HR: 11:50h
AN: T32B-07    [Abstracts]
TI: Maturity and Activity of Faults in a Backarc Region of Southwest Japan Inferred From Integration of Topography, Paleoseismology, Coseismic Slip and Fault Rocks
AU: * Sugiyama, Y
EM: sugiyama-y@aist.go.jp
AF: Active Fault Research Center, Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Fusejima, Y
EM: fusejima.y@aist.go.jp
AF: Active Fault Research Center, Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Miyashita, Y
EM: yukari-miyashita@aist.go.jp
AF: Active Fault Research Center, Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Kobayashi, K
EM: kenkoba@gs.niigata-u.ac.jp
AF: Graduate School of Science and Technology, Niigata University, 8050 Igarashininomachi, Niigata, 950-2102 Japan
AU: Miyawaki, A
EM: a-miyawaki@hanshin-consul.co.jp
AF: Hanshin Consultants Co., Ltd., 3-24-4 Minami-Otsuka, Toshima-ku, Tokyo, 170-0005 Japan
AB: Back arc region of southwest Japan (San_fin district) is characterized by relatively low strain rate and sparse distribution of major active faults with high slip rate. The San_fin district, however, has been frequently hit by _gmedium-sized_h damaging earthquakes since the historical age, including the recent 2000 western Tottori earthquake (Mw 6.6). Our integrated study of topography, paleoseismology, coseismic slip and fault rocks revealed that the 2000 western Tottori earthquake was produced by a fault system with low maturity and low activity, but long history at least since the Miocene. The low maturity is indicated by narrow shear zones (several mm to cm wide) and poor traceability (several ten to hundred meters long) of their topographic expressions. We think that the low maturity of the fault system caused sporadic occurrence of coseismic surface ruptures only within a 6-km-long area, which is far smaller than the seismologically estimated ca. 20-km-long source fault. Trenching survey identified a preceding rupture event around 28 ka, and post-earthquake geodetic survey revealed 70 cm to 1 m coseismic slip. Based on the data, average slip rate of the fault system responsible for the 2000 earthquake is estimated to be less than 0.1 mm/y. Fault rock study has showed that NW-striking mesoscopic faults in Paleogene granite are distributed more densely in the aftershock area than the outside area. Cataclasites are characteristically distributed within the aftershock area, and a large number of Miocene andesite dikes have intruded into the granite in the fault-controlling NW direction. The facts indicate that the fault system responsible for the 2000 western Tottori earthquake has a long formation history at least since Miocene time. Inoue et al. (2002) have revealed that a 4-km-long parallel fault located about 2 km west of the aftershock zone ruptured between AD 770 and 1260, suggesting the relation of this event to the AD 880 historical earthquake. Our integrated survey also has identified two parallel fault systems about 10 km long in the vicinity of the 2000 aftershock area, 5 km southwest and 9 km northeast, respectively. Maturity of the latter fault system inferred from shear zone characteristics and topographic expressions is as low as the fault system that caused the 2000 earthquake. Excavation survey indicates that the fault system has repeatedly ruptured at intervals of more than several ten thousand years. Rather dense distribution of low-maturity and low-activity faults might cause frequent _gmedium-sized_h damaging earthquakes in the San_fin district.
DE: 7221 Paleoseismology (8036)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8010 Fractures and faults
DE: 8104 Continental margins: convergent
SC: Tectonophysics [T]
MN: Fall Meeting 2005