山口県中西部で発生した3被害地震と周辺断層付近の応力変化

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Three destructive inland earthquakes of JMA magnitudes 5 to 6 have successively occurred in central-western Yamaguchi Prefecture during the recent 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 W. Yaumeyama-Lake Ohara fault system. In this paper, 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 W. Yaumeyama-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. Because faults approach their points of stress failure with increases in stress, the seismic risk of the parts of the faults that constitute the W. Yaumeyama-Lake Ohara fault system may be elevatecl after these events. The stress increase may shorten the period until the occurrence of the next earthquake for their stress-increased fault and may also trigger seismic activity along them.
2001-08-31

山口県中西部で発生した3被害地震と周辺断層付近の応力変化

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