Chapter 13 Bedrock structure around faults and its relation to earthquake disaster

The frequency-wavenumber and horizontal-to-vertical ratio spectral analyses of microseisms (long-period microtremors), and the bandpass filtering of Bouguer gravity anomalies, were conducted to elucidate the subsurface structure around the eastern end of the severely damaged belt-like zone in the Kobe-Hanshin area which appeared during the 1995 Hyogokennanbu (Kobe) earthquake. A close relationship was found between the fault-related bedrock configuration thus obtained and the distribution of earthquake damage: a narrow zone of severe damage was located 1.5-2 km apart from the basin edge where the depth to bedrock changes abruptly by several hundreds to a thousand meters. The relationship was atributed to the amplification of ground motions due to the specific configuration of bedrock, that is, focusing of seismic waves and/or interference between incident S-waves and surface-waves secondarily generated at the basin edge. It is pointed out, as a lesson learned from the Hyogoken-nanbu earthquake disaster, that the investigation of fault-related 3-D bedrock configuration is an urgent issue for hazard mapping of an urban area on a sedimentary basin.