HR: 08:00h
AN: T11A-01 INVITED     [PDF]
TI: Deep geometry and evolution of the northern part of Itoigwa-Shizuoka Tectonic Line active fault system, Central Japan, revealed by Seismic profiling
AU: * Sato, H
EM: satow@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Ikeda, Y
EM: ikeda@eps.s.u-tokyo.ac.jp
AF: Dept. Earth & Planet. Sci., Univ. Tokyo, 1-3-7 Hongo, Tokyo, 113-0033 Japan
AU: Iwasaki, T
EM: iwasaki@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Matsuta, N
EM: matsuta@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Takeda, T
EM: takeda@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Kawasaki, S
EM: kawasaki@jgi.co.jp
AF: JGI, Inc., 1-5-21 Otsuka, Tokyo, 112-0012 Japan
AU: Kozawa, T
EM: tkozawa@jgi.co.jp
AF: JGI, Inc., 1-5-21 Otsuka, Tokyo, 112-0012 Japan
AU: Elouai, D
EM: driss@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Elouai, D
EM: driss@eri.u-tokyo.ac.jp
AF: Dept. Earth & Planet. Sci., Hiroshima Univ., 1-3-1 Kagamiyama, Higashi Hiroshima, 739-8526 Japan
AU: Hirata, N
EM: hirata@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Kawanaka, T
EM: taku@jgi.co.jp
AF: JGI, Inc., 1-5-21 Otsuka, Tokyo, 112-0012 Japan
AB: The northern Fossa Magna (NFM) is a Miocene rift system produced in the final stages of the opening of the Sea of Japan. It divides the major structure of Japan into SW and NE portions. The Itoigawa-Shizuoka Tectonic Line (ISTL) bounds the western part of the northern Fossa Magna and forms an active fault system showing the one of the largest slip rates in the Japanese islands. Based on the paleo-seismological data, the ISTL active fault system was evaluated to have the highest seismic risk among active faults within inland Japan. A quantitative understanding of active tectonic processes, including crustal deformation and related destructive earthquakes, is important in reducing seismic hazards through precise estimation of strong ground motions. The structure of the crust, especially the deep geometry of active fault systems, is the most important piece information required to construct such a dynamic model. In this context, the seismic reflection profiling was performed across the northern part of the ISTL active fault system by three seismic lines. Obtained seismic sections are interpreted based on the pattern of reflectors, surface geology and velocity model by refraction analysis, using the balanced cross section technique. The 68-km-long Itoshizu 2002 seismic section across the northern middle part of the ISTL active fault system suggest that the Miocene NFM basin was formed by an east dipping normal fault with shallow flat (6 km), deeper ramp (6 $<$ETH$>$ 15 km) and deeper flat at 15 km in depth. This unique geometry is interpreted that this low-angle normal fault was produced by Miocene high thermal regime, estimated from the thick volcanic rocks at the base of the basin fill. Namely, the normal fault reflects the brittle-ductile boundary in Miocene. Consequently, since the Pliocene, the basin fill was strongly folded by the reverse faulting along the pre-existing normal faults in the Pre-Neogene rocks. The reverse faults in the basin fill produced fault-related folds on their hanging wall. Westward migration of thrusting is recognized by shallow high-resolution seismic section and tectonic geomorphology. The ISTL active fault is an emergent thrust dipping 30 degrees to the east and no evidence is observed showing late Quaternary faulting along the fault, runs parallel to the ISTL and located east of it. Based on the balanced geologic cross-section, the total amount of Miocene extension is ca. 45 km and the total amount of shortening is ca. 25 km. If we assume that the shortening deformation has continued since 5 Ma at constant rate, the horizontal slip rate is calculated as 5 mm/y. The late Quaternary slip rate in the northern part of the ISTL active fault system based on very shallow seismic profiling and drilling shows similar value. The 7-km-long seismic section (Matsumoto 2002) across the middle part of ISTL active fault system also suggests that east-dipping fault geometry at gentle dip. To summarize deep geometry of the ISTL active fault system is strongly controlled by the Miocene extensional structure.
DE: 7223 Seismic hazard assessment and prediction
DE: 8005 Folds and folding
DE: 8107 Continental neotectonics
DE: 9320 Asia
DE: 9604 Cenozoic
SC: Tectonophysics [T]
MN: 2003 Fall Meeting