CHARACTERISTICS OF THE SEISMIC QUIESCENCE AND ACTIVATION PATTERNS BEFORE THE M = 7.2 KOBE EARTHQUAKE, JANUARY 17, 1995

Abstract

We investigated the characteristics of the seismicity changes associated with the M=7.2 Kobe (Japan) earthquake, January 17, 1995 by applying the Region–Time–Length (RTL) algorithm to the earthquake catalog of the Japan meteorological agency (JMA). The weighted coefficients associated with all three parameters (time, place and magnitude) of earthquakes are taken into account in this algorithm. The JMA catalog after eliminating aftershocks is complete for events with M≥3 in most parts of the Japanese islands during 1977–1995. The RTL parameters at the epicenter indicated that a seismic quiescence (a decrease of seismicity compared to the preceding background rate) started in 1993 and reached its bottom in May 1994. An activation stage (an increase of seismicity compared to the preceding background rate) with duration of about seven months followed. Our detailed investigations indicated that the RTL anomaly during 1993–1994 would not be an artificial effect due to the influence of the changes of some threshold parameters or the changes of the JMA seismological network. The spatial distribution of quiescence during 1993–1994 revealed a significant anomaly in a broad region around the epicenter of the Kobe earthquake. Following the quiescence stage, an activation zone, which was in order of the rupture length, was obtained around the epicenter during May–December 1994. The evolutions of spatial distributions of seismic quiescence and activation suggested that above anomalies around the epicentral zone would have reasonable correlation with the preparation of the Kobe earthquake. The primary characteristics of the seismicity changes prior to the Kobe earthquake are similar to those obtained for other large earthquakes in Kamchatka and Hokkaido. Therefore, the seismicity changes revealed in this study may give better understanding of the seismogenic process of the Kobe earthquake and provide useful information for seismic risk estimation.