HR: 1330h
AN: T22A-0495    [PDF]
TI: Paleoseismicity on the Dense Network of Holocene Submarine Faults in Beppu Bay, Southwest Japan
AU: Shimazaki, K
EM: nikosh@eri.u-tokyo.ac.jp
AF: Earthq. Res. Inst., Univ. Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: * Matsuoka, H
EM: matsuoka@cc.kochi-u.ac.jp
AF: Kochi University, 2-5-1, Akebono, Kochi, 780-8072 Japan
AU: Okamura, M
EM: mako-ok@cc.kochi-u.ac.jp
AF: Kochi University, 2-5-1, Akebono, Kochi, 780-8072 Japan
AU: Chida, N
EM: nchida@cc.oita-u.ac.jp
AF: Ooita University, 700 Tannnohara, Ooita, 870-1192 Japan
AB: Beppu Bay, approximately 30 km by 15 km in size, contains a complex network of Holocene submarine faults whose total length amounts to 230km. They are normal dip-slip fault with left-lateral strike-slip component. The maximum vertical offset accumulated in the past 7,300 years exceeds 20 m. A detailed study on paleoseismicity on one of the faults shows a feature of the time-predictable recurrence, i.e., the larger the vertical offset, the longer the following inter-event time. Branching features can be often recognized near the end of fault and the consistency in branching direction of neighboring faults suggest repeated rupture propagation in the same direction. A detailed examination of high-resolution seismic profiling of branch indicates a repeat of branching and a slow transition of rupture from an old branch to a new one. The central Beppu-Bay fault running WNW to ESE in the center of the bay forms the northern boundary of the major graben structure of the bay. The Asamigawa fault in the west of the bay, running parallel to the central Beppu-Bay fault, has been considered as the southern boundary, but its eastern continuation was not clear. Recent seismic profiling carried out by Chida et al. (2003) showed an existence of Holocene normal fault beneath the city of Oita whose population is 440,000 and interpreted it as a part of the southern boundary. Our high-resolution shallow-water profiling survey revealed the submarine portion of the southern boundary fault, filling a gap between two subaerial faults. We continuously sample marine sediments down to a subbottom depth of 20m by piston coring and correlate specific features of sediment, 20 volcanic ash layers, a few features of magnetic susceptibility and coarse fraction together with C-14 ages of echinoids, pelecypods, and plant remains on the both sides of a targe fault to estimate the date and vertical offset of paleoearthquakes.
DE: 7223 Seismic hazard assessment and prediction
DE: 8010 Fractures and faults
DE: 8123 Dynamics, seismotectonics
DE: 9320 Asia
DE: 9604 Cenozoic
SC: Tectonophysics [T]
MN: 2003 Fall Meeting