Chapter 12 Changes in Coulomb failure function due to the occurrence of the M7.2 Kobe Earthquake of January 17, 1995, as a possible measure of the change in seismicity

Abstract

Abstract We calculated changes in Coulomb failure function (CFF) for a geodetic fault model of the M7.2 Kobe Earthquake of January 17, 1995, to investigate the possible relationship between the occurrence of the main shock and the following increase in seismicity in the surrounding area. The modeled fault, which was derived from geodetic data, consists of six nearly vertical planes trending in the NE-SW direction with dominant right-lateral strike-slip and some thrust components. We calculate CFF changes for several focal mechanisms which are usually seen in the Kinki district or are consistent with the type of active faults there, and compare its distribution with the seismicity observed after the main shock. Calculated CFF for right-lateral slip on NE-SW trending vertical planes or left-lateral slip on NW-SE trending vertical planes, which are dominant focal mechanisms in this area, increases along the northeastern extension of the source region and around the Yamasaki fault, a conjugate fault located northwest of the source region. In the former region a drastic increase in number of earthquakes were observed after the main shock. The activation of seismicity was also observed near the Yamasaki fault. CFF decreases in the north of the source region, where the size of earthquakes became smaller than before the main shock. CFF may decrease a little in the Wakayama region, where no notable changes in seismicity were observed. The Kobe Earthquake may have loaded stresses which might lead to right-lateral slip on the eastern half of the Arima-Takatsuki Tectonic Line and segments of the Median Tectonic Line in the Wakayama and Tokushima regions. Since these faults have no record of M7 or greater events during at least 400 years, we should be aware of the seismicity around these tectonic lines.