Three destructive inland earthquakes in the central-western Yamaguchi Prefecture, southwest Japan and accompanied stress changes to their adjacent faults

Three destructive inland earthquakes of JMA magnitudes M j=5–6 have successively occurred in central-western Yamaguchi Prefecture during the past 15 years. The 1987 M j=5.2 Central Yamaguchi Prefecture earthquake, the 1991 M j=6.0 Suo-nada earthquake, and the 1997 M j=6.1 Northern Yamaguchi Prefecture earthquake were each strong enough to cause moderate damage to houses. The epicenters of these three earthquakes were located along the active W. Yauneyama–Lake Ohara fault system (WLFS). In this paper, we calculated the stress changes in the Coulomb Failure Function (CFF) induced by the three successive earthquakes in order to evaluate the seismic risk in and around this fault system. Our calculations indicated that due to these three earthquakes, the stress approximately increased from 10- to 30-km-wide area along the WLFS. The maximum stress increase was estimated to be 0.46 MPa at the Sakota–Ikumo fault. There are also stress increases at the East Mitsugatake fault. In contrast, post-seismic stress decreased along the Tokusa–Jifuku, the Lake Ohara, and the Shibuki faults. No stress change occurred on the Kikugawa fault. Because faults approach their points of stress failure with increases in stress, the seismic risk of parts of the faults that constitute the WLFS system may have been elevated after these events. The stress increase may shorten the period until the occurrence of the next earthquake along these stress-increased faults and may also trigger seismic activity along them.