Aftershock Distribution of the 1995 Hyogo-ken Nanbu Earthquake beneath the A waji-shima Island

Following the 1995 Hyogo-ken Nanbu earthquake, six seismic recorder sites were deployed in the northern part of the Awaji-shima island during the period between 8 and 29 August 1995. Analyzing continuous seismic recordings, we find an activity of micro-earthquakes with local magnitude below 2. Total number of well-located aftershocks is 448. The epicentral distribution and its NE-SW azimuthal trend are similar to those obtained just after the main shock. The trend is interrupted and changed by the Nojima fanit at the east-shore of the Awaji-shima island. The epicentral locations of aftershocks are generally coincident with surface active fault traces: e. g., the Nojima fault, the Higashiura fault, and the Asano fault. An aftershock cluster just beneath the south margin of the Nojima fault is found at depths between about 2 and 10km by dipping toward inland direction with an angle of 80°. We also find an aftershock cluster beneath the Higashiura fault, which did not slip coseismically at the mainshock. The cluster can be traced down to about 9km depth. Such distributions of aftershocks seem to indicate that these faults are structurally independent of each other down to this depth.