LONG-TERM SEISMOGENIC PROCESS FOR MAJOR EARTHQUAKES IN SUBDUCTION ZONES

A qualitative physical process for the long-term seismogenesis of majo r earthquakes in subduction zones is proposed on the basis of quantita tive empirical evidence that swarms, mainshocks acid aftershocks are c losely related phenomena. The relations, which have been identified in the comprehensive, long-term catalogues of New Zealand and Japan, rep resent swarms as predictors of mainshocks with respect to location, ti me and magnitude. Clustering of swarms and of mainshock/aftershock eve nts is allowed for. With a database of 15 sequences of swarms, mainsho cks and aftershocks, tests are being conducted with the object of refi ning the relations and evaluating them as a possible means of practica l synoptic forecasting. Three sequences have culminated in major earth quakes since the tests began, and the systematic study now relates a t otal of 36 swarms with 29 mainshock/aftershock events. These empirical results strengthen and quantify the connection between swarms and maj or earthquakes, which several authors have demonstrated by means of nu merical/physical modelling. The proposed seismogenic process includes swarms, mainshocks and aftershocks as separate event stages which are related by predictability. Interevent conditions are specified accordi ng to the Mogi criteria for the medium; cracks at which fractures subs equently occur constitute nonuniformity in the Mogi sense, and post-ea rthquake healing restores uniformity. Where the Gutenberg-Richter rela tion occurs, it is accepted as possible evidence of deterministic chao s and unpredictability; as a corollary, the process is noncyclical. Th e principle of scaling is held to apply except when modified by large- scale boundaries in the medium. Subduction zones and some other locali ties where water is abundant are indicated by the main empirical studi es as favourable to the occurrence of swarms. Fluid overpressuring is therefore proposed as a mechanism for the self-triggering of swarms, a nd this is supported by additional examples of the predictive relation s occurring in conditions of high fluid pressure, including the vicini ty of large man-made reservoirs. The process can be tested by systemat ic studies in other subduction regions, given adequate catalogues for quantifying the algorithm. It also has implications for other tectonic environments, with swarms replaced by cognate, more protracted seismi city precursors. (C) 1998 Elsevier Science B.V. All rights reserved.