GEOLOGICAL CHARACTERISTICS AND PROBLEMS IN AND AROUND OSAKA BASIN AS A BASIS FOR ASSESSMENT OF SEISMIC HAZARDS

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Abstract
A number of civil engineering structures in a large area Kobe-Hanshin and Awaji in southwest Kinki, southwest Japan, were seriously damaged by the 1995 Hyogoken-Nambu earthquake. Geological studies were made to reveal both geological and geotechnical characteristics in and around the Osaka sedimentary basin. A geological map of the Osaka Plain and the surrounding area was compiled for the geotechnical engineering use.A Bouguer gravity anomaly map is presented with additional gravity measurements for an evaluation of the subsurface geologic structure. Data on the depth of the basement rocks was collected from information on deep drilling for the geologic survey or hot springs, reflection seismic exploration and surface geological survey. A configuration map of the basement rock forming the bottom of the Osaka basin was also compiled from the depth data collected and an analysis of the correlation between the depth and the Bouguer anomaly was attempted to outline the relief image.In order to assess the contribution of underground structure with faults to the distribution of structural damage, the focusing process of seismic wave was examined by using the ray theory applied to model the Uemachi Fault zone, which is located in a dense urban region in the Osaka area. As a result, the efficiency of the focusing based on a common underground structure with fault was substantiated qualitatively.