TITLE OF PROJECT : Study on reactivation mechanism of geological faults and prediction of bedrock disaster in the western Chugoku District, southwest Japan

为减轻内陆地震引发的地震灾害，对东北-西南向西高山湖大原断裂系统的活动断层和地质断层进行了地震危险性评价。对所有活动断层和地质断层的组成进行了地形调查和野外填图，明确了碎裂岩的特征和地表分布以及断层伴生的规范带。根据断层岩的构造分析，构成山原湖西断裂系统的断层在上新世中期由左旋运动反转为右旋运动。山口县中西部在15年间先后发生了3次JMA 5 ~ 6级的破坏性内陆地震。1987年山口县中部发生5.2级地震后，1991年山口县北部发生6.0级地震和1997年山口县北部发生6.1级地震，地震强度足以对房屋造成中等程度的破坏。这三次地震的震中位于活跃的西山下-奥原湖断裂系统沿线。计算了三次连续地震引起的库仑破坏函数（CFF）的应力变化，以评价该断裂系统内部及其周围的地震危险性。计算结果表明，由于这三次地震的发生，沿山山湖奥原断裂系统约10 ~ 30公里宽的区域内应力增加。Sakota-Ikumo断层的最大应力增量估计为0.46 MPa。东Mitsugatake山断层的应力也增加了。相反，震后应力沿德佐-吉福、大原湖和涩木断裂减小。在菊川断层上没有产生应力变化

Seismic risk of active and geologic faults belonging to the NE-SW-trending West Yauneyama Lake Ohara fault system in Western Chugoku Distirct was evaluated, to mitigate seismic hazards that were induced by inland earthquakes. Topographical surveys and field mapping were performed for all the constituent active and geologic faults and characteristics and surface distributions of cataclasite and gauge zones accompanying those faults were clarified. Based on the structural analysis of the fault rocks, faults which constitute the West Yauneyama Lake Ohara fault system has inverted into the right-lateral motion from the left-lateral motion during the Middle PlioceneThree destructive inland earthquakes of JMA magnitudes 5 to 6 have successively occurred in central-western Yamaguchi Prefecture dubbing the receipt fifteen years. Following the 1987 M=5.2 Central Yamaguchi Prefecture earthquake, the M=6.0 Suo-nada earthquake and the M=6.1 Northern Yamaguchi Prefecture earthquake in 1991 and 1997, respectively, were strong enough to cause moderate damage on houses. The epicenters of these three earthquakes were located along the active West Yauneyama-Lake Ohara fault system. We calculated the stress changes in the Coulomb Failure Function (CFF) induced by the three successive earthquakes, in order to evaluate seismic risk in and around this fault system. This calculation indicated that by the occurrence of these three earthquakes the stress increased in an approximately 10 to 30-km wide area along the Vest Yauneyama Lake Ohara fault system. The maximum stress increase was estimated to be 0.46 MPa at the Sakota-Ikumo fault. The stress also increased at the Eastern Mt. Mitsugatake fault. In contrast, the post-seismic stress decreased along the Tokusa-Jifuku, the Lake Ohara and the Shibuki faults. No stress change was produced on the Kikugawa fault