Shocks and Rocks Seismologyin the Plate Tectonics Revolution Thestory of earthquakesand the great earth science revolutionof the1960s by Jack Oliver Irving PorterChurch Professor Emeritus Departmentof GeologicalSciences and Institutefor the Studyof the Continents(INSTOC) CornellUniversity Ithaca, NY AMERICAN GEOPHYSICAL UNION WASHINGTON, Historyof GeophysicsVolume 6 Publishedunder the aegis of the AGU History Committee Library of CongressCataloging-in-Publication Data Oliver,J. E. (JackErtle), 1923- Shocksand Rocks: seismology in the plate teetonics revolution: thestory of earthquakesand the great earth science revolution of the 1960s / Jack Oliver. p. cm.--(Historyof geophysics:v. 6) Includesbibliographical references (p.-) andindex. ISBN 0-87590-280-4 1. Platetectonics. 2. Seismology.I. Title. QE511.4.045 1996 551.1 '36'09-de20 96-2723 CIP ISSN: 8755-1217 ISBN: 0-87590-280-4 Copyright1996 American Geophysical Union. Short excerpts may be reprintedin scientificbooks and journals if thesource is properlycited; all otherrights reserved. Printed in the United States of America AmericanGeophysical Union 2000 Florida Avenue, N.W. Washington,D.C. CONTENTS Preface Acknowledgements xiii The Pre-1960s 1 Observingthe Shaking Learningabout the Earth and about Earthquakes 13 3 Artificial Sources 23 The 1960s 4 ContinentalDrift and SeaFloor Spreading, The Forerunners of Plate Tectonics 31 TheOrigin and Early Days of theLamont Geological Observatoryand Its Programin EarthquakeSeismology 41 Scienceby HypothesisTesting 53 Scienceby Serendipity 61 Scienceby Synthesis 75 After the 1960s 9 Full-FledgedPlate Tectonics 91 10 Strategy,Philosophy, and Things Like That 105 11 Epilogue 125 Bibliography 127 Index 131 Appendix:Seismology and the New GlobalTectonics PREFACE Duringthe decade of the1960s, science of thesolid earth underwent an astonishingand awesomeupheaval. In justa few years,geoscientists con- structeda newway of describingand understanding the dynamics of ever- changingearth, past and present, and so found a routeto explanationfor how most,if not all, of the greatfeatures of the earth'ssurface that have harboredand plaguedand enchantedhumans throughout their existence cameto be. Continents,ocean basins, mountain ranges, deep sea trenches, earthquakes,and volcanoes suddenly became explicable as consequences of earthmovements that, on a globalscale, have a remarkablysimple and readilyunderstandable pattern. The long-sought key to the ponderous and agonizinglyslow movements of earththat, over millennia, have deftly shapedour surroundings was found during that decade, or somost scien- tiststhink today, more than a quarterof a centurylater. For thosescientists studying the earth,and a few skepticsaside, the 1960swere a time of astonishmentand discovery,of delightand euphoria, ofpride, and of stimulation reinforced by successafter success. Word of the achievementsspread quickly throughoutscience and beyond. The upheavalwould be called "the plate tectonics revolution," "plate tectonics" beingthe term invented to describethe essence of thenew dynamics. It has beenclaimed by somethat the discoveryof platetectonics is the greatest achievementever of modernearth science, although those who hold brief in thisregard for oneor two otheroutstanding revelations such as recogni- tionof thegreat age of theearth or thepostulating of thetheory of evolu- tion might challengethat statement. In anycase, the achievements of the 1960s were outstanding and their storyseems to merit telling, not once from the singular perspective of a par- ticularscience historian, but multiplyfrom the diverseperspectives of sci- entistsand otherswho witnessedand/or participatedin that revolution. Thisbook is onesuch attempt. It focuseson themany critical contributions of thefield of seismologyto thenew dynamicsduring the early stages of the revolutionand it is writtennot by an historianbut by a participant.As such,it shouldcomplement the now numerousother histories of the dis- coveryof platetectonics, most of whichreport, but none of whichis solely devoted to, the role of seismology. In an effortto keepthe text flowing smoothly, and to holdits lengthin check,I citeonly key referencesand discussnon-seismological topics in rathergeneralized fashion. For those who would like additional reading on thesubject as seen from a perspectivedifferent from mine or with empha- sis differentfrom mine, or who would like a more completeset of ences, I include at the end of this section a list of some of the available lit- erature.It is a measureof the breadthof interestin the subjectto note that the following categoriesare representedin the list of authors:science writer, historianof science,philosopher of science,sociologist of science, earth scientist,non-earth scientist, participant in the revolution,and non- participantin the revolution.Their views of the revolutionand their pur- posesare sometimescommon, sometimes diverse, but they all sharerecog- nition and appreciationof a major happeningin science.Some 25 years after the fact, it should be clear and obviousto the reader that the prime purposeof this book is not to add glory to the revolutionand its partici- pants, but rather to describejust how a scienceworks when things are goingwell, in the hopeof acceleratingthe pacetoward greatdiscoveries of the future. As is the casein most, if not all, scientificrevolutions, or paradigm shifts to use modern terminology,the essenceof plate tectonicsand the characteristicsthat distinguishit from previoustheories on the nature of earthdynamics are remarkablysimple. In contrastto the staticearth mod- els that dominatedearlier geologicthinking, plate tectonicscalls for hori- zontal movementsof rock massesat or near the earth's surfacethrough hugedistances (perhaps thousands or tensof thousandsof kilometers)over long intervalsof time (perhapstens or hundredsof millionsof years).The correspondingspeeds of a few cm/yr for a land masssuch as a continent, althoughextremely slow in normalday to day context,are sufficientlyhigh geologicallyspeaking to startlethose accustomed to thinking of rocksor land massesas immobileand fixed in globalperspective. The comingof plate tectonics,then, corresponded to a shiftin empha- sis of geologicthought from what has been termed the "fixist" schoolin which rocksare thought to remainthroughout their existencenear the place where they were formed, to the "mobilist" schoolin which rocks are thoughtto movelaterally and geologicallyrapidly through significant frac- tionsof earth'scircumference. In this sense,plate tectonicsis like its earlier but neverwidely acceptedforerunner, the conceptof continentaldrift. Plate tectonicsis built upon,and incorporatessome of the basicelements of, con- tinentaldrift, but differsfundamentally from early or orthodoxcontinental drift theoryin that the prime entitiesthat moveas units are not definedby theborders of the continentsbut arethe mostlylarger and usuallyless obvi- ous segmentsof the earth'ssurface that have come to be known as the "plates." Takentogether, fewer than a dozenlarge plates form a mosaicthat cov- ersvirtually the entiresurface of the earth.Obvious earth featuressuch as continents,oceans, islands and mountainsare, in the very simplestper- spective,not the prime elementsin the dynamicmodel; it is the lessreadi- ly apparentplates that arethe basicunits, and it is their movementsthat can be describedin simple geometricalfashion. The lesserunits suffer more complex and intricate secondaryeffects that make the story of earth dynamicsmore comprehensive,more varied, more interesting,and often more confounding,but thoseeffects now fall into placeand can be under- stoodas consequencesof the simpleprimary motionsof the basicplates. The effectsof the ponderousmotions are awesome.Driven by heat escapingfrom the earth'sdeep interior,the elementsof the mosaic,i.e. the plates,are continuallyin motion and continuallymodifying their shapes throughaddition of new surfacematerial at somesegments of platebound- aries and destructionof old at others.Huge land massesriding atop the platesmay travel from onehemisphere to another,or from the tropicsto the arctic,and may be split apart, or sheared,or deformedand piled atop one anotherin collision,thereby providing a settingfor a variety of lessergeo- logicprocesses that build the featuresof rocks,terrains, and landscapesthat humansfind familiar and beautifuland essentialto supportof civilization. That so simple a basicmodel of earth was found to accountfor surround- ings so complexand so perplexingas thosewe find in the real earth is a strikingexample of successin sciencethrough synthesis, and also of the beautyof explanationthrough simplicity that characterizesgreat advances in scienceand that merits attentionwell beyond the inner circlesof earth science. In fact,and as is the casein mostrevolutions in thought,the beautiful simplicityof the plate tectonicsconcept is a sourcenot only of pride and elationfor the earth sciencecommunity, but also of mild embarrassment and consequenthumility. Why, indeed, didn't we, and couldn't we, earth scientistsarrive at that simple solutionearlier? How had we misdirected and immobilizedour thinkingso that somethingthat in retrospectseems so obviousremained hidden for so long?What might we have doneto make this revolutionoccur at an earlier time and in lessabrupt and more pre- dictablefashion? Such questions suggest that someintrospection on just how the new order cameabout is appropriateand may well be instructive. Well, how, indeed,did we manageto achievethis