Minute Locating of Faulting beneath Kobe and the Waveform Inversion of the Source Process during the 1995 Hyogo-ken Nanbu, Japan, Earthquake Using Strong Ground Motion Records.

Abstract

We estimated the source process of the 1995 Hyogo-ken Nanbu (Kobe), Japan, earthquake based on: 1) locating buried fault planes in the Kobe area by examining particle motion at observation stations in the near-source area and 2) multi-time window linear waveform inversion of strong ground motion seismograms. S-wave particle motion diagrams created theoretically show reverse rotation in the horizontal plane at two stations located on opposite sides of the intersection of the earth's surface and the extension of the buried fault plane. We simulated ground motion at various locations surrounding the buried fault plane and compared their particle motions with observed records to obtain constraints on the location of the fault plane. At least two planes are needed for the rupture area northeast of the epicenter if the causative fault is assumed to be made up of a few perfectly planar structures, and the rupture extended at least 26 km northeast from the epicenter.Using a fault model consisting of three planes, two planes on the Kobe side and on the Awaji side, and assuming a step-over at the Akashi strait based on the aftershock distribution, we performed a waveform inversion. The main rupture extended about 45 km horizontally. Three regions had relatively large moment releases: 1) around the rupture starting point; 2) in the shallow (less than 10 km) part of the Nojima fault, which extends along the northwest shore of Awaji Island: and 3) deep (about 10 km) under Kobe City. Even though the second subevent on the Nojima fault had a large moment release, it did not generate pulsive waves, because its rise time was long. The two remarkable pulses seen in the seismograms in the Kobe area came from the first and third subevents.