Seismic reflection profiling across the seismogenic fault of the 1995 Kobe earthquake, southwestern Japan

Abstract

The 1995 Kobe (Hyogo-ken Nanbu) earthquake (Mw 6.9) surface rupture appeared along the Nojima fault on the northwest coast of Awaji Island. The aftershock epicentres in northern Awaji Island are not aligned with the surface faulting. suggesting a complicated active fault structure. To reveal the structure of the seismogenic fault and its associated active faults, a perpendicular 41.6-km-long seismic reflection survey was undertaken across northern Awaji Island. The Trans-Awaji seismic reflection profile reveals the fault geometry beneath the area. Awaji Island was uplifted by movement on the Kariya fault along its east coast, which produced an asymmetry in Neogene basin that is shallower in the western part than in the eastern part. The faults beneath Awaji Island show a ‘pop-up’ or a ‘positive flower structure’ and the basement is bounded by two active faults, the Nojima along the west coast and the Kariya along the east coast. Both are high-angle, reverse faults with a right-lateral, strike-slip movement. The cluster of aftershocks in the northern part of Awaji Island has a Y-shaped hypocentral distribution. The Nojima and Kariya faults thus appear to connect in the middle of the upper crust at about 7 km depth, forming a seismogenic master fault. Three major seismic events have been reported on the active faults in northern Awaji Island: a prehistoric event dated 2000 yr BP, the 1596 Keicho-Fushimi earthquake, and the 1995 Kobe earthquake. Based on the structural relationship of the active faults that we observed in this study, we suggest that these events occurred on the proposed master fault at mid-crustal depth.