Microearthquake Seismology and Seismotectonics of South...

Microearthquake Seismology andSeismotectonics of South Asia

Microearthquake Seismology andSeismotectonics of South Asia

By

J.R. KayalFormer Deputy Director General (Geophysics)

Geological Survey of India, Kolkata, IndiaEmeritus Scientist (CSIR)

Jadavpur University, Kolkata, Indiaand

Adjunct ProfessorIndian School of Mines, Dhanbad, India

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Foreword

Hardly a week passes without our learning of natural geologic disastersomewhere in the world, be it a volcanic eruption, landslide, or destructiveearthquake. The prominent public notice given to such events is not only theresult of better communications, but also results from the increased impactof these events on a growing human population. In recent years, the populationhas increased greatly in regions of active tectonics. Northern India and thesurrounding areas are prime examples. The consequence is that people andtheir man-made structures are concentrated close to active faults and steep,landslide-prone terrains. In just the past several years, even moderateearthquakes with seismic magnitudes less than 6.5 have killed as many as20,000 people precisely because these earthquakes occurred directly beneathpopulation centres in central India.

The greater Himalayan region, including the Ganges Plain, is a primeexample of the coexistence of a pronounced geological hazard with a growinghuman population. Due in part to the spectacular topography, the region haslong attracted scientific investigations, and may be considered as the birthplaceof modern studies of earthquake hazards. R.D. Oldham (1858-1936) of theGeological Survey of India played a prominent role in the development ofmodern studies of historical seismicity, active faulting and seismic waveanalysis. Oldham published extensively on the earthquakes and the geologyof India, including his report entitled “Catalogue of Indian earthquakes fromthe earliest time to the end of A.D. 1869” (Mem. Geol. Surv. India, 19, 1882,52 pp.) and his massive “Report on the great earthquake of 12th June, 1897”(Mem. Geol. Surv. India, 29, 1899, 1379 pp.). Later, Middlemiss reported onthe great Kangra earthquake of 1905 (Mem. Geol. Surv. India, 32, 1905, 36pp.). D.N. Wadia (1883-1969) was a pioneer in geologic and tectonic studies,and the Wadia Institute of Himalayan Geology in Dehradun bears his name.A.N. Tandon and his colleagues at the India Meteorological Departmenthave carried out detailed studies of major earthquakes that have occurredsince the 1950’s.

The present comprehensive monograph by J.R. Kayal is, therefore, acontinuation of the proud tradition established by several outstandinggeologists and seismologists who have devoted their scientific lives to studying

the underlying causes of earthquake hazards in the greater Himalayan region.Undoubtedly, investigations such as the present one by J.R. Kayal will providethe necessary basis for a safer coexistence of mankind and geological hazardin this region. It is also my wish that this monograph, like that of Oldham(1899), will inspire young readers to join this exciting path of scientificdiscovery.

Walter D. Mooney, PhDSenior Research Seismologist

United States Geological Survey (USGS)Menlo Park, California USA

and Seismology Section PresidentAmerican Geophysical Union (AGU)

vi Foreword

Preface

This book is a humble effort to share my experiences in the field of seismologyand seismotectonics. During my PhD study in early 1980s at the VictoriaUniversity of Wellington (VUW), New Zealand, I came across only onebook on Microearthquake Seismology by Lee and Stewart (1981). We gethundreds of books on theoretical seismology and thousands of research paperson theoretical as well as on observational seismology, but not many bookson observational seismology or seismotectonics. I felt this demand in mystudent life as well as in my professional life. I think it is appropriate to havesuch a reference book on South Asia region, that may help to foster morecommunication between seismologists and geologists at national andinternational levels.

Instrument seismology in microearthquake networks has developed veryrapidly, from analog system to 24-bit digital to broadband system within afew years, say within the time span of 1970-2000. Understanding andinterpretation of observational seismology, however, need to keep an equalpace with the higher quality of seismograms and earthquake data. The authorhas worked with both systems, analog and digital. Now various softwaresystems are also available for seismic data analysis. This requires upgradedtraining and understanding. For example, fault-plane solutions of earthquakesmay now be quickly obtained using software, but its judgement andinterpretation need proper understanding, training and experience. Particularlywith respect to newcomers in the field, I noticed this gap.

In this humble effort, I tried to bridge the gap. I tried to explain the basicsand then the interpretation with many case histories of earthquake study inthe Indian subcontinent. This work took much longer than I expected. Myofficial commitments and field as well as research and teaching activitieskept me too busy to complete this book earlier, but made the book richerwith time as more and more data were available. During the last decadethere were seven devastating earthquakes in the Indian subcontinent (1991Uttarkashi, 1993 Killari, 1997 Jabalpur, 1999 Chamoli, 2001 Bhuj, 2004Andaman-Sumatra and 2005 Kashmir earthquakes). The author happened tolead all the investigations, for the Geological Survey of India, except the2005 event. Investigations of these events made the book more interesting

viii Preface

with more case histories and more understanding of the earthquake processesin different tectonic regions of South Asia.

I share my lifetime experiences in this book, and have tried to reviewseismotectonic researches, as much as possible, in this region of South Asia.It is a huge job to review all research publications, and I confess that acomplete review was not possible. Much remains to be done in the nextvolume of the book. Any comments and feedback from readers are welcomedvery much and may help me in my future endeavour.

I thank my professor, guide and philosopher, Late Prof. Frank F. Evisonwho introduced me to the field of Seismology as his PhD student under aCommonwealth Scholarship scheme at the Victoria University of Wellington,New Zealand and Prof. R.K. Verma (Retd. Professor, Indian School of Mines,Dhanbad), who first encouraged me to take up this work about 10 years ago.I have made an attempt to honour their esteemed expectation and love to me.My family support provided me to carry out this huge task, without whichit would not have been possible. Thanks to my colleagues and coauthors ofmy publications who worked with me in the field and in the laboratory.

This book reviews some principles and applications of microearthquakenetworks and an overview of the published results of the field surveyscarried out by the author in the Geological Survey of India and by others indifferent organizations/national and international institutes in various partsof the Indian sub-continent. In this volume, seismotectonics of the Himalayas,peninsular India, northeast India and adjoining areas including Bangladesh,Sri Lanka, Indo-Burma ranges, and Andaman-Sumatra regions are discussed.

Different types of earthquakes in the light of plate tectonics and variousseismic waves generated by earthquakes are described in Chapter 2. Inaddition, magnitude and intensity scales, earthquake hazard and the greatearthquakes of India are briefly discussed in this chapter. Chapter 3 containsmicroearthquake field surveys and data analysis. It describes microearthquakerecording system, data acquisition, data processing, microearthquake location,seismic tomography etc. Since excellent books are available on theoreticalseismology, theoretical part is dealt in minimum. A special emphasis onfault-plane solution is given in Chapter 4. Readable accounts on fault-planesolution, particularly for the microearthquake network data, seems to belacking in the present literature. Although software is now available to obtainfault-plane solution, an attempt is made to present this topic in an introductorymanner for clear understanding of this very important technique. Chapter 5sheds some light on seismotectonics of the northwestern, western, central(Nepal) and eastern Himalayas (Sikkim and Bhutan), the 2500 km longseismic belt, from Hindu Kush in the northwest to Arunachal Himalaya inthe northeast. The recent aftershock investigations of the Uttarkashi earthquakeof October 20, 1991, the Chamoli earthquake of March 28, 1999 and resultsof several permanent and temporary microearthquake network data shed newlight on the seismotectonic models of the Himalayas.

Preface ix

Chapter 6 deals with seismotectonics of northeast India and adjoiningsouth Asia region, which includes northeastern Himalaya, Burmese Arc,Shillong Plateau, Assam Valley, Tripura fold belt and adjoining areas likeTibet, southeast China, Indo-Burma ranges, Bengal basin (Bangladesh) etc.Seismotectonics of the Andaman-Sumatra region is briefly discussed in thischapter in view of the 2004 mega thrust (Mw 9.3) event; a detailed discussionof the Andaman-Sunda arc needs to be dealt separately. The seismologicaldata and the results of several temporary microearthquake surveys andpermanent digital networks in various parts of northeast India are reviewed.A detailed study of 3-D velocity structure, fractal dimension, b-value mappingin northeast India region is also given in this chapter. Pop-up as well astransverse tectonics in the Shillong Plateau is critically reviewed. Chapter 7describes seismotectonics of the intraplate earthquakes of peninsular Indiawith particular references to the Killari (Latur) earthquake (M 6.3) ofSeptember 30, 1993, Jabalpur earthquake (M 6.0) of May 22, 1997 and thedevastating Bhuj earthquake (M 7.7) of January 26, 2001. Results of a fewtemporary microearthquake surveys and permanent network data in the shieldareas are also discussed. The seismic tomography results of the 1993 Killariearthquake and the 2001 Bhuj earthquake source areas shed new light on thegenerating process of the intraplate earthquakes.

To make this book more useful a glossary and a subject index are given.I hope that this work will provide some guidance and useful data togeoscientists and teachers who are involved in this field, and to the beginnerswho are aiming to start their career as a field seismologist or earthquakegeologist.

J.R. Kayal

About the Author

J.R. Kayal, former Deputy Director General (Geophysics), Geological Surveyof India, Kolkata, is Emeritus Scientist (CSIR), Jadavpur University, Kolkataand Adjunct Professor, Indian School of Mines, Dhanbad, and guest facultyat the Tezpur University, Assam, and UNESCO training courses in SouthAsia. He did his M.Sc. Applied Geophysics in 1969 from the Indian Schoolof Mines, Dhanbad, and PhD from the Victoria University of Wellington,New Zealand on a Commonwealth Scholarship (1979-83). He has beenawarded National Mineral Award (1994) by Govt of India and awardedFellow by several scientific societies in India and abroad. He is a visitingscientist/professor to several national and international universities/institutes,and leads several national and international projects in earthquake seismology.

Acknowledgements

Since this book focuses on developments in our understanding of earthquakescience, with a particular reference to Indian subcontinent, most of the earthscientists, whose work is discussed in this book, are my colleagues andfriends, and I am proud to be a part of this respectable geoscience communityin India and abroad. I owe my sincere gratitude to my friends and earthscience colleagues all over the world whose names are sprinkled through thepages of this book. My attempt to name them all would exceed the spacelimitations of this short section.

The list of my immediate colleagues who worked with me all along maynot be too long and more amenable to enumeration. G. Karunakar, DevenChowdhury and Moly Mitra gave me working hand to carry a big part of theload in preparing the manuscript. Without Karunakar’s help in graphics, somany figures would not have been possible to illustrate. I owe my sinceregratitude to my dear colleagues late Reena De, Sagina Ram, Pankaj MalaBhattacharya, S.G. Gaonkar, O.P. Mishra, O.P. Singh, Pranab Chakraborty,G.K. Chakraborty, S.N. Chowdhury, B. Srirama, P.R. Rao, Balaram Das,Debangshu Banerjee, P.C. Das, M.K. Rai, R.P. Singh, A.K. Pawa, C.S. Verma,C.B.K. Sastry, L.K. Das, B. Banerjee, C.S. Venkiteswaran, T.R. Goswami,T.K. Sinha, A. Roy, S.K. Roy, D.R. Nandy, R.K. Chaturbedi, Prabhas Pande,Sujit Dasgupta and many others who worked with me in field investigations.A major part of this book is based on the field investigations that the authorled during the last three decades.

The UNESCO and USGS nominated me as a member of the South AsiaSeismic Analysis team since 2001 and as a visiting faculty for its trainingcourses since 2005 that provided many information and direct interactionswith many friends of South Asian countries. Leadership opportunities inseveral international projects like IGCP 411, IGCP 474 and Indo-RussianILTP project helped me in making a comprehensive review work to theextent possible.

I must acknowledge the direct encouragement, help and inspiration thatI received from many respectable leaders and earth scientists in India andabroad, like R.N. Bose, S.K. Acharyya, S.K. Mazumdar, Late S.N. Saha,R.K. Verma, V.K. Gaur, T.M. Mahadevan, H.K. Gupta, B.K. Rastogi,

V.P. Dimri, S.K. Guha, K.N. Khattri, Y. Sreedahar Murthy, SagarikaMukhopadhyay, Saurabh Baruah, S.N. Bhattacharya, Manoj Mukhopadhyay,C.P. Rajendran, Kusala Rajendran, N. Purna Chandra Rao, V. K. Gahalaut,S.S. Rai, Ravi Kumar, P. Mandal, Paramesh Banerjee, S.K. Nath, PradeepTalwani, Shamita Das, Walter Mooney, Peter Suhadolc, Dapeng Zhao, EuanSmith, Martin Reyners, Russel Robinson, S.K. Singh, David Gubbins, JamesMori, Roger Bilham, E.R. Engdahl, Susan Haugh – to name a few.

I was fortunate to have all supports from my family, brothers, sisters, andrelatives. I dedicate this book to my wife (Sunanda), son (Saurav) and daughter(Dahlia), for simple reasons that any husband or parent who has written abook would understand.

xii Acknowledgements

Contents

Foreword vPreface viiAbout the Author xAcknowledgements xiAbbreviations xvii

1. Introduction 11.1 Classification of Earthquakes and Frequency-Magnitude

Relation 11.2 Historical Development of Earthquake Seismology and

Global Network 21.3 Historical Development of Microearthquake Network 31.4 Applications of Microearthquake Network 5

2. Earthquakes and Seismic Waves 6

Earthquakes

2.1 Types or Causes of Earthquakes 62.2 Teleseismic, Regional and Local Earthquakes 122.3 Foreshocks, Aftershocks and Earthquake Swarms 142.4 Earthquake Magnitude, Intensity and Energy 162.5 Some Useful Definitions in Earthquake Seismology 252.6 Earthquake Effects and Hazards 292.7 Great Earthquakes of India 362.8 Large and Damaging Earthquakes of India 422.9 Permanent Seismological Observatories in India 46

2.10 Indian Earthquake Catalogues 502.11 Seismic Zoning Map of India 51

Seismic Waves

2.12 Types of Seismic Waves 532.13 Seismic Waves and Ground Shaking 602.14 Seismic Phases at the Rock Boundaries 612.15 Seismic Phases at Short Distances 63

2.16 Travel Times in a Layered Earth 662.17 Teleseismic Waves and Interior of the Earth 702.18 Wave Attenuation 762.19 Seismic Diffraction 77

3. Microearthquake Recording and Data Analysis 813.1 Seismometers: Theory and Practice 823.2 Seismographs: Recording Systems 943.3 Development of Modern Seismic System 1013.4 Station Distribution and Site Selection 1073.5 Analog Record Processing and Record Keeping 1093.6 Digital Data Management 1103.7 Basic Data for Analysis 1113.8 Earthquake Location 1133.9 Computer Programs for Earthquake Location 118

3.10 Limitations of Accuracy in Location 1253.11 Earthquake Relocation 1283.12 Seismic Tomography and Earthquake Location 1313.13 Receiver Function 1403.14 Seismic Waveform Inversion and Modelling 1413.15 Microearthquake Magnitude 1443.16 Earthquake Phenomena: Power Law Relations 146

4. Dynamics of Faulting and Fault Plane Solution 1524.1 Classification of Faults 1524.2 Force and Stress 1554.3 Criteria of Fracture 1574.4 Griffith’s Theory of Crack Propagation 1584.5 Dynamics of Faulting 1594.6 Elastic Rebound Theory 1624.7 Equal Area Projection and Fault Plane Solution 1644.8 Moment Tensor Solution 1744.9 Earthquake Mechanisms and Plate Tectonics 176

5. Himalayas, Pamir-Hindu Kush and Foredeep Region 1805.1 Introduction 1805.2 Geological Process 1835.3 Geophysical Data in Himalayas 1855.4 Conceptual Tectonic Models 1945.5 Himalayan Foredeep 1995.6 Large Earthquakes in the Himalaya 2035.7 Western Syntaxis and Pamir-Hindu Kush Seismicity 2045.8 Northwest and Western Himalayan Seismicity 2205.9 Central and Eastern Himalayan Seismicity 242

xiv Contents

5.10 Seismotectonic Perspective 2495.11 Seismic Hazard Potential 261

6. Northeast India, Myanmar, Bangladesh andAndaman-Sumatra Region 2666.1 Introduction 2666.2 Geological Structure 2686.3 Gravity Anomaly 2776.4 Crustal Structure 2796.5 Seismological Data 2826.6 Microearthquake Data 2976.7 Seismotectonic Perspective 3156.8 Strong Motion Data 3266.9 Seismic Hazard Potential 327

6.10 Tectonics and Seismicity of Andaman-Sumatra Arc 334

7. Seismotectonics of Peninsular India and Sri Lanka 3487.1 Introduction 3487.2 Geological Evolution 3527.3 Geophysical Features 3657.4 Past Seismic Activity 3857.5 Recent Seismicity: Microearthquake Networks 4007.6 Aftershock Investigations: Recent Damaging Earthquakes 4147.7 Sri Lanka: Tectonic Configuration 4427.8 Conclusion and Recommendation 447

References 450Index 496

Contents xv

Abbreviations

ABF Allah Bund FaultADC Analog to Digital ConvertorADMB Aravalli Delhi Mobile BeltAIN Angle of IncidenceAK AchankovilAMD Atomic Mineral DivisionANF Alokananda FaultARL Anjar Rapar LineamentASR Andaman Spreading RidgeBARC Bhaba Atomic Research CentreBARS Belt of Active Rift SystemsBA Burmese ArcBB Broad BandBC Bhandra CratonBIS Bureau Indian StandardBN BundelkhundBL Bhachau LineamentBS Bastar CratonBSF Barwani Sukta FaultBT Basement ThrustBTF Basement Thrust FrontCBR Carlsberg ridgeCCR Central Core RegionCESS Centre for Earth Science StudiesCGS Coast and Geodetic SurveyCIS Central India SutureCMB Core Mantle BoundaryCMP Common MidpointCMT Centroid Moment TensorCRB Cambay Rift BasinCRS Central Recording StationCSIR Council of Scientific and Industrial ResearchCWPRS Central Water and Power Research StationDAFB Delhi Aravalli Fold Belt

xviii Abbreviations

Db.F Dhubri FaultDD Double DifferenceDF Dauki FaultDSS Deep Seismic SoundingsDST Department of Science and TechnologyDVP Deccan Volcanic ProvinceEDC East Dharwar CratonEGF Empirical Green’s FunctionEGGB Eastern Ghat Granulite BeltEGMB Eastern Ghat Mobile BeltER East RidgeERI Earthquake Research Institute (Japan)ESC European Seismological CommissionGBF Great Boundary FaultGDSN Global Digital Seismic NetworkGERI Gujarat Engineering Research InstituteGL Goalpara lineamentGMT Greenwich Mean TimeGPS Global Positioning SystemGR Godavari RiftG-R Gutenberg and RichterGSHAP Global Seismic Hazard Assessment ProgrammeGSI Geological Survey of IndiaGSN Global Seismographic NetworkGTF German Task ForceHFF Himalayan Frontal FaultHFT Himalayan Frontal ThrustHFU Heat Flow UnitHRV Harvard UniversityHSB Himalayan Seismic BeltHVZ High Velocity ZoneHYB HyderabadIASPEI International Association of Seismology and Physics

of the Earth’s InteriorIBF Island Belt FaultIGAP Indo-Gangetic Alluvial PlainsIGP Indo-Gangetic PlainsIIT (K) Indian Institute of Technology (Kharagpur)IIT(R) Indian Institute of Technology (Roorkee)ILTP Integrated Long Term Program (Indo-Russian)IMD India Meteorological DepartmentINDEPH International Deep Profiling in Tibet and HimalayaIRIS Incorporated Research Institute for SeismologyISC International Seismological CentreISPP Indo Stanvac Petroleum ProjectISR Institute of Seismological Research

Abbreviations xix

IST Indus Suture ThrustJ-B Jeffreys-BullenJ&K Jammu & KashmirJHD Joint Hypocentre DeterminationKaF Karakuram FaultKC Karnataka CratonKLP KalpaKMF Kutch Mainland FaultKRB Kutch Rift BasinKSEB Kerala State Electricity BoardKSZ Koyna Seismic ZoneLET Local Earthquake TomographyLP Long PeriodLTA Long Term AverageLVZ Low Velocity ZoneMBT Main Boundary ThrustMCS Mercalli-Cancani-SiebergMCT Main Central ThrustMERI Maharashtra Engineering Research InstituteMEQ Micro EarthquakeMG Mahanadi GrabenMHSB Main Himalayan Seismic BeltMIL MilneM-K Malda KishenganjMKN MakniMKT Main Karakuram ThrustMM Modified MercalliMMT Main Mantle ThrustMoho Mohorovi�i�M-S Munger-SaharsaMSK Medvedev-Sponheuer-KarnikMSR Munger Saharsa RidgeMT MagnetotelluricNCSN Northern California Seismic NetworkNEIC National Earthquake Information CenterNEZ North Escarpment ZoneNGRI National Geophysical Research InstituteNIED National Research Institute for Earth Science and

Disaster PreventionNKF North Kathiwar FaultNNF Narmada North FaultNPF Nagar Parkar FaultNRB Narmada Rift BasinNSF Narmada South FaultNSL Narmada-Son LineamentOBS Ocean Bottom Seismograph

xx Abbreviations

OIL Oil India LimitedONGC Oil & Natural Gas Commission (now Corporation)PGA Peak Ground AccelerationPF Patna FaultRF Rossi ForelRIS Reservoir Induced SeismicityRMS Root Mean SquareRRL - J Regional Research Laboratory - JorhatRSVP Rajmahal-Sylhet Volcanic ProvinceRTS Reservoir Triggered SeismicitySC Singhbhum CratonSCB Saurashtra Continental BlockSCR Stable Continental RegionSCSI Small Computer System InterfaceSCSN Southern California Seismic NetworkSEPB South East Platformal BlockSEZ South Escarpment ZoneSGT Southern Granulite TerrainSIRT Simultaneous Iterative Reconstruction TechniqueSMA Strong Motion ArraySOI Survey of IndiaSONATA Son-Narmada-Tapti LineamentSP Short PeriodSPS Samples Per SecondSRN SrinagarSRO Seismic Research ObservatoriesSTA Short Term AverageSTF Source Time FunctionSVD Singular Value DecompositionTAN TandonTHDC Tehri Hydro Development CorporationTHF Trans-Himadri FaultTL Tapti LineamentUNESCO United Nations Educational, Scientific and Cultural

OrganizationUPS Uninterrupted Power SupplyUSGS United States Geological SurveyVBB Very BroadbandVUW Victoria University of WellingtonWAF West Andaman FaultWCF West Coast FaultWDC West Dharwar CratonWIHG Wadia Institute of Himalayan GeologyWK WajrakarurWSZ Warna Seismic ZoneWWSSN World Wide Seismograph Station Network

1Introduction

Seismology is the study of generation, propagation and recording of elasticwaves or seismic waves in the Earth (and other celestial bodies) and of thesource, which produces them. The sources can be natural earthquakes orman-made sources of deformational energy that generate the seismic waves.Study of the seismic waves of the Earth provides the highest resolution ofthe internal Earth structure that no other geophysical method can provide. Itis a branch of geophysics with the highest societal impact, both in assessingand reducing earthquake hazards, and in understanding seismotectonics, thestructure and dynamic processes of the Earth.

1.1 CLASSIFICATION OF EARTHQUAKES ANDFREQUENCY-MAGNITUDE RELATION

Seismologists had used the words like small, medium or large to describe anearthquake size. After introduction of the Richter magnitude-scale (Richter,1935), the classification has been more definite. Hagiwara (1964) classifiedthe earthquakes, according to magnitude, as follows:

Magnitude (M) ClassificationM � 8 Great earthquake7 � M < 8 Major or Large earthquake5 � M < 7 Moderate earthquake3 < M < 5 Small earthquake1 � M < 3 MicroearthquakeM < 1 Ultra-microearthquake

All earthquakes of magnitude less than 3 are generally called micro-earthquakes. Gutenberg and Richter (1941) found that the frequency ofearthquake occurrence is related to their magnitudes. The frequency-magnituderelation is given by: log10 N = a – bM, where N = number of earthquakesof magnitude M or greater, a and b are constants. The parameter b is oftenreferred to b-slope or b-value.

2 Microearthquake Seismology and Seismotectonics of South Asia

Many studies of earthquake statistics show that in the normal cases thevalue of b is approximately 1.0, which means that the number of earthquakesincreases tenfold for each decrease of one magnitude unit. Thus, if an areaproduces one earthquake of magnitude 4 in a month, it would produce 10earthquakes of magnitude 3, 100 earthquakes of magnitude 2 and 1000earthquakes of magnitude 1 in one month. Therefore, smaller the magnitudeof the earthquakes, greater the amount of seismic data one can record.Microearthquake networks are designed to take an advantage of the abovefrequency-magnitude relation. Table 1.1 illustrates the global observations offrequency of occurrence of earthquakes since 1900, and Table 1.2 illustratesthe number of earthquakes located by the National Earthquake InformationCenter (NEIC), US Geological Survey (USGS) for the period 1990-2002and the estimated loss of lives.

Table 1.1: Frequency of occurrence of earthquakes based on observationssince 1990, NEIC, USGS

Description Magnitude Average Annually

Great 8 and higher 1Major 7 - 7.9 18Strong 6 - 6.9 120Moderate 5 - 5.9 800Small 4 - 4.9 6,200 (estimated)Minor 3 - 3.9 49,000 (estimated)Microearthquakes <3.0 Magnitude 2 – 3: about 1000 per day

Magnitude 1 – 2: about 8000 per day

(Note: Description or classification as per USGS)

Table 1.2: Number of earthquakes worldwide, 1990-2000, NEIC, USGS

Magnitude 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

8.0 to 9.9 0 0 0 1 2 3 1 0 2 4 1 1 07.0 to 7.9 12 11 23 15 13 22 21 20 14 23 14 15 136.0 to 6.9 115 105 104 141 161 185 160 125 113 113 158 126 1335.0 to 5.9 1635 1469 1541 1449 1542 1327 1223 1118 979 982 1345 1243 10374.0 to 4.9 4493 4372 5196 5034 4544 8140 8794 7938 7303 6998 8045 8084 9034Estimated 51916 2326 3814 10036 1038 7949 419 2097 8928 22711 231 21357 1711 Deaths

1.2 HISTORICAL DEVELOPMENT OF EARTHQUAKESEISMOLOGY AND GLOBAL NETWORK

Seismology is relatively a young science, which is basically developed withthe evolution of theory of elasticity and development of instrumental andobservational data base. Although the Chinese had a first operational seismic-wave detector (seismoscope) around 132 AD, the theoretical seismology was

Introduction 3

considerably ahead of observational seismology until late 1800s. The evolutioncame with the Hook’s law indicating proportionality between stress andstrain, which was followed by equations of elasticity by Navier and Canchyin 1921-22. In 1830, Poisson used the equations of motion and laws ofelasticity to show two fundamental types of waves propagate through theinterior of homogeneous solids. These two waves are P wave (Primary orCompressional wave) and S wave (Secondary or Shear wave). In 1887, LordRayleigh discovered existence of surface wave called Rayleigh wave, and in1911 a second type of surface wave was discovered by A.E. Love, called Lovewave. Both these waves are the results from the interaction of P and S waveswith boundary conditions on the body with no shear stress of the surface.

International efforts led to the invention of the first seismometer by FilippoCecchi in Italy in 1875 (Lay and Wallace, 1995). The sensitivity of earlyseismometer improved rapidly, and in 1889 accurate recording of seismicwaves from a distant earthquake was possible in Potsdam, 15 minutes afterthe earthquake in Japan, demonstrating that the Earth behaves as a nearlyelastic body in the frequency band of most seismic observations.

In early 1960s, a World Wide Seismograph Station Network (WWSSN)was established with identical sets of short-period and long-period three-component seismographs. This global network was equipped with veryaccurate timing by crystal clocks and standardized instrumentation; about120 stations were in operation. The impact of the WWSSN was tremendous,coming at the time of plate tectonics revolution, when accurate seismicrecordings and earthquake location were critical. In around 1975, the WWSSNwas superseded by the Seismic Research Observatories (SRO), and eventuallythe network was digitally upgraded to the Global Digital Seismic Network(GDSN) in 1980s jointly by the Incorporated Research Institute for Seismology(IRIS) and the US Geological Survey (USGS). Due to large ocean surfaceas well as political situations, the network was not well distributed. Thenetwork is, however, growing continuously in collaboration with the Frenchproject GEOSCOPE together with instruments deployed by Australia, Canada,Europe and Japan. The ocean basins, of course, cause substantial gaps in thecoverage, and Ocean Bottom Seismographs (OBS) are planned to minimizethese gaps. In mid 2004, the Global Seismographic Network (GSN) surpassesits design goal (Park et al., 2005); a total of 136 GSN stations are now inoperation from south pole to Siberia, and from Andaman basin to the seafloor of the northeast Pacific ocean. All GSN broadband data are freelyavailable through the internet (www.iris.edu).

1.3 HISTORICAL DEVELOPMENT OFMICROEARTHQUAKE NETWORK

In terms of historical development, microearthquake networks evolved fromtemporary expeditions for aftershock study of large earthquakes to


reconnaissance surveys, and finally mobile arrays to permanent telemeterednetworks. By the discovery of the earthquake frequency-magnitude relation(Gutenberg and Richter, 1941), methods and techniques to studymicroearthquakes have been developed by various groups of seismologist inseveral countries, like Japan, South Africa, USA and USSR. Althoughimplementation of technological advances in instrumentation, datatransmission and data processing in microearthquake networks was made in1970s, many pioneering investigations were carried out in many countriesmuch early. A detailed account of historical development is given by Lee andStewart (1981). A brief outline is given here.

Japanese seismologists first developed portable and sensitive seismographsto study aftershocks, and Asada (1957) first reported a detailed study ofmicroearthquakes. In the 1960s many microearthquake networks wereestablished in Japan. These networks consisted 5-15 stations each and werereported by many groups (e.g. Suzuki et al., 1979).

In USA, the first microearthquake study was reported by Sanford andHolmes (1962) in a manner similar to Asada (1957). J.P. Eaton first developeda telemetered network for precise location of microearthquakes in Hawaii(Eaton, 1962). Since then several groups in the United States beganmicroearthquake surveys, notable among them were J. Oliver and his group(e.g. Oliver et al., 1966), J.N. Brune and his group (e.g. Brune and Allen,1967). A detailed study of the aftershocks of the 1966 Parkfield earthquakeby Eaton et al. (1970a) demonstrated the usefulness of microearthquakenetworks in understanding the earthquake generating process, and made agreat impact to develop the Central California Telemetric Network on largescale (Eaton et al., 1970b).

In India, microearthquake investigation started in 1978 for geothermalexploration in Puga valley and also to cater the need of river valley projectsin 1980s in northwestern Himalaya (IMD, 1980 and 1988). K.N. Khattri andhis group in the University of Roorkee (now Indian Institute of Technology,Roorkee), and S.N. Saha and his group in the Geological Survey of India(GSI) made a collaborative microearthquake investigation in northeast Indiaregion in 1980s (Khattri et al., 1983). The investigation in northeast Indiawas extended by the author and his group in the GSI (e.g. Kayal, 1987;Kayal and De, 1987). Two permanent telemetric networks were establishedin late 1980s on experimental basis, one by the then University of Roorkeein Garhwal Himalaya, and the other by the NGRI in northeast India. Theaftershock study of the 1991 Uttarkashi earthquake in the Garhwal Himalaya,and the aftershock study of the 1993 Killari (Latur) earthquake in Maharashtra,in particular, made tremendous impact on modernisation of the permanentseismological observatories as well as microearthquake networks in India. Agreat deal of effort has been made by the Department of Science andTechnology (DST) to upgrade the permanent seismological observatories ofthe India Meteorological Department (IMD) to GSN standard, and to establish

Introduction 5

more permanent observatories and telemetric networks in different earthquakeprone zones of the country. Many institutes and universities in the countryare also funded by the DST for running microearthquake networks andpermanent observatories.

1.4 APPLICATIONS OF MICROEARTHQUAKENETWORK

Microearthquake networks are powerful tool for detailed seismologicalinvestigations. These networks generate large quantity of seismic data becauseof high occurrence-rate of microearthquakes. Applications of microearthquakenetworks are numerous; to name a few:

(i) mapping seismicity and active faults/lineaments,(ii) studying focal mechanism of earthquakes, tectonic stress etc.,

(iii) investigating velocity structure of the crust and upper mantle,(iv) exploring geothermal resources,(v) investigating aftershocks, swarm activity etc.,

(vi) developing mining industry, hydel-dam projects, nuclear power plantsetc.,

(vii) investigating Reservoir Induced Seismicity (RIS) and other man-madeinduced seismicity,

(viii) monitoring volcanic seismicity,(ix) monitoring earthquake precursors, etc.

Results from these studies are integrated with other geophysical andgeological field investigations for better understanding of earthquakeprocesses, seismicity, seismotectonics, crustal structure, precursors etc. inSouth Asia region. A large quantity of local microearthquake and regionalearthquake data are analysed to understand seismotectonics of the Himalayafrom the westerm syntaxis, Pamir-Hindu Kush to eastern syntaxis, ArunachalHimalaya, and a 2500 km long Himalayan seismic belt. The seismic activityin the northeast India region as well as the intense activity along the Indo-Burma and Andaman-Sumatra arcs are well studied using the recent digitalseismic network data at the atypical continental subduction zone beneath theIndo-Burma ranges and at the typical oceanic plate (Indian Ocean) subductionbeneath the Andaman-Sumatra arc. The complex intraplate tectonics ofpeninsular India, Bangladesh and Sri Lanka are also well studied; the typicalintraplate seismicity, RIS, aftershocks, swarms and active faults areinvestigated using the microearthquake network data.

2Earthquakes and Seismic Waves

EARTHQUAKES

Earthquakes are exciting and sometimes devastating. A large earthquakeattracts general attention because it causes damage to houses and other worksof man, or breaks up and changes the ground surfaces. To the engineers, theearthquakes of great interest are those which affect the stability and safetyof the buildings and other constructions. To the geologists, the earthquakephenomena of interest are the evidence of faulting or other tectonic processand the relation of earthquakes to the geological structures. And for thegeophysicists or seismologists, every earthquake, large or small, presents aproblem of explanation and interpretation of every wriggle of the seismicwaves produced by the earthquakes. Earthquake seismology is thus definedto be a dual science, a border line field between geology and geophysics. Itcalls for sound physical and mathematical thinking, and a strong backgroundof geological concepts.

2.1 TYPES OR CAUSES OF EARTHQUAKES

Earthquakes can be divided into four categories based on their mode ofgeneration. These are discussed below:

2.1.1 Tectonic Earthquakes

Tectonic earthquakes are by far the most common earthquakes. Theseearthquakes are produced when the rocks fracture by various geologicalforces because of constant geological reshaping of the Earth. Tectonicearthquakes are most important to study the structure and interior of theEarth, and they are of much social significance as they pose greatest hazard.

Earthquakes and Seismic Waves 7

Sensitive instruments register major tectonic earthquakes at large distancesdemonstrating that such events disturb the whole Earth.

Plate tectonics concept is the most recent and quite satisfying explanationfor majority of the earthquakes. The plate tectonic hypothesis is defined ina series of papers between 1967 and 1971 (e.g. Elsasser, 1967; McKenzie,1967 and 1969; Oliver and Isacks, 1967; Isacks and Molnar, 1971 etc.).Readers are referred to read a few excellent books on Plate Tectonics, e.g.Le Pichon et al. (1976), Condie (1982) etc. The basic idea in plate tectonicsis that the Earth’s outermost part, called lithosphere, consists of several largeand fairly stable slabs; these are called plates. Boundaries of these plates arethe seismic belts of the world. The major plates and seismic belts are shownin Fig. 2.1. The plates move horizontally relative to adjoining plates on alayer of softer rock, called the asthenosphere. Lava is continually upwellingat the mid-oceanic ridges. This rock slowly moves across the Earth’s surfaceas a new sea-floor on either side of the ridge. In this way plates move likea conveyer belt over the asthenosphere, and get cooled as they get furtheraway from the ridges. The mid-oceanic ridges are called spreading zones oraccreting plate boundaries or divergent plate boundaries (Fig. 2.2).

Fig. 2.1 World map showing relation between tectonic plates, seismicity (red dots)and volcanoes (triangle). Red boundary indicates spreading zone, yellow subduction

zone, blue transcurrent and white diffuse zone(website: http://www.geol.binghamton.edu/faculty/jones).

Now the question arises, if a new plate is constantly being created at mid-oceanic ridges what happens to older plate? Since the Earth probably remainsthe same size over quite a long period of geological time, the moving plates


must be absorbed at some places; these are convergent plate boundaries. Theburial ground of plates is believed to be the ocean trenches, where thesurface layers of rock plunge into Earth’s interior. This process is known assubduction. The other type of convergent plate boundary forms the continent-continent collision zone. The third type of plate boundary is the transcurrentboundary, where the plates move past one another, without either convergenceor divergence. Transform faults or strike-slip faults are the examples oftranscurrent boundaries.

Fig. 2.2 (a) Schematic diagram showing oceanic plate spreading out from themid-ocean ridge and sinking under the oceanic trench, magma rises upwards abovethe subduction zone. (b) I: overriding plate, II: subducting plate. a: zone of bending,where tensional stress is dominant, b: zone of thrusting and c: sinking portion ofthe lithosphere, where compressional stress is dominant. The stress regimes areshown by two arrows, inward pointed arrows indicate zone of compression and

outward arrows tension.

Subduction Zone EarthquakesAs the oceanic plate bends downwards at the ocean trenches, fractures causeshallow earthquakes within the subducting plate as well as in the overridingplate (Fig. 2.2); these are called interplate earthquakes. In the process of itsdownward movement, additional forces are generated causing furtherdeformation and fracturing, thus giving rise to deep focus earthquakes. Thedeeper earthquakes, which occur in the subducting plate, define a cleardipping seismic zone, called Benioff zone. On the average, frequency ofoccurrence of earthquakes in this zone declines rapidly below 200 km depth.

(a)

(b)


Some earthquakes are, however, as deep as 700 km in some of the subductionzones, e.g. the Tonga Islands, Java Islands, the New Hebrides etc. Earthquakes,generally, with foci between 70-300 km deep are called intermediate focusand those below this depth are called deep focus. Along with the deep focusearthquakes, the chemical composition of rocks in the subducting platechanges. The molten fraction of rocks may be stored for a time in magmachambers, which are huge reservoirs underneath volcanic vents. From thesepressure chambers magma moves upwards from time to time as lava. Thus,a volcanic arc is a typical characteristic in a subduction zone (Fig. 2.2).Further, interaction between the two plates, overriding continental plate andthe subducting oceanic plate, near the trench causes folding, faulting andthrusting in the continental crust, and makes an exposure of ophiolite (oceaniccrust) at the surface.

Collision Zone EarthquakesTectonic earthquakes are also produced at the edges of the interacting plateswhere the plates collide head on, and rise as mountain chains, e.g. theHimalayan belt in Indo-Eurasian collision zone, the Alpine belt in the easternEurope, the Zagros thrust system in Iran (Fig. 2.1). The interplate earthquakesin collision zones are usually shallow (<100 km), and dominated by lowangle thrusting, which is a manifestation of lithospheric shortening. These lowangle faults/thrusts can generate large earthquakes as we see in the Himalayanbelt.

Divergent and Transcurrent Plate Boundary EarthquakesIn general, divergent and transcurrent plate boundaries are characterized bymuch shallower seismicity (focal depth less than 30 km). The mid-oceanicridges, the divergent zones, are the manifestation of shallow extensionalfaulting (Fig. 2.3). One of the interesting features of mid oceanic ridges isthat they are offset by lineament known as fracture zones. Wilson (1965)proposed the concept of transform faults or strike-slip faults that accommodateridge spreading. Continental strike-slip or transcurrent plate boundary mayalso be observed in continent-continent or oceanic-continental situation (Fig.2.3). The most famous example in oceanic-continental situation is the SanAndreas fault in California, which separates the North America and Pacificplates. Transcurrent plate boundary in continental settings is extremelyimportant from seismic hazard point of view. A few examples of suchcontinental transcurrent faults are the Anatolian fault in Turkey, Alpine faultin New Zealand and the Sagaing fault in Burma (Fig. 2.3). These transcurrent-fault earthquakes are shallow and sometimes produce large fatal earthquakes,e.g. the 1989 Loma Prieta earthquake on the San Andreas fault (Catchingsand Kohler, 1996).


Intraplate EarthquakesAbout 60 percent of the world’s seismicity is mapped by the interplateearthquakes at the plate boundaries as stated above, and about 10 percent ismapped at the spreading zones. The world’s seismicity map, however, showsthat earthquakes do take place within the plate, far from the plate boundariesas well (Fig. 2.1). These earthquakes are called intraplate earthquakes, andthey arise from the localised system of forces. The intraplate and interplateearthquakes differ in two important ways. First, the recurrence interval ofintraplate earthquakes is generally much longer than those of the interplateearthquakes, and second, the intraplate earthquakes typically have muchhigher stress drops. Three large/fatal intraplate earthquakes (M 6.0~7.7) thathave occurred in peninsular India during the last decade are discussed inChapter 7.

Hypocentres of the earthquakes in the intraplate zones rarely occur deeperthan 15-20 km. Deepest earthquakes indicate the base of the seismogeniczone, or bottom of the fault. Within the seismogenic zone the crust deformseither by stable or unstable sliding on the existing fault(s), or as a brittlematerial that fails generating a new fault, when subjected to stresses greaterthan the strength of the material. The strength depends on both temperatureand pressure. The strength increases to a depth of about 15-20 km, and thendecreases rapidly. Below the seismogenic zone stable sliding or ductiledeformation takes place.

2.1.2 Volcanic Earthquakes

Volcanic earthquakes are defined as the earthquakes which occur or arelinked up with the volcanic activity. These earthquakes occur in two ways.

Fig. 2.3 World stress map (Zoback, 1992); inward pointed arrows indicatecompression (thrust faulting), outward arrows tension (normal and strike slip faulting).


First, often before an eruption minor seismic activity increases in the vicinityof the volcano. Some kilometres below the volcanic vent, very hot viscousmagma moves sluggishly under high steam pressure through a network ofveins and arteries from one storage chamber to another. Due to this motion,various parts of the surrounding rock become hotter and more strained as themagma pushes them. These forces fracture the neighbouring rocks, and thestrain is relieved by small or moderate earthquakes. Second, sometimes faultrupture precedes the motion of magma and eruption of lava. The earthquakewaves from the rupturing fault may shake up the molten material in thestorage reservoir beneath the volcano. In a way, similar to the violent shakingof a bottle of soda pop, steam and gas that have previously dissolved in themagma begin to boil off and accelerate the escape of lava and gaseousmaterial. This, in turn, would disturb the unstable equilibrium of the magmabelow the vent, and stimulate local volcanic earthquakes.

2.1.3 Induced Earthquakes

Some earthquakes are induced; this means that human activity triggers theearthquakes. These are: (i) Reservoir Induced Seismicity (RIS), (ii) Nuclearexplosion, (iii) Rock burst, fluid injection, etc.

Reservoir Induced SeismicityReservoir induced earthquakes have been reported in conjunction with theimpoundment of water or rapid water level changes behind large dams (Guptaand Rastogi, 1976). Since these earthquakes are triggered along pre-existingand pre-stressed tectonic faults, they show typical characteristics of doublecouple sources (see Section 4.6.1).

The December 10, 1967 Koyna earthquake (M 6.7) in India that causeda loss of about 200 human lives, is attributed to RIS for the Koyna dam(Gupta and Rastogi, 1976). It is unlikely that simply weight of the water,which would only add a tiny fraction of the total stress at a depth 3 or 5 kmbelow the surface, causes the RIS. A more likely explanation is that the porepressure increases because of the hydrostatic head of the reservoir. The RISwas first observed with the filling of Lake Mead behind Hoover Dambeginning in 1935 (Simpson, 1976). Before the reservoir was filled, thebackground seismicity was low; the seismicity began to rise in 1936, withoccurrence of an earthquake M 5.0 in 1940. RIS is mostly shallow focus,less than 6 km. Damaging earthquakes M>6.0, that are referred to the RIS,occurred at Hsinfeugkiang, China in 1962, at Kariba, Zambia-Zimbabweborder in 1963, at Kremasta, Greece in 1966 and at Koyna, India in 1967(Gupta, 1992). In a recent book on Reservoir Induced Earthquakes by Gupta(1992) all the necessary information on RIS has been given. In his recent


review Gupta (2002), however, preferred the term RTS (Reservoir TriggeredSeismicity) rather than the RIS.

Nuclear ExplosionThe other man-made earthquake is the nuclear explosion. Theoretically theseismic waves generated by an underground nuclear explosion should bevery different from shear motion that characterizes double-couple sourcesfor natural earthquake. The nuclear explosion produces outward directedcompressional motions in all directions while tectonic earthquake producesmotions of different amplitude and polarity in different directions (see Section4.6.2). So, the seismograms from an explosion should not have SH or Lovewaves, but many explosions do have SH type energy. This energy is thoughtto be the release of pre-existing strain by the explosion. A very good casestudy of the Indian nuclear explosion at the Pokhran-test site, Rajasthan onMay 11, 1998, was made by Sikka et al. (1998). The estimated magnitudeof this explosion was mb = 5.4. They did not report SH wave.

Rock Burst, Fluid Injection etc.The third type of induced earthquakes are generated by rock burst, fluid orstream injection etc. Collapsing of the roof of mines and caverns, the so-called phenomena of rock-burst, occurs due to induced stress around themine workings, and it produces seismic waves. Readers are referred to areview book on Mining Seismology by Gibowicz and Kijko (1994) fordetailed information. Similarly, fluid or steam injection process in the oilfields for secondary production also causes minor tremors (e.g. Talebi andCornet, 1987; Talebi et al., 1998). In a recent book, Guha (2000) has givena detailed review of induced seismicity.

2.2 TELESEISMIC, REGIONAL AND LOCALEARTHQUAKES

Earthquakes may be further classified into three types depending on thedistance from source to the seismograph station. Since the character ofseismograms depends on epicentral distance, the nomenclature for seismicphases is also distant dependent.

2.2.1 Teleseismic Earthquakes

The earthquakes, which are recorded by a seismograph station at a greaterdistance, are called teleseismic earthquakes. These are very often calledteleseisms. By international convention the epicentral distance is required tobe more than 1000 km for a teleseism. Lay and Wallace (1995), however,define teleseismic distance as � > 30°. An example of a tele-seismogram is

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