HR: 0800h
AN: T11A-0354    [Abstracts]
TI: Deep-Sea Turbidite Evidences on the Recurrence of Large Earthquakes Along the Eastern Nankai Trough
AU: * Ikehara, K
EM: k-ikehara@aist.go.jp
AF: Geological Survey of Japan, AIST, Tsukuba Central 7, Higashi 1-1-1, Tsukuba, 305-8567 Japan
AU: Ashi, J
EM: ashi@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, University of Tokyo, Minamidai 1-15-1, Nakano, Tokyo, 164-8639 Japan
AB: Large earthquakes (M larger than 8) have occurred along the Nankai Trough plate boundary. To understand the nature of the earthquakes and their recurrence intervals, turbidites in the deep-sea sediments were examined. Many turbidite sands were observed in cores collected from some slope basins and forearc basins along the eastern Nankai Trough. Frequent and rhythmical occurrence of turbidite sands suggested periodical failures of submarine slopes. For example, fifteen and thirteen turbidites were intercalated in two sediment cores from two slope basins along the Tokai Thrust, which is a out-of-sequence thrust in the Nankai accretional prism off Tokai. Geological and topographic setting of the basins suggests that these turbidites were of earthquake origin. Depositional age of each turbidite layer determined by radiocarbon ages of planktonic foraminifera in hemipelagic mud indicated that large earthquakes along the eastern Nankai Trough might have occurred periodically every 100-150 years, which is the same as the known intervals of large intraplate earthquakes from the historical and archaeological data, during the last 3000 years. The other rhythmical occurrences of turbidites found in the foot of slope in the northern Kumano Trough. The sites locate the western extension of Enshu Fault, which is a strike-slip fault along the Japanese coast. Judging from the averaged sedimentation rates of hemipalegic mud in the northern Kumano Trough suggested that the depositional intervals of turbidites might be around 200-250 years, which is two-times longer than the known interval of intraplate earthquakes along the eastern Nankai Trough. On the other hand, the depositional intervals of turbidites in a core from the Omine Ridge, which is a outer ridge of the Kumano Trough and locates near the OST, might be around 1000 years based on radiocarbon ages of planktonic foraminifera in hemipelagic mud. These facts indicated difference on response of deep-sea turbidite to the large earthquake related to geological and sedimentological settings.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3022 Marine sediments: processes and transport
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3070 Submarine landslides
DE: 7221 Paleoseismology (8036)
SC: Tectonophysics [T]
MN: Fall Meeting 2005