HR: 1340h
AN: T13C-1377    [Abstracts]
TI: Active Normal Faults on the Forearc Slope of the Eastern Nankai Subduction Zone
AU: * Arai, K
EM: ko-arai@aist.go.jp
AF: Geological Survey of Japan, AIST, AIST Tsukuba Central 7, 1-1-1, Higashi, Tsukuba, 305-8567
AU: Okamura, Y
EM: okamura-y@aist.go.jp
AF: Geological Survey of Japan, AIST, AIST Tsukuba Central 7, 1-1-1, Higashi, Tsukuba, 305-8567
AU: Ikehara, K
EM: k-ikehara@aist.go.jp
AF: Geological Survey of Japan, AIST, AIST Tsukuba Central 7, 1-1-1, Higashi, Tsukuba, 305-8567
AU: Ashi, J
EM: ashi@ori.u-tokyo.ac.jp
AF: ORI, The University of Tokyo, 1-15-1 Minamidai, Nakano-ku, 164-8639
AB: Many low angle thrusts run parallel to Nankai Trough on the lower slope of the eastern Nankai subduction zone, whereas normal faults have been developed on the shelf to upper shelf slope (Yamaji et al., 2003). "Central Uplift" composed of a row of 5-10 km long anticlines trending northeast to southwest divides the thrust zone in the lower slope and normal fault zone in the upper slope. The uplift has been inferred to have developed along the Enshu fault which has right-lateral transpressional movement. The normal faults are distributed in the area about 70 km long and 15 km wide on the outer shelf to shelf slope. The faults dip {70-80 $^{\circ}$} north, and have maximum downthrown offsets up to 0.2 second in two-way travel time. On the hanging wall, wedge shaped growth strata fill the depression. Seismic reflection and calcareous nanno fossil age indicate that the normal faults began to be active during 0.91-0.45 Ma (middle Pleistocene). Some faults dislocate the shelf surface that was formed by the sea level fall in the last glacial period, suggesting that the normal faults are still active and have a potential of earthquakes. It is important to study palepseismicity of the normal faults, because they are located near coast. We conducted detailed mapping and sampling from seven sites across one of the normal faults using Navigable Sampling System (NSS) during the R/V Kaiyo (KY03-11) cruise. Seabeam mapping clearly showed a trace of the fault as a linear step of seafloor about 10 m high. NSS has two video cameras which provide seafloor image to operators on line, so we could select a suitable place to take samples. We towed NSS across the fault observing seafloor and collected sediment cores from 5 sites on hanging wall and 2 sites on foot wall. We identified the steep slope on the image, but observed similar seafloor on the both sides of the fault and could not find evidence for cold seepage. Approximately two meters long cores were sampled on the both sides of the fault. One of the core collected from the footwall is composed of semi-consolidated sandy silt overlain by sandy silt. The boundary between the lithologic change indicates the unconformity that is characterized by cobbles encrusted by algae. The semi-consolidated sandy silt contains 0.91-0.45 Ma calcareous nanno fossil, suggesting that the sediments can be correlated to the base of growth strata. On the other hand, the core of hanging wall of the fault composed of alternation of silt and sandy silt. We tried to correlate the sandy silt above the unconformity on the footwall to the sediments on the hanging wall, but could not find key horizon. The average sedimentation rate of the hanging wall is estimated to be about 10 cm/kys, and we infer that slip rate of the fault is larger than the sedimentation rate. At this time, we could not estimate the paleosismicity of the fault, but we will mention further study on cruise of this September using NSS and discuss the potential of NSS for the study of paleoseismicity of offshore active faults.
DE: 7221 Paleoseismology
DE: 7230 Seismicity and seismotectonics
DE: 7294 Instruments and techniques
DE: 3025 Marine seismics (0935)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting