Aftershock Distribution of the January 17, 1995 Hyogo-ken Nanbu Earthquake Determined by the JHD Method.

Abstract

On January 17, 1995, the Hyogo-ken Nanbu (Kobe) earthquake of magnitude 7.2 occurred in the Kansai district near Kobe, Japan. After the quake, the Net-Hyogo group set temporal seismic stations in and around Kobe and Awaji Island. Using P and S arrival data from temporal observations, we determined the aftershock distribution of the Kobe earthquake. We used the JHD (joint hypocenter determination) method which eliminates mislocation of hypocenters caused by errors of the simplified model of 1-dimension velocity structure and heterogeneity near stations. From the analysis, we obtained final results of the aftershock distribution. First, the hypocenters thus obtained form a thin flat zone in the Kobe area but form a complex feature in the Awaji area. The aftershock zone strikes N51°E and the total area of the aftershock zone is 60 km wide and 15 km deep. Second, in its northeastern half (Kobe and central area), the aftershock zone forms nearly a simple plane. Rupture did not appear at the surface in the Kobe area while a number of aftershocks were located underneath the surface trace of pre-existing active faults. Third, in the southwestern half (Awaji area), it shows complicated features. The aftershock zone dips northwestward near the northern tip of Awaji Island but southeastward in the central part of the Awaji area. Although the rupture trace appeared on the surface along the Nojima Fault which runs on the western coast of the island, no aftershocks have yet been found to occur immediately beneath the surface rupture. The aftershock zone is located 3 km to the east of the Nojima Fault. It indicates that the fault geometry at the surface does not correspond to the distribution of aftershocks in the Awaji area. These results show that surface fault traces do not necessarily connect directly with the underground fault geometry estimated by aftershock distribution if the fault dips vertical. Pre-shocks occurred several hours before the main shock in the central part of the fault region where the initial rupture of the main shock started at the a depth of 17 km. Hypocenters of the pre-shocks and the initial rupture are located at the lower edge of the aftershock zone. This implicates that the precursory rupture process of the Hyogo-ken Nanbu earthquake began at the base of the seismogenic zone.