Characteristics of the seismic quiescence and activation patterns before the M=7.2 Kobe earthquake, January 17, 1995

We investigated the characteristics of the seismicity changes associated wi th the M = 7.2 Kobe (Japan) earthquake, January 17, 1995 by applying the Re gion-Ti me-Length (RTL) algorithm to the earthquake catalog of the Japan me teorological agency (JMA). The weighted coefficients associated with all th ree parameters (time, place and magnitude) of earthquakes are taken into ac count in this algorithm. The JMA catalog after eliminating aftershocks is c omplete for events with M greater than or equal to 3 in most parts of the J apanese islands during 1977-1995. The RTL parameters at the epicenter indic ated that a seismic quiescence (a decrease of seismicity compared to the pr eceding 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 b e an artificial effect due to the influence of the changes of some threshol d parameters or the changes of the JMA seismological network. The spatial d istribution of quiescence during 1993-1994 revealed a significant anomaly i n a broad region around the epicenter of the Kobe earthquake. Following the quiescence stage, an activation zone, which was in order of the rupture le ngth, was obtained around the epicenter during May-December 1994. The evolu tions of spatial distributions of seismic quiescence and activation suggest ed that above anomalies around the epicentral zone would have reasonable co rrelation with the preparation of the Kobe earthquake. The primary characte ristics of the seismicity changes prior to the Kobe earthquake are similar to those obtained for other large earthquakes in Kamchatka and Hokkaido. Th erefore, the seismicity changes revealed in this study may give better unde rstanding of the seismogenic process of the Kobe earthquake and provide use ful information for seismic risk estimation. (C) 2001 Elsevier Science BN. All rights reserved.