Aspects of Pacific Seismicity

198 Pages·1991·6.26 MB·English

by  Emile A. Okal (eds.)

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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