HR: 1330h
AN: T12C-0489    [PDF]
TI: Preliminary Report Of Paleoseismological Study On The Kuromatsunai Lowland Fault Zone, Southwest Hokkaido, Northern Japan.
AU: * Shimokawa, K
EM: k.shimokawa@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, site7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Azuma, T
EM: t-azuma@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, site7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Sugiyama, Y
EM: sugiyama-y@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, site7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Sangawa, A
EM: sangawa.a@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, site7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Kuwabara, T
EM: t-kuwabara@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, site7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Okumura, K
EM: kojiok@hiroshima-u.ac.jp
AF: Hiroshima University, 1-2-3 Kagamiyama, Higashi-Hiroshima, 739-8511 Japan
AU: Kurosawa, H
EM: kurosawa-hideki@oyonet.oyo.co.jp
AF: OYO Co., 2-2-19 Daitakubo, Minami-ku, Saitama, 336-0015 Japan
AU: Miwa, A
EM: miwa-atsushi@oyonet.oyo.co.jp
AF: OYO Co., 2-2-19 Daitakubo, Minami-ku, Saitama, 336-0015 Japan
AB: The Kuromatsunai lowland fault zone in southwest Hokkaido is one of the major active faults in Japan. This 30-km-long and 5-km-wide fault zone consists of many N-S-trending, several-km-long thrust faults, which are closely related to the folding structure of the thick sediments of late Miocene to early Pleistocene age. The folding structure has been formed by E-W compression due to the plate convergence after E-W extension related to the opening of the Japan Sea. However, there is no historical document that recorded large earthquakes in this area. To reveal the recent activity of the Kuromatsunai lowland fault zone, we conducted paleoseismological study including trenching and coring in the northern and southern parts of the fault zone. A trench on the Shirozumi fault, one of the main fault strands in the northern part, exposed a nearly horizontal, sharp contact between an eastward-dipping silt bed of 45,000 years BP and an overlying weathered aeolian volcanic ash layer. It is not clear, however, whether the contact is a low-angle fault of tectonic origin or a slip surface of landslide origin. Another trench on a dissected flexural scarp of the Oshamanbe fault in the southern part exposed the unconformity between an eastward-dipping silt layer of 45,000 to 50,000 years BP and an overlying gravel layer of 4,000 to 9,000 years BP. Liquefaction traces were also found in the gravel layer. The coring survey near the trench site shows that the vertical displacement of the silt layer attains about 10 m. We are planning to conduct seismic reflection survey to image the subsurface structure of this fault zone.
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