HR: 11:00h
AN: T32C-03    [Abstracts]
TI: Estimating Slip Rate and Last Event Timing for the Blind Fukaya Fault System in Greater Metropolitan Tokyo Through Precise Analyses of Continuously Cored Boreholes
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: Mizuno, K
EM: k4-mizuno@aist.go.jp
AF: Active Fault Research Center, Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Sugai, T
EM: sugai@k.u-tokyo.ac.jp
AF: Graduate School of Frontier Science, University of Tokyo, 7-3-1 Hongo, Tokyo, 113-0033 Japan
AU: Hachinohe, S
EM: a1096357@pref.saitama.lg.jp
AF: Center for Environmental Science in Saitama, 914 Kamitanadare, Kisai Town, 347-0115 Japan
AU: Nakazato, H
EM: h_nakazato@nkk.affrc.go.jp
AF: National Institute for Rural Engineering, 2-1-6 Kan'nondai, Tsukuba, 305-8609 Japan
AU: Matsushima, H
EM: hmatsu@nenv.k.u-tokyo.ac.jp
AF: Graduate School of Frontier Science, University of Tokyo, 7-3-1 Hongo, Tokyo, 113-0033 Japan
AU: Ishiyama, T
EM: t-ishiyama@aist.go.jp
AF: Active Fault Research Center, Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AB: The NW-trending Fukaya fault system is located along the northwestern margin of the Kanto Plain where greater metropolitan Tokyo is expanding. The fault system comprises an 80-km-long array of left-stepping west-side-up reverse faults. The faults appear as flexures or fault-related folds at the ground surface and are widely concealed beneath fluvial deposits. Our boring and trenching surveys in 2000 and 2001 revealed a 2-to-6-ka earthquake event on the fault system. However, paleoseismological data such as the last event timing, recurrence interval and rupture extent are still not constrained enough for reliable probabilistic evaluation of future earthquakes from the fault system. In addition, historical and archeological data suggest a large earthquake in AD 818 in the nearby area, but no evidence is known for an earthquake in the period between 2 ka and 6 ka. Under the circumstances, we have conducted a complementary study for the Fukaya fault, an about 38-km-long main segment of the fault system. We extracted a 173-m-long continuous core from the downthrown side of the southernmost part of the fault concealed beneath the alluvial plain in the Fukiage area. The core is subdivided into five stratigraphic units 20 to 50 m thick. The upper part of each unit includes fine-grained deposits, which are correlative to Marine Isotope Stage (MIS) 1, 5, 7, 9 and 11, respectively, in descending order. The average subsidence rate in the Fukiage area is estimated to be around 0.35 to 0.4 m/ky on the assumption that the base of the core (153 m below the sea level) is ca. 430 ka (base of MIS 11). The upthrown side seems to be absolutely uplifted because middle or late Pleistocene fluvial terraces are distributed at the level about 20 m high from the present alluvial surface. Consequently, the vertical slip rate of the southernmost part of the Fukaya fault is estimated to be more than 0.35 m/ky. We also conducted a precise stratigraphic correlation of Holocene deposits for a boring array comprising 12 existing and 3 newly extracted cores, and extending over 600 m across the flexure zone in the Fukiage area. Each newly extracted core records a liquefaction trace at the same stratigraphic horizon. Two of the three cores preserve liquefied sands jetting out to the ground surface at that time. We identify the liquefaction event, which is dated to be 2,700 to 3,000 years BP, as the latest faulting event on the Fukaya fault in the Fukiage area. The event timing on the southernmost Fukaya fault is consistent with a 2-to-6-ka event on the northern and middle parts of the Fukaya fault system. The result suggests a possibility that a 2.7-to-3-ka earthquake ruptured the whole extent of the Fukaya fault system.
DE: 7221 Paleoseismology
DE: 7223 Seismic hazard assessment and prediction
DE: 8107 Continental neotectonics
DE: 7212 Earthquake ground motions and engineering
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting