HR: 1340h
AN: V33D-1494    [Abstracts]
TI: Late Quaternary Activity of active faults in the Unzen Graben, western Kyushu, Japan, based on tectonic geomorphology
AU: * Takemura, K
EM: takemura@bep.vgs.kyoto-u.ac.jp
AF: Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Noguchibaru, Beppu, 874-0903 Japan
AU: Matsuoka, A
EM: a6065@n-koei.co.jp
AF: Nihon Koei, 5-4 Koujimachi, Chiyoda-ku, Tokyo, 102-8539 Japan
AU: Tsutsumi, H
EM: tsutsumh@kugi.kyoto-u.ac.jp
AF: Department of Geophysics, Graduate School of Science, Kyoto University, Kitashirakawa Oiwakecho, Sakyo-ku, Kyoto, 606-8502 Japan
AB: The Unzen Graben, which is bounded by normal faults to the north and south, is located on the western end of the Beppu-Shimabara Graben, west Kyushu, Japan. The faults in the Unzen Graben have developed with the growth of the Unzen volcanoes and dislocated volcanic materials such as lavas and pyroclastic deposits. The detailed location and vertical offsets of the active faults have been reported by previous studies, but the timing of faulting was poorly constrained. We estimated the vertical offsets based on topographic profiling, and calculated vertical slip rate based on recently obtained ages of volcanic rocks and sediments. In addition, we excavated two trenches at Chijiwa Town (western part of Unzen Graben) in order to reveal the timing of recent faulting events, and also found several outcrops across the active faults. Based on these surveys, the active faults in the Unzen Graben can be divided into four groups. Two groups of faults, which are located on the northern and southeastern margins of the Unzen Graben, have vertical slip rates as high as 0.13 to 1.0 mm/yr. A group of faults inside the Unzen Graben has slip rate 0.085 to 0.46 mm/yr except for the Akamatsudani fault which has slip rate 0.91 to 1.7 mm/yr. Three groups of faults mentioned above show cumulative vertical offsets and their rates of activity tends to be higher around the edifice of the Fugen volcano. On the other hand, the group outside the Unzen Graben is 0.026 to 0.068 mm/yr.
DE: 8107 Continental neotectonics
DE: 8109 Continental tectonics--extensional (0905)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting