HR: 08:35h
AN: T11A-03    [PDF]
TI: Reverse faulting Events of the Ohchigata Fault Zone, Central Japan
AU: * Azuma, T
EM: t-azuma@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, site7, 1-1-1 Higashi, Tsukuba, 3058567 Japan
AU: Shimokawa, K
EM: k.shimokawa@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, site7, 1-1-1 Higashi, Tsukuba, 3058567 Japan
AU: Mizuno, K
EM: k4-mizuno@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, site7, 1-1-1 Higashi, Tsukuba, 3058567 Japan
AU: Sugiyama, Y
EM: sugiyama-y@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, site7, 1-1-1 Higashi, Tsukuba, 3058567 Japan
AU: Sugito, N
EM: nsugito@kugi.kyoto-u.ac.jp
AF: Department of Geophysics, Kyoto University, Kitashirakawa-oiwake-cho, Sakyo-ku, Kyoto, 606-8502 Japan
AU: Tsutsumi, H
EM: tsutsumh@kugi.kyoto-u.ac.jp
AF: Department of Geophysics, Kyoto University, Kitashirakawa-oiwake-cho, Sakyo-ku, Kyoto, 606-8502 Japan
AB: The Ohchi plain is one of the NE-SW trending tectonic depressions located at the base of the Noto Peninsula protruding into the Japan Sea. Tectonic setting of this area has been changed from the tensional stress field related to the opening of the Japan Sea during Miocene into the compressional stress field after Pliocene. Geologists in 1930's discussed that geological structure of the both margins of this plain are controlled by normal faults or folds. In 1970's, geomorphologists referred that those are reverse faults and active during late Quaternary period based on the study on the terrace deformations. Our seismic reflection profiling surveys in 2001 and 2002 clearly imaged thrust faults dipping approximately 30 degrees to the mountain side beneath both of the margins of this plain. In 2003, we excavated a trench on the fault along the southwestern plain margin. The result shows that this fault has repeatedly been reactivated during late Pleistocene and the last faulting occurred around 3,000 years ago. Active faults along the margins of this plain are called the Ohchigata Fault Zone. Based on the topographic study using the aero-photographs, several tectonic landforms are identified on the alluvial terraces traversed by the Sekidosan fault along the southern margin of this plain. This fault strikes N45E and has produced a scarp about 2 m high at Mijiro site. In order to obtain the information on faulting history, we excavated a trench across the fault scarp at this site. On the trench walls, a reverse fault dipping 10 to 45 degrees to the southeast was exposed. On the upthrown side of the fault, thin terrace deposits in Holocene overlie late Pleistocene beds intercalating 2 layers of widespread volcanic ash, that may correspond to AT (29 ka) and Aso-4 (85-90 ka), respectively. The Pleistocene beds are extensively deformed and partly overturned near the fault. On the downthrown side, a layer of sand and gravel dated at around 3,000 yrs BP by 14C dating method (AMS) is vertically displaced about 2.2 m by this fault.
DE: 7221 Paleoseismology
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 9320 Asia
SC: Tectonophysics [T]
MN: 2003 Fall Meeting