© Birkhliuser Verlag, Basel, 1999 Pure appl. geophys. 155 (1999) 203-205 0033-4553/99/040203-03 $ 1.50 + 0.20/0
I Pure and Applied Geophysics
Introduction Seismicity Patterns, their Statistical Significance and Physical Meaning
The topic of seismicity patterns is one in which significant advances in the understanding of seismotectonics and earthquake hazard are being made, and in which controversies have recently developed. This special issue grew out of a workshop attended by forty-one participants on May 11/12, 1998 in Nikko, Japan. The collection of papers we ultimately generated does not cover all aspects of problems related to seismicity patterns, but a fair part of them. In a few cases, authors wrote papers especially for this workshop, however, in general, they submitted papers they fortuitously worked on at the time. Nevertheless, this collection of papers shows what issues currently top the priority list in this field of research. The workshop was sponsored by the Geophysical Institute of the University of Alaska, Fairbanks (UAF), in order to develop contacts within the international community of experts in this field, who may participate in collaborative research at the International Arctic Research Center (IARC) in the future. The IARC was established as a part of the common agenda between Japan and the Untied States and is located at UAF. It is a research facility that will house thirty scientists and which opened its doors in December 1998. This two-day meeting of minds with opposing views took place in the oldest hotel in Japan, the Kanaya Hotel, within a few stones throw of the grave of Iyeyasu Tokugawa, the first Shogun, amidst the serene beauty of the waterfall rich Nikko mountain area. In addition to thirty-four formal lectures, two extended evening discussions, each led by an advocate and an opponent, were conducted on the topics of "Foreshocks, their Recognition and Properties" and "Can Earthquakes be Predicted," respectively. The topic of foreshocks and moment release increase was discussed in six papers from an observational and modeling point of view. K. Yamaoka, T. Ooida and Y. Ueda reported that in May 1993 they interpreted a seismicity rate increase as a potential foreshock sequence and estimated the probable occurrence time of the main shock by fitting a time to failure curve to it. In real time they estimated the expected magnitude as 5, and installed a strong-motion instrument that recorded the M 5.1 earthquake which occurred ten days after the expected date, at about 5 km from it. Studies estimating the time of main-shock failure from the regionally observed increase of moment release as a function of time, including the correct prediction of the 1996 M 7.9 Delaroff Islands earthquake, were summarized by S. Jaume and L. Sykes.
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P. Reasenberg demonstrated that in Cascadia earthquakes are four times more likely to be foreshocks than in California. Many speakers emphasized the regional differences in all earthquake parameters, and it was generally understood that basic models of the earthquake occurrence must be modified for regional application. The idea that the focal mechanisms of foreshocks may differ from that of background activity was advocated by Y. Chen and identified by M. Ohtake as possibly the thus far most neglected property of foreshocks, in efforts to identify them. S. Matsumura proposed that focal mechanism patterns of small earthquakes may differ characteristically near locked fault segments into which fault creep is advancing. Considerable discussion was devoted to the status of the seismic gap hypothesis because M. Wyss argued that the occurrence of the M 7.9, 1986, Andreanof Islands earthquake was a confirmation of Reid's rebound theory of earthquakes and thus of the time predictable version of the gap hypothesis, whereas Y. Kagan believed he could negate this view by presenting a list of nine earthquake pairs with M> 7.4, moment centroid separation of less than 100 km, and time difference less than about 60% of the time he estimated it would take plate motions to restore the slip of the first event. Most participants did not accept this table as conclusive evidence supporting the idea that the energy for very large main shocks can be drawn from the same volume within short-time intervals, because, when high-quality data are available, it can be shown that rupture areas of very large neighboring shocks usually abut, or, if they overlap, the overlapping segments are locations of slip deficiency. K. Shimazaki presented evidence that showed that the slip distribution in the previous event is important to estimate the occurrence time of the next, a result that again was pointed out by B. Shaw in his rupture models. Several authors discussed evidence and models for seismic quiescence. J. Zschau illustrated that in an elastic layer overlaying a viscous one, slow stress changes (e.g., due to creep) propagate to considerably longer distances than rapid ones (e.g., coseismic stress changes). N. Kato and T. Hirasawa presented a model in which, applying constitutive laws of failure, creep events on a megathrust could start and stop without generating a main shock, thus potentially explaining swarms and seismic quiescence episodes not preceded or followed by a main shock. Several papers dealt with statistical models for seismicity patterns, such as the point process model by Y. Ogata, while other papers dealt with the statistical evaluations of observations. For example, V. Kossobokov established that the M 8 hypothesis passed the ongoing real-time test at the 99% confidence level, if M> 7.9 events were considered, but failed it (85% confidence level) when M> 7.5 earthquakes were included. The test of the swarm hypothesis by D. Rhoades and F. Evison is an example of rigorous testing of a hypothesis. Nonetheless, several attendees criticized it because a single excellent success generated such a large bonus that it alone could validate their hypothesis. The consensus was that a hypothesis cannot be validated on the basis of one event.
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Computer models of earthquake generation and interaction were based on constitutive laws of friction (J. Dieterich, B. Shaw) and on pore pressure variations (J. Miller, T. Yamashita). Both types of models produce many aspects of earthquake sequences realistically, although different models concentrated on different parameters. It seems that these models have recently become more realistic and useful for understanding the earthquake generation process. J. Dieterich proposed that his model could be tested quantitatively. The b-value oftheJrequency-magnitude relation was discussed by several authors, with a controversy developing because Y. Kagan argued that the factor of two differences in b-values, shown by S. Wiemer over lO-km distances along the San Andreas fault, must be due to catalog errors. He proposed that b-values are always approximately equal to I, because he observed constant b-values in the Flinn-Engdahl regions of the world. The new problem of self-organized criticality (SOC) in seismicity received insufficient discussion at the workshop. However, papers authored by C. Sammis and S. W. Smith and S. Jaume and L. Sykes in this volume summarize the current status of SOC models. Although everyone realizes that most analyses of seismicity patterns are sensitive to errors in the earthquake catalogs, relative few search quantitatively for changes in reporting. R. Zufiiga and S. Wiemer demonstrated new problems that can exist in catalogs. From the vigorous debate at the workshop, and from the papers collected in this special issue, it is evident that seismicity patterns are currently a controversial topic. It seems that all participants at the workshop believe that considerable information regarding the generation of large earthquakes may be locked in seismicity patterns, and that we are making progress in learning how to extract it and put this knowledge to use.
Max Wyss Geophysical Institute University of Alaska Fairbanks Alaska U.S.A. E-mail:
[email protected] Akihiko Ito University of Utsunomiya Utsunomiya Tochigi Japan E-mail:
[email protected]
Kunihiko Shimazaki Earthquake Research Institute University of Tokyo Tokyo Japan E-mail:
[email protected]
