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Aspects of Pacific Seismicity PDF

198 Pages·1991·6.26 MB·English
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Aspectsof Pacific Seismicity Editedby EmlleA.Okal 1991 Springer Basel AG Reprint from Pure and Applied Geophysics (PAGEOPH), Volume 135 (1991), No. 2 Editor's address: Emile A. Okal Northwestern University Department of Geological Sciences Evanston, IL 60208 USA Library of Congress Cataloging-in-Publication Data Aspects of Pacific seismicity / edited by Emile A. OkaI. p. cm. Published also as v. 135, no. 2 of Pure and Applied Geophysics. 1. Seismology-Pacific Area. I. Okal, Emile A. QE537.2.P2A76 1991 551.2'2'091823-dc20 Deutsche Bibliothek Cataloging-in-Publication Data Aspects ofpacific seismicity / ed. by Emile A. OkaI. - Basel Boston ; Berlin : Birkhäuser, 1991 NE: Okal, Emile A. [Hrsg.] This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, reprinting, re-use of illustrations, broadcasting, reproduction by photocopying machine or similar means, and storage in data banks. Under § 54 of the German Copyright Law where copies are made for other than private use a fee is payable to »Verwertungsgesellschaft Wort«, Munich. ©1991 Springer Basel AG Originally published by Birkhäuser Verlag AG Basel in 1991. ISBN 978-3-0348-5641-6 ISBN 978-3-0348-5639-3 (eBook) DOI 10.1007/978-3-0348-5639-3 Contents 167 Introduction, E. A. Okal 169 Circum-Pacific seismic potential: 1989-1999, S. P. Nishenko 261 Intraplate seismicity ofthe Pacific Basin, 1913-1988, M. E. Wysession, E. A. Okal and K. L. Miller PAGEOPH, Vol. 135, No. 2 (1991) 0033-4553/91/020167 -02$1.50 + 0.20/0 © 1991 Birkhäuser Verlag, Basel Introduction This special issue of PAGEOPH regroups two extensive studies of the seismicity of the Pacific region. They are intended to provide a comprehensive update on our understanding of the occurrence of seismicity along the plate boundaries of the Pacific Basin, as wel1 as in its interior. The first paper, "Circum-Pacific Seismic Potential, 1989-1999", by Stuart P. Nishenko, is an assessment of the present seismic potential in 96 circum-Pacific plate boundary zones, in the form of the specific conditional probabilities for the occurrence of large to great earthquakes along these segments, during a number of time windows extending from 1989 to 2009. Both in its goals, its approach, and its general philosophy, this study fol1ows in the steps of a previous study by Nishenko and three co-workers, published 12 years aga in PAGEOPH (McCann et al., 1979). The paper builds on the experience acquired in the past decade, notably by the detailed seismo10gical study of great historical earthquakes, but also through significant progress in the recognition and dating of the geological evidence for pre-instrumental events; in addition, we have obviously learned a great deal from the large subduction events of the past decade (Colombia, 1979; Mexico and Chile, 1985; Aleutian 1986, to name a few). The paper conc1udes by ranking the seismic gaps with the highest probability of activity in the next ten years, and the next twenty years, respectively. The second paper, "Intraplate Seismicity of the Pacific Basin, 1913-1988", by Michael E. Wysession, Emile A. Okal, and Kristin L. Mil1er, presents a thorough compilation of al1 seismicity reported as intraplate in the Pacific Basin, since the inception of the regular listing of seismological observations in the International Seismological Summary. In addition to about 800 earthquakes be\onging to wel1- defined temporal and spatial swarms, approximately 900 events were critical1y analyzed and most of them relocated. In the end, only 45% proved to be genuinely intraplate, with the remainder a mixture of plate boundary events erroneously listed as intraplate, poorly constrained solutions for which an interplate location cannot be ruled out, and blatant errors resulting from typographicalor other systematic errors upon compilation. The paper also catalogs al1 available focal mechanisms and presents some statistics on the evolution with time of the detection of intraplate earthquakes, as wel1 as of the accuracy of the location process. 168 Introduction PAGEOPH, Both papers in the issue are obviously intended and expected to serve as basic reference for future investigations of these problems. Emile A. Okal REFERENCE MCCANN, W. R., NISHENKO, S. P., SYKES, L. R., and KRAUSE, J. (1979), Seismic Gaps and Plate Tectonics: Seismic Potential Jor Major Boundaries, Pure Appl. Geophys 1/7, 1082-1147. PAGEOPH, Vol. 135, No. 2 (1991) 0033-4553/91/020169-91$1.50 + 0.20/0 © 1991 Birkhäuser Verlag, Basel Circum-Pacific Seismic Potential: 1989-1999 STUART P. NISHENK01 Abstract - The seismic potential for 96 segments of simple plate boundaries around the circum- Pacific region is presented in terms of the conditional probability for the occurrence of either large or great interplate earthquakes during the next 5, 10, and 20 years (i.e., 1989-1994, 1989-1999 and 1989-2(09). This study represents the first probabilistic summary of seismic potential on this scale, and involves the comparison of plate boundary segments that exhibit varying recurrence times, magnitudes, and tectonic regimes. Presenting these data in a probabilistic framework provides a basis for the uniform comparison of seismic hazard between these differing fault segments, as weil as accounting for individual variations in recurrence time along a specific fault segment, and uncertainties in the determination of the average recurrence time. The definition of specific segments along simple plate boundaries relies on the mapping of earthquake rupture zones as defined by the aftershock distributions of prior large and great earthquakes, and historic descriptions of feit intensities and damage areas. The 96 segments are chosen to represent areas likely to be ruptured by "characteristic" earthquakes of a specified size or magnitude. The term characteristic implies repeated breakage of a plate boundary segment by large or great earthquakes whose source dimensions are similar from cyc1e to cycle. This definition does not exc1ude the possibility that occasionally adjacent characteristic earthquake segments may break together in a single, larger event. Conversely, a segment mayaiso break in aseries of smaller ruptures. Estimates of recurrence times and conditional probabilities for characteristic earthquakes along segments of simple plate boundaries are based on I) the historic and instrumental record of targe and great earthquake oc,currence; 2) paleoseismic evidence of recurrence from radiometric dating of Holocene features produced by earthquakes; 3) direct calculations of recurrence time from the size of the most recent characteristic event and the long-term rates of plate motion assuming the validity of the time-predictable model for earthquake recurrence; and 4) the application of a lognormal distribution for the recurrence times of large and great earthquakes. . Time-dependent estimates of seismic potential are based on a physical model of earthquake occurrence which assumes that the probability for an earthquake is low immediately following the occurrence of a characteristic earthquake and increases with time as the stress on the fault segment recovers the stress drop of the event. This study updates earlier work on seismic gaps by explicitly inc1uding both recurrence time information and the temporal proximity to the next event as factors in describing earthquake hazards. Currently, 11 out of 96 regions have a high (i.e., ~ 50%) probability ofrecurrence during the next 10 years and are characterized by either fairly short (i.e., less than 30-40 years) recurrence times or long elapsed times relative to the average recurrence time. The majority of these segments are located in the southwest Pacific (Vanuatu, New Guinea, and Tonga). When a longer time window is considered (e.g., 20 years or 1989-2009), 30 out of 96 regions have a high potential. Many of these regions are located near areas of high population density. These determinations do not preclude rupture of other fault segments, with less than a 50% chance in 10 or 20 years, or large and great earthquakes in areas we have I National Earthquake Information Center, United States Geological Survey, Denver, CO 80225, U.S.A. 170 Stuart P. Nishenko PAGEOPH, not studied in detail. While this study has summarized the seismie potential for a large number of regions aroul}d the eireum-Pacifie, there are still a number of geographie and seismotectonie regions that need to be eonsidered, inciuding Indonesia, the Philippines, New Zealand, and the eountries that surround the Caribbean basin. Key words: Cireum-Pacifie, earthquake foreeasting, earthquake predietion, eharaeteristie earth- quakes, probability, seismie hazards. Introduction In the past 30 years great strides have been made in the fields of seismology and geophysies towards understanding the occurrenee of large and great earthquakes along simple plate boundaries. These advanees have recently led to the development of long-term earthquake foreeasts for speeifie fault zones. The applieability of these teehniques and ideas to at least some areas of the cireum-Paeifie region was demonstrated by the suecessful foreeast ofthe great (Ms 7.8) 1985 Valparaiso, Chile earthquake (Nishenko, 1985). The loeations of other suecessful earthquake fore- easts and predietions sinee 1940 are shown in Figure I. At present, national earthquake predietion pro grams in the United States and Japan have identified speeifie areas for intensive study, based on regularities in the patterns of historie earthquake oeeurrenee and expeetations of similar sized events in the near future (i.e., Parkfield, California [BAKUN and LINDH, 1985] and the Tokai District, Japan [MOGI, 1981]). On a broader seale, The WORKING GROUP ON CALIFORNIA EARTH- QUAKE PROBABILITY (1988, 1990) reports represent the first Federally sanetioned regional probabilistic forecasts for earthquake activity in the Uni ted States. It is generally known which population centers and sites of eritieal faeilities around the circum-Pacific region have experieneed destruetive large (Ms 7.0-7.7) and great (M w 7.7-9.3) earthquakes in the historie past. These same loealities are also eandidates for the inevitable reeurrenee of similar earthquakes and tsunamis at so me future time. Henee, while it is of aeademic interest to know how long it has been sinee a prior destruetive earthquake occurred at a partieular loeation; it is more important, from a societal perspeetive, to know when the next damaging event will oeeur. This report summarizes the known seismie history for 123 seismie gaps around • Plate I Cireum-Pacifie Seismie Potential 1989-1999. Colors portray the time-dependent eonditional probability for the reeurrence of either large (7.0< M, < 7.7) or great (M" M w > 7.7) shallow, plate boundary earthquakes during the time interval 1989-1999. See text for loeation of segment boundaries and expeeted magnitudes. Probabilities are eonditional on the event not having occurred prior to 1989 and are represented by dark blue (0-20%), green (20-40%), yellow (40-60%), and red (60-100%). Light blue areas are those regions with no historie record of great earthquakes. Specifie dates and magnitudes refer to those areas with ineomplete historie records. See text for recent forecast updates. Vol. 135, 1991 Circum-Pacific Seismic Potential 171 .. ' ,"

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