Synchronous seismicity changes in and around the northern Japan preceding the 2003 Tokachi-oki earthquake of M8.0

Abstract

The epidemic-type aftershock sequence (ETAS) model predicts the normal seismic rate that superposes the empirical laws of decreasing aftershock activity. Anomalies (lowering and enhancement) of the seismic rate from the rate predicted by the ETAS model are found to have occurred concurrently during 1996, in three interplate aftershock sequences and in inland (intraplate) seismicity, in and around northern Japan. Each anomaly is explained by change in Coulomb failure stress, caused by aseismic slip that is assumed to have occurred on the plate interface of the 2003 Tokachi-oki rupture of M8.0. The slip is also supported by a significant change in the distribution of fault mechanisms of moderate earthquakes before and after 1996 in northern Japan. Using the occurrence data of strong earthquakes of M5.0 or larger for the long period 1926-2003, a similar relative quiescence lasting about 10 years is indicated not only preceding the recent 2003 earthquake but also preceding the 1952 and 1968 great Tokachi-oki earthquakes. Copyright 2005 by the American Geophysical Union.