HR: 0800h
AN: T11D-1294    [Abstracts]
TI: Seismic reflection profiling around the hypocentral area of the 2003 Miyagi-ken Hokubu earthquake (Mj6.4): Reactivated thrust faulting of a Miocene normal fault.
AU: * YOKOKURA, T
EM: taka.yokokura@aist.go.jp
AF: Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: YAMAGUCHI, K
EM: yamaguchi-k@aist.go.jp
AF: Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: KANO, N
EM: n.kano@aist.go.jp
AF: Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: YOKOTA, T
EM: yokota-t@aist.go.jp
AF: Institute for Geo-Resources and Environment, Geological Survey of Japan, AIST, Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: TANAKA, A
EM: akiko-tanaka@aist.go.jp
AF: Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: OHTAKI, T
EM: t-ohtaki@aist.go.jp
AF: Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AB: The 2003 Miyagi-ken Hokubu (northern Miyagi) earthquake occurred on July 26, which was preceded by the largest foreshock of Mj5.6 and was followed by the largest aftershock of Mj5.5. Although these earthquakes were not so large in magnitude, they caused large damages. The earthquakes occurred just beneath the Asahiyama hills, where exist the active Asahiyama flexure. Aftershock observations delineate a clear fault plane that extends toward the Sue hills in the east, not toward the Asahiyama hills. However neither surface ruptures nor active fault assocciated with the earthquakes were observed in this region. To clarify both the surface extension of the fault and geologic structure of this region, we conducted 17km-long seismic reflection profiling, using a 17.5-ton vibrator. Geologically, this region was subjected rapid EW extension in middle Miocene and thus produced rift basin was filled by the Matsushima-wan Group (syn-rift sediments) which was bounded by a normal fault, the Ishinomaki-wan fault, in the eastern side of the basin. The Matsushima-wan Group was unconformably overlain by the Shida Group (Miocene post-rift sediments). The Shida Group was unconformably overlain by the Pliocene and post-Pliocene sediments. Deformed Pliocene strata show thrust faulting, indicating EW compression after early Pliocene. Detailed data processing reveals that the seismic profile is essentially concordant with the structure inferred from surface geology. A west-dipping fault with about 50 degrees is found beneath the southeastern extension of the Sue hills where the Ishinomaki-wan fault was supposed to extend. The deeper part of the fault extends toward the earthquake fault plane determined by aftershocks and the shallower part shows a thrust-like structure, which indicate basin inversion using this fault. Thus the 2003 Miyagi-ken Hokubu earthquake occurred as reactivated thrust faulting of the Miocene normal fault bounding the eastern side of the rift basin.
DE: 7221 Paleoseismology
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
DE: 8107 Continental neotectonics
DE: 0935 Seismic methods (3025)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting