Aftershocks and Faults of the Hyogo-ken Nanbu Earthquake beneath Akashi Strait.

Abstract

We carried out a seismic marine survey around Akashi Strait employing reflection and side-scan sonar imaging methods and an ocean bottom seismograph (OBS) array in order to closely examine the aftershocks in the immediate vicinity of the rupture initiation area of the 1995 Hyogo-ken Nanbu earthquake. A total of 71 aftershocks and some mechanisms were determined using OBS data for a period of 1 month with land temporary station data starting 12 days after the main shock. We identified a number of seismologically characteristic clusters to understand the heterogeneity that seem to exist around Akashi Strait. We determined that two major faults, the Nojima Fault and the Rokko fault system, are right-stepping within the strait cause en-echelon faultings in between. The Nojima Fault seems to terminate by deflecting westward in the strait. Focal mechanisms of these events are generally concordant with an E-W compression stress field, which is the tectonic trend. Events beneath the northern part of Awaji Island occur as a cluster dipping eastward, but mechanisms vary.