mapped in Figure 1-2. Each plateextends to a depth of about 80 kilometers.

1. THE NATURE OF EARTHQUAKE GROUND MOTION 7

seismic hazard along continental collisionmargins at tectonic plates has not as yetreceived detailed attention.

10'

20'

30'

40'

30'

90'

90'

PRESENT

80'

71 MILLIONYEARS AGO

80'70'

70'

o40'

40'

40'

10'

10'

20'

30'

30'

Figure 1-3. Continued drift ofthe Indian plate towardsAsian plate causes major Himalayan earthquakes. (FromThe Collision Between India and Eurasia, by Molnar andTapponnier. Copyright 1977 by ScientifIC American, Inc.All rights reserved)

While a simple plate-tectonic theory is animportant one for a general understanding ofearthquakes and volcanoes, it does not ~xplain

all seismicity in detail, for within continentalregions, away from boundaries, largedevastating earthquakes sometimes occur.These intraplate earthquakes can be found onnearly every continent.

Moving plates of the Earth's surface (seeFigures 1-2 and 1-3) provide mechanisms for agreat deal of the seismic activity of the world.Collisions between adjacent lithospheric plates,destruction of the slab-like plate as it descendsor subducts into a dipping zone beneath islandarcs (see Figure 1-4), and spreading along mid­oceanic ridges are all mechanisms that producesignificant straining and fracturing of crustalrocks. Thus, the earthquakes in thesetectonically active boundary regions are calledplate-edge earthquakes. The very hazardousshallow earthquakes of Chile, Peru, the easternCaribbean, Central America, Southern Mexico,California, Southern Alaska, the Aleutians theKuriles, Japan, Taiwan, the Philippines,Indonesia, New Zealand, the Alpine-Caucasian­Himalayan belt are of plate-edge type.

As the mechanics of the lithospheric platesbecome better understood, long-termpredictions of place and size may be possiblefor plate-edge earthquakes. For example, manyplates spread toward the subduction zones atrates of from 2 to 5 centimeters (about one totwo inches) per year. Therefore in active arcslike the Aleutian and Japanese Islands andsubduction zones like Chile and westernMexico, knowledge of the history of largeearthquake occurrence might flag areas thatcurrently lag in earthquake activity.

Many large earthquakes are produced by slipalong faults connecting the ends of offsets inthe spreading oceanic ridges and the ends ofisland arcs or arc-ridge chains (see Figure 1-2).In these regions, plates slide past each otheralong what are called transform faults.Considerable work has been done on theestimation of strong ground motion parametersfor the design of critical structures inearthquake-prone countries with eithertransform faults or ocean-plate subductiontectonics, such as Japan, Alaska, Chile andMexico. The Himalaya, the Zagros and Alpineregions are examples of mountain rangesformed by continent-to-continent collisions.These collision zones are regions of highpresent day seismic activity. The estimation of

8 Chapter 1

Figure 1-4. A sketch of the Earth's crust showing mid-oceanic ridges and active continental margin along a deep trench

One example of such earthquakes is theDashte-e-Bayaz earthquake of August 31, 1968in north-eastern Iran. In the United States, themost famous are the major earthquake series of1811-1812 that occurred in the New Madridarea of Missouri, along the Mississippi Riverand the 1886 Charleston, South Carolinaearthquake. One important group for example,which seems to bear no simple mechanicalrelation to the present plate edges, occurs innorthern China.

Such major internal seismic aCtIVItyindicates that lithospheric plates are not rigid orfree of internal rupture. The occurrence ofintraplate earthquakes makes the prediction ofearthquake occurrence and size difficult in

many regions where there is a significantseismic risk.

1.3.2 Dilatancy in the Crustal Rocks

The crust of the continents is a rocky layerwith average thickness of about 30 km butwhich can be as thick as 50 km under highmountain ranges. Under the ocean, the crustalthickness is no more than about 5 km.

At a depth in the crust of 5 kilometers or so,the lithostatic pressure (due to the weight of theoverlying rocks) is already about equal to thestrength of typical uncracked rock samples atthe temperature (5000 C) and pressureappropriate for that depth. If no other factorsentered, the shearing forces required to bring

1. THE NATURE OF EARTHQUAKE GROUND MOTION 9

about sudden brittle failure and frictional slipalong a crack would never be attained; rather,the rock would deform plastically. A wayaround this problem was the discovery that thepresence of water provides a mechanism forsudden rupture by reduction of the effectivefriction along crack boundaries. Nevertheless,in a normal geological situation, such as thecrust of coastal California, temperaturesincrease sufficiently fast so that at crustaldepths greater than about 16 km the elasticrocks become viscoelastic. Strain is thenrelieved by slow flow or creep rather than bybrittle fracture. The part of the crust above thistransition point is the seismogenic zone.

Studies of the time of travel of P and Swaves before the 1971 San Fernandoearthquake indicated that four years before itoccurred, the ratio of the velocity of the P wavesto the velocity of the S waves decreased rathersuddenly by 10 percent from its average valueof 1.75. There was, thereafter, a steady increasein this ratio back to a more normal value. Oneexplanation is the dilatancy model. This statesthat as the crustal rocks become strained,cracking occurs locally and the volume of rockincreases or dilates. Cracking may occur tooquickly for ground water to flow into the dilatedvolume to fill the spaces so the cracks becomevapor-filled. The consequent fall in porepressure leads to a reduction mainly in P wavevelocities. Subsequent diffusion of groundwater into the dry cracks increases the porepressure, and provides water for lubricationalong the walls of the cracks, while at the sametime, the P wave velocity increases again (seeFigure 1-31 in Section 1.10 below).

The full implications and relevance of thedilatancy theory of earthquake genesis are notyet clear, but the hypothesis is attractive in thatit is consistent with precursory changes inground levels, electrical conductivity and otherphysical properties which have been noted inthe past before earthquakes. The theory has apotential for forecasting earthquakes undercertain circumstances. For example,measurement of the P wave velocity in thevicinity of large reservoirs before and after

impounding of water might provide a moredirect method of indicating an approachingseismic crisis near dams than is now available.

1.3.3 Explosions

Ground shaking may be produced by theunderground detonation of chemicals or nucleardevices. When a nuclear device is detonated ina borehole underground, enormous nuclearenergy is released. Underground nuclearexplosions fired during the past several decadesat a number of test sites around the world haveproduced substantial artificial earthquakes (upto magnitude 6.0). Resultant seismic waveshave traveled throughout the Earth's interior tobe recorded at distant seismographic stations.

1.3.4 Volcanic Earthquakes

As Figure 1-4 shows, volcanoes andearthquakes often occur together along themargins of plates around the world. Likeearthquakes, there are also intraplate volcanicregions, such as the Hawaiian volcanoes.

Despite these tectonic connections betweenvolcanoes and earthquakes, there is no evidencethat moderate to major shallow earthquakes arenot essentially all of tectonic, elastic-reboundtype. Those earthquakes that can be reasonablyassociated with volcanoes are relatively rare andfall into three categories: (i) volcanicexplosions, (ii) shallow earthquakes arisingfrom magma movements, and (iii) sympathetictectonic earthquakes.

Among the three categories, Category (iii),tectonically associated with volcanoes, is moredifficult to tie down, as cases which may fit thiscategory, are rare. There is no report ofsignificantly increased volcanic activity in thegreat 1964 Alaska earthquake, but PuyehueVoi.::ano in the Andes erupted 48 hours after thegreat 1960 Chilean earthquake. One mightsuppose that in a large earthquake the groundshaking would set up waves in reservoirs ofmagma; the general compression and dilatationof the gaseous liquid melt may trigger volcanicactivity.

10

1.3.5 Collapse Earthquakes

Collapse earthquakes are small earthquakesoccurring in regions of underground cavernsand mines. The immediate cause of groundshaking is the sudden collapse of the roof of themine or cavern. An often observed variation isthe mine burst. This rock rupture happens whenthe induced stress around the mine workingscauses large masses of rock to fly off the mineface explosively, producing seismic waves.Mine bursts have been observed, for example,in Canada, and are especially common in SouthAfrica.

An intriguing variety of collapseearthquakes is sometimes produced by massivelandsliding. For example, a spectacularlandslide on April 25, 1974, along the MantaroRiver, Peru, produced seismic waves equivalentto a magnitude 4.5 earthquake. The slide had avolume of 1.6 xl09 cubic meters and killedabout 450 people.

1.3.6 Large Reservoir-InducedEarthquakes

It is not a new idea that earthquakes mightbe triggered by impounding surface water. Inthe 1870's, the U.S. Corps of Engineers rejectedproposals for major water storage in the SaltonSea in southern California on the grounds thatsuch action might cause earthquakes. The firstdetailed evidence of such an effect came withthe filling of Lake Mead behind Hoover Dam(height 221 meters), Nevada-Arizona,beginning in 1935. Although there may havebeen some local seismicity before 1935, after1936 earthquakes were much more common.Nearby seismographs subsequently showed thatafter 1940, the seismicity declined. The foci ofhundreds of detected earthquakes cluster onsteeply dipping faults on the east side of thelake and have focal depths of less than 8kilometers.

In Koyna, India, an earthquake (magnitude6.5) centered close to the dam (height 103

Chapter 1

meters) caused significant damage onDecember 11, 1967. After impounding began in1962, reports of local shaking became prevalentin a previously almost aseismic area.Seismographs showed that foci wereconcentrated at shallow depths under the lake.In 1967 a number of sizable earthquakesoccurred, leading up to the principal earthquakeof magnitude 6.5 on December 11. This groundmotion caused significant damage to buildingsnearby, killed 177 persons, and injured morethan 1,500. A strong motion seismograph in thedam gallery registered a maximum accelerationof 0.63g. The series of earthquakes recorded atKoyna has a pattern that seems to follow therhythm of the rainfall (see Figure 1-5). At least acomparison of the frequency of earthquakes andwater level suggests that seismicity increases afew months after each rainy season when thereservoir level is highest. Such correlations arenot so clear in some other examples quoted.

In the ensuing years, similar case historieshave been accumulated for several dozen largedams, but only a few are well documented.Most of these dams are more than 100 metershigh and, although the geological framework atthe sites varies, the most convincing examplesof reservoir induced earthquakes occur intectonic regions with at least some history ofearthquakes. Indeed, most of the thousands oflarge dams around the world give no sign ofearthquake induction. A poll in 1976 showedthat only four percent of large dams had anearthquake reported with magnitude greaterthan 3.0 within 16 kilometers of the dam.

Calculation shows that the stress due to theload of the water in even large reservoirs is toosmall to fracture competent rock. The bestexplanation is that the rocks in the vicinity ofthe reservoir are already strained from thetectonic forces so that existing faults are almostready to slip. The reservoir eiti ler adds a stressperturbation which triggers a slip or theincreased water pressure lowers the strength ofthe fault, or both.

Weekly frequency of earth tremors(at Koyna Nagar Observatory)

1. THE NATURE OF EARTHQUAKE GROUND MOTION

Inflow hydrograph, Shivajisagar L\ke

Reservoir water levels, Shivajisagar Lake

222°1_ 2140.] 2060

- 19801900 -----,---------,-------,-------.,.------

1---1963-ot·I-·--1964--....1-1.--1965--......-f41·--1966--....1-1'--1967 ·1320-.".

176, ".64.......:; 229-t:

:" 157' ':

Figure 1-5. The relationship between reservoir level and local seismic activity at Koyna Dam. (From Earthquakes, byBruce A. Bolt. Copyright 1978,1999 W.H. Freeman and Company. Used with permission.)

11

Figure 1-6. Normal fault at the Corinth Canal, Greece.(Photo courtesy ofL. Weiss.)

1.4 Earthquake fault sources

Field observations show that abrupt changesin the structure of rocks are common. In someplaces one type of rock can be seen butting upagainst rock of quite another type along a planeof contact. Such offsets of geological structureare called jaults(l-4). Clear vertical offset oflayers of rock along an exposed fault in the wallof the Corinth canal, Greece, can be seen inFigure 1-6.

Faults may range in length from a fewmeters to many kilometers and are drawn on ageological map as continuous or broken lines(see Figure 1-7). The presence of such faultsindicates that, at some time in the past,movement took place along them. Suchmovement could have been either slow slip,which produces no ground shaking, or suddenrupture (an earthquake). Figure 1-8 shows oneof the most famous examples of sudden faultrupture slips of the San Andreas fault in April1906. In contrast, the observed surface faulting

12

, SAN DIEGO.·

Chapter 1

Figure 1-7. A simplified fault map of California. (From The San Andreas Fault, by Don L. Anderson. Copyright 1971 byScientific American, Inc. AI rights reserved.)

1. THE NATURE OF EARTHQUAKE GROUND MOT10N 13

Figure 1-8. Right-lateral horizontal movement of the San Andreas Fault in the 1906 earthquake the Old Sir Francis Highway.(Photo by G.K Gilbert, courtesy of USGS.)

of most shallow focus earthquakes is muchshorter and shows much less offset. Indeed, inthe majority of earthquakes, fault rupture doesnot reach the surface and consequently is notdirectly visible. Geological mappings andgeophysical work show that faults seen at thesurface sometimes extend to depths of tens ofkilometers in the Earth's crust.

It must be emphasized that most faultsplotted on geological maps are now inactive.However, sometimes previously unrecognizedactive faults are discovered from fresh groundbreakage during an earthquake. Thus, a faultwas delineated by a line of cracks in open fieldssouth of Oroville after the Oroville dam,California earthquake of August 1, 1975. Thelast displacement to occur along a typical faultmay have taken place tens of thousands or evenmillions of years ago. The local disruptiveforces in the Earth nearby may have subsidedlong ago and chemical processes involvingwater movement may have healed the ruptures,

particularly at depth. Such an inactive fault isnot now the site of earthquakes and may neverbe again.

In seismology and earthquake engineering,the primary interest is of course in active faults,along which rock displacements can beexpected to occur. Many of these faults are inwell defined plate-edge regions of the Earth,such as the mid-oceanic ridges and youngmountain ranges. However, sudden faultdisplacements can also occur away from regionsof clear present tectonic activity (see Section1.3.1).

Fault displacement in an earthquake may bealmost entirely horizontal, as it was in the 1906San Francisco earthquake along the SanAndreas fault, but often large vertical motionsoccur, (Fig. 1-9) such as were evident in the1992 Landers earthquake. In California in the1971 San Fernando earthquake, an elevationchange of three meters occurred across theruptured fault in some places.

14 Chapter 1

Figure 1-9. Normal fault scarp associated with the 1992 Landers, California, earthquake (Photo by Dr. Marshall Lew).

1. THE NATURE OF EARTHQUAKE GROUND MOTION 15

POOTWALL

FAULT LINE

LEFT LATERAL NORMAL FAULT(LEFT OBLIQUE NORMAL FAULT)

(e)

(d) LEFT LATERAL FAULT

NORMAL FAULT

(0) REVERSE FAULT LEFT LATERAL REVERSE FAULT(I) (LEFT OBLIQUE EVEBSE FAULT)

Figure 1-10. Diagrammatic sketches of fault types

The classification of faults depends only onthe geometry and direction of relative slip.Various types are sketched in Figure 1-10. Thedip of a fault is the angle that fault surfacemakes with a horizontal plane and the strike isthe direction of the fault line exposed at theground surface relative to the north.

A strike-slip fault, sometimes called atranscurrent fault, involves displacements ofrock laterally, parallel to the strike. If when westand on one side of a fault and see the motionon the other side is from left to right, the fault isright-lateral strike-slip. Similarly, we canidentify left-lateral strike-slip.

16

A dip-slip fault is one in which the motion islargely parallel to the dip of the fault and thushas vertical components of displacement. Anormal fault is one in which the rock above theinclined fault surface moves downward relativeto the underlying crust. Faults with almostvertical slip are also included in this category.

A reverse fault is one in which the crustabove the inclined fault surface moves upwardrelative to the block below the fault. Thrustfaults are included in this category but aregenerally restricted to cases when the dip angleis small. In blind thrust faults, the slip surfacedoes not penetrate to the ground surface.

In most cases, fault slip is a mixture ofstrike-slip and dip-slip and is called obliquefaulting.

For over a decade it has been known thatdisplacement in fault zones occurs not only bysudden rupture in an earthquake but also byslow differential slippage of the sides of thefault. The fault is said to be undergoing tectoniccreep. Slippage rates range from a fewmillimeters to several centimeters.

The best examples of fault creep come fromthe San Andreas zone near Hollister, California,where a winery built straddling the fault trace isbeing slowly deformed; in the town, sidewalks,curbs, fences and homes are being offset. Onthe Hayward fault, on the east side of SanFrancisco Bay, many structures are beingdeformed and even seriously damaged by slowslip, including a large water supply tunnel, adrainage culvert and railroad tracks thatintersect the zone.

Horizontal fault slippage has now also beendetected on other faults around the world,including the north Anatolian fault at Ismetpasain Turkey and along the Jordan Valley rift inIsrael. Usually, such episodes of fault slip areaseismic-that is, they do not produce localearthquakes.

It is sometimes argued that a large damagingearthquake will not be generated along a faultthat is undergoing slow fault slip, because theslippage allows the strain in the crustal rocks tobe relieved periodically without sudden rupture.However, an alternative view is also plausible.

Chapter 1

Almost all fault zones contain a plastic clay-likematerial called fault gouge. It may be that, asthe elastic crystalline rocks of the deeper cruststain elastically and accumulate the energy to bereleased in an earthquake, the weak gougematerial at the top of the fault zone is carriedalong by the adjacent stronger rock to the sideand underneath. This would mean that the slowslip in the gouge seen at the surface is anindication that strain is being stored in thebasement rocks. The implication of this view isthat, on portions of the fault where slippageoccurs, an earthquake at depth could result fromsudden rupture, but surface offset would bereduced. On the portion where slippage is smallor nonexistent, offsets would be maximum. Aprediction of this kind can be checked afterearthquakes occur near places where slippage isknown to be taking place.

Sometimes aseismic slip is observed at theground surface along a ruptured fault that hasproduced an earlier substantial earthquake. Forexample, along the San Andreas fault break inthe 1966 earthquake on June 27 near Parkfield,California, offset of road pavement increased bya few centimeters in the days following themain earthquake. Such continued adjustment ofthe crustal rock after the initial major offset isprobably caused partly by aftershocks and partlyby the yielding of the weaker surface rocks andgouge in the fault zone as they accommodate tothe new tectonic pressures in the region.

It is clear that slow slippage, when it occursin built up areas, may have unfortunateeconomic consequences. This is another reasonwhy certain types of structures should not bebuilt across faults if at all possible. When suchstructures including dams and embankmentsmust be laid across active faults, they shouldhave jointed or flexible sections in the faultzone.

1. THE NATURE OF EARTHQUAKE GROUND MOTION

P WAVE

Figure 1-11. Ground Motion near the ground surface due to P waves. (From Nuclear Explosions and Earthquakes, byBruce A. Bolt. Copyright 1976 W. H. Freeman and Company. Used with Permission.)

17

1.5 SEISMIC WAVES

Three basic types of elastic waves make upthe shaking that is felt and causes damage in anearthquake(l-l). These waves are similar in manyimportant ways to the familiar waves in air,water, and gelatin. Of the three, only twopropagate within a body of solid rock. Thefaster of these body waves is appropriatelycalled the primary or P wave. Its motion is thesame as that of a sound wave, in that, as itspreads out, it alternately pushes (compresses)and pulls (dilates) the rock (see Figure 1-11).These P waves, just like sound waves, are ableto travel through both solid rock, such as granitemountains, and liquid material, such as volcanicmagma or the water of the oceans.

The slower wave through the body of rock iscalled the secondary or S wave. As an S wavepropagates, it shears the rocks sideways at rightangles to the direction of travel (see Figure 1­12). Thus, at the ground surface S waves can

produce both vertical and horizontal motions.The S waves cannot propagate in the liquidparts of the Earth, such as the oceans and theiramplitude is significantly reduced in liquefiedsoil.

The actual speed of P and S seismic wavesdepends on the density and elastic properties ofthe rocks and soil through which they pass. Inmost earthquakes, the P waves are felt first(l-5).The effect is similar to a sonic boom that bumpsand rattles windows. Some seconds later the Swaves arrive with their significant componentof side-to-side motion, so that the groundshaking is both vertical and horizontal. This Swave motion is most effective in damagingstructures.

The speed of P and S waves is given interms of the density of the elastic material andthe elastic moduli. We let k be the modulus ofincompressibility (bulk modulus) and Jl be themodulus of rigidity and p be the density. Thenwe have(l-5) for P waves;

18 Chapter 1

The third general type of earthquake wave iscalled a surface wave because its motion isrestricted to near the ground surface. Suchwaves correspond to ripples of water that travelacross a lake. Most of the wave motion islocated at the outside surface itself, and as thedepth below this surface increases, wavedisplacements become less and less.

Surface waves in earthquakes can be dividedinto two types. The first is called a Love wave.Its motion is essentially the same as that of Swaves that have no vertical displacement; itmoves the ground side to side in a horizontalplane parallel to the Earth's surface, but at rightangles to the direction of propagation, as can beseen from the illustration in Figure 1-13. Thesecond type of surface wave is known as aRayleigh wave. Like rolling ocean waves, thepieces of rock disturbed by a Rayleigh wavemove both vertically and horizontally in avertical plane pointed in the direction in whichthe waves are travelling. As shown by thearrows in Figure 1-14. Each piece of rockmoves in an ellipse as the wave passes.

Surface waves travel more slowly than bodywaves and, of the two surface waves, Lovewaves generally travel faster than Rayleighwaves. Thus, as the waves radiate outwardsfrom the earthquake source into the rocks of theEarth's crust, the different types of wavesseparate out from one another in a predictablepattern.

Figure J-J2. Ground motion near the ground surface dueto S waves. (From Nuclear Explosions and Earthquakes,by Bruce A. Bolt. Copyright 1976 W. H. Freeman andCompany. Used with Permission.)

LOVE WAVE

An illustration of the pattern seen at a distantplace is shown in Figure 1-15. In this example,the seismograph has recorded only the verticalmotion of the ground, and so the seismogramcontains only P, S and Rayleigh waves, becauseLove waves are not recorded by verticalinstruments.

When the body waves (the P and S waves)move through the layers of rock in the crust theyare reflected or refracted at the interfacesbetween rock types, as illustrated in Figure 1­16a. Also, whenever either one is reflected orrefracted, some of the energy of one type isconverted to waves of the other type (see Figure1-16b).

(1-1)

(1-2)

Jk+ 411V = 3

p P

forS waves;

Figure J-J3. Ground motion near the ground surface dueto Love waves. (From Nuclear Explosions andEarthquakes, by Bruce A. Bolt. Copyright 1976 W. H.Freeman and Company. Used with Permission.)

1. THE NATURE OF EARTHQUAKE GROUND MOTION

_0_0-'.---

19

­'-

Figure J_J5. A seismograph record of the vertical component of a distant earthquake (third trace on bottom) on which thearrival of P, S and Rayleigh waves are marked. (Time increases from left to right.)

0f\

RAYLEIGH WAVE

Figure 1-14. Ground motion near the ground surface dueto Rayleigh waves. (From Nuclear Explosions andEanhquakes, by Bruce A. Bolt. Copyright 1976 W. H.Freeman and Company. Used with Pennission.)

When P and S waves reach the surface ofthe ground, most of their energy is reflectedback into the crust, so that the surface isaffected almost simultaneously by upward anddownward moving waves. For this reasonconsiderable amplification of shaking typicallyoccurs near the suiface-sometimes doubling theamplitude of the upcoming waves.

This surface amplification enhances theshaking damage produced at the surface of theEarth. Indeed, in many earthquakes mineworkers below ground report less shaking thanpeople on the surface.

ROCK DISCONTINUITY (OR BOUNDARY I

b

Figure J-J6. Reflection, refraction, and transformation ofbody waves. (From Nuclear Explosions and Eanhquakes,by Bruce A. Bolt. Copyright 1999 W. H. Freeman andCompany. Used with pennission.)

20 Chapter 1

Table 1-1. Magnitudes of Some Recent Damaging Earthquakes

Date Region Deaths Magnitude (Ms)

December 7, 1988 Spitak, Armenia 25,000 7.0

August 1, 1989 West Iran, Kurima District 90 5.8

October 17, 1989 Santa Cruz Mountains, Lorna Prieta 63 7.0

June 20, 1990 Caspian Sea, Iran Above 40,000 7.3

March 13, 1992 Erzinean, Turkey 540 6.8

July 16, 1990 Luzon, Phillipines 1,700 7.8

July 12, 1993 Hokkaido, Japan 196 7.8

September 29, 1993 Killari, India 10.000 6.4

January 17, 1994 Northridge, C~lifornia 61 6.8

January 16, 1995 Kobe, Japan 5400 6.9

August 17, 1999 Izmit, Turkey 16,000 7.4

September 21, 1999 Chi Chi, Taiwan 2,200 7.6

For P and S waves in sediments, Q is about 500and 200, respectively.

The above physical description isapproximate and while it has been verifiedclosely for waves recorded by seismographs at aconsiderable distance from the wave source (thefar-field), it is not adequate to explain important

Another reason for modification of theincoming seismic wave amplitudes near theground surface is the effect of layers ofweathered rock and soil. When the elasticmoduli have a mismatch from one layer toanother, the layers act as wave filters amplifyingthe waves at some frequencies anddeamplifying them at others. Resonance effectsat certain frequencies occur.

Seismic waves of all types are progressivelydamped as they travel because of the non-elasticproperties of the rocks and soils. Theattenuation of S waves is greater than that of Pwaves, but for both types attenuation increasesas wave frequency increases. One usefulseismological quantity to measure damping isthe parameter Q such that the amplitude A at adistance d of a wave frequency f (Hertz) andvelocity C is given by:

A = AOe - (!ljd / qc )(1-3)

details of the heavy shaking near the center of alarge earthquake (the near-field). Near a faultthat is suddenly rupturing, the strong groundshaking in the associated earthquake consists ofa mixture of various kinds of seismic waves thathave not separated very distinctly. Tocomplicate the matter, because the source ofradiating seismic energy is itself spread outacross an area, the type of ground motion maybe further mixed together. This complicationmakes identification of P, S and surface waveson strong motion records obtained near to therupturing fault particularly difficult. However,much progress in this skill, based on intensestudy and theoretical modeling, has been madein recent years. This advance has made possiblethe computation of realistic ground motions atspecified sites for engineering designpurposes(l-6).

A final point about seismic waves is worthemphasizing here. There is considerableevidence, observational and theoretical, thatearthquake waves are affected by both soilconditions and topography. For example, inweathered surface rocks, in alluvium and water­filled soil, the size of P, S and surface wavesmay be either increased or decreased dependingon wave frequency as they pass to and along thesurface from the more rigid basement rock.