Establishment of a zoning method for fault bedrocks based on the strain partitioning of multiple faults and seismic properties of fault rocks

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NE-SW-trending, 105-km long Lake Ohara -W. Yauneyama fault system (OYFS) is developed in western Chugoku District. This fault system is composed of multiple faults. The Yamaguchi -Izumo seismic zone exists along the OYFS. The movement of active faults may be related to a formational mechanism of these basins, because the basins are located at bending parts of the faults. On the other hand, in this study, bedrocks which are affected by a fault movement and are in the vicinity of a fault are defined as "fault bedrock".In order to obtain a basic data for mitigating bedrock disaster caused by inland earthquakes, this study establishes a zoning method of seismic hazards for the fault bedrock. We select the OYFS as a case study, and the contents of the investigation and results are as follows.1. Configuration of in plane view of the constituent fault of the OYFS is revealed, and strain partitioning of each constituent fault is analyzed by a combination microscopic observations of fault rocks and fault process zones.2. Velocities, Vp and Vs, of the fault rock and the process zone are measured on rock samples in a laboratory and on outcrops in situ, respectively, and seismic properties of the fault bedrock are obtained. By combining the seismic property with the microscopic observation of the fault rock, a classification criterion of the fault bedrock is developed. Based on the classification criterion, a classification map of fault rocks is made up in the area along the OYFS.3. Three-dimensional configuration of the multiple fault system is reconstructed by the combination of regional geology information, geophysical data and the result (1).4. Using a two-dimensional finite element method, fault movement is simulated on the reconstructed model of the multiple fault system, and the seismic risk is evaluated.