HR: 1340h
AN: T13C-1374    [Abstracts]
TI: Characteristics of deep-sea turbidites by the 1994 Hokkaido-nansei-oki earthquake
AU: * Ikehara, K
EM: k-ikehara@aist.go.jp
AF: Geological Survey of Japan, AIST, Tsukuba Central 7, Higashi 1-1-1, Tsukuba, Iba 305-8567 Japan
AU: Usami, K
EM: usami@imslab.co.jp
AF: Information and Mathematical Science Laboratory, 2-43-1 Ikebukuro, Toshima, 171-0014 Japan
AB: To understand the recurrence intervals of earthquakes in a peculiar area, it is very important to describe the phenomena at the sea-bottom occurring during an earthquake event. Based on surface ship and submersible surveys, submarine slope failures were a important phenomena on sea-bottom around the rupture area of the 1994 Hokkaido-nansei-oki earthquake. Earthquake-triggered turbidites were observed in and near the rapture area. The turbidite had sharp basal contact with underlay brown hemipelagic mud, and showed clear fining-upward grading structure from lower fine sand to upper clayey silt and parallel and cross lamination. No surface oxidized layer was observed, indicating very recent deposits. Benthic foraminifera in turbidite sands suggested that the sand was shallow water origin such as shelf to upper slope. Spatial distribution of turbidite thickness and grain size in the Shiribeshi Trough suggested major contribution from the southeastern steep slope with some minor contribution through submarine canyons opening to the northeastern trough floor. Such estimation was supported by grain fabric analysis using anisotropy of magnetic susceptibility. The similar turbidite sand layers were found in a sediment core collected from central Shiribeshi Trough. Radiocarbon age determinations and tephrochronology suggested that the averaged recurrence interval of turbidites was inferred as 1000-1500 years. The recurrence interval is almost the same as those of most active on land faults along the Japan Sea coast indicating higher seismic activity along the plate convergent margin at the eastern margin of Japan Sea.
DE: 7221 Paleoseismology
DE: 8105 Continental margins and sedimentary basins
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3022 Marine sediments--processes and transport
DE: 1744 Tectonophysics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting