Seismic Risk Evaluation of Inland Active Faults

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FOr seismic risk evaluation of active faults inland Japan, we carried out detailed mapping of active faults especially distributed in densely populated areas such as three metropolitan districts and other major cities. The maps in 1 : 25,000 scale are published from Geographical Institute of Japan, Ministry of Construction.The surface fault ruptures associated with the 1995 Hyogoken-nambu earthquake have been mapped and trenched in order to clarify their extent, slip distribution, mode of faulting and paleoseismic history.We proposed a method to predict rupture propagation which is closely related with strong ground motions and resulting earthquake damage based on an interdependent correlation between the branching direction of the surface ruptures associated with several large historical earthquakes and the direction of their propagation. As patterns of surface traces of active faults are results of repeated earthquake faulting, the branching of several active faults with well-defined branching such as the active faults in Kobe-Osaka area are examined to propose that the direction of their future rupture propagation.We carried out paleoseismological studies on several distinctive active faults using conventional trench excavation method. Newly developed research method and equipment so-called Geo-slicer for more effective paleoseismological field studies, were tested on active fault traces of two most active fault systems in Japan, the Itoigawa-Shizuoka Tectonic Line and the Median Tectonic Line, and obtained promising results. For seismic risk evaluation, the active fault system along the Median Tectonic Line in Shikoku were examined by mini-trenching and Geo-slicing, and aseismic for a large earthquake is revealed based on its paleoseismic history.