Large Earthquakes in Subduction Zones: Segment Interaction and Recurrence Times

Abstract

Subduction zones generate most of the world's seismicity, and all of the largest earthquakes. This overview of large earthquakes in subduction zones consists of two parts: a review of the occurrence of large events in different tectonic regimes of subduction zones, and the timing of large interplate underthrust events. Our global review shows that large earthquakes have occurred in all intra-plate environments from the outer-rise down to 650 km depth, except for the fore-arc region of the upper plate in mature subduction zones. It seems that the seismogenic plate interface is an efficient concentrator of seismicity, though large earthquakes do occur just trenchward and also downdip of the interplate coupled zone. We focus on two aspects of the temporal occurrence of interplate events: a brief analysis of the composite global occurrence of great events, and then a brief review and analysis of the methodology of long-term earthquake forecasting, followed by a suggestion to improve the methodology. Occurrence times of the greatest interplate events in the 20th century are clustered more than expected from random occurrence. However, a “waiting time” analysis of the 40 great interplate events (M>8) in the 20th century shows that their origin times are consistent with a model of independent random occurrence. The key to earthquake forecasting methodology is the accurate determination of recurrence time for each plate boundary segment for the current earthquake cycle. Observations of large earthquake occurrence show great variability in rupture mode and recurrence times. Mechanical models that include interaction between adjacent plate boundary segments produce synthetic event catalogs with variable rupture modes and recurrence times, similar to observed earthquake sequences. One robust “rule” extracted from these simulations is that if the rupture mode changes from one great event to several smaller events, then the first smaller event will occur in the epicentral segment of the great event with a recurrence time that is shorter than the average time for that segment. This “rule” appears to explain four examples of sooner-than-expected large earthquakes: the 1942 event in Ecuador; the 1986 event in central Aleutians; the 1994 event in Sanriku, Japan; and the 1995 event in Kuriles Islands.