Preservation of submarine event deposits: Where is the most preferable area to reconstruct the past earthquake/tsunami events from marine sediment records?

Thursday, 18 December 2014
Ken Ikehara, Marine Geology Research Group, Geological Survey of Japan, AIST, Tsukuba, Japan, Toshiya Kanamatsu, JAMSTEC Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan, Juichiro Ashi, University of Tokyo, Bunkyo-ku, Japan, Michael Strasser, ETH Swiss Federal Institute of Technology Zurich, Zurich, Switzerland, Kazuko Usami, Geological Survey of Japan, AIST, Tsukuba, Japan, Yasuyuki Nakamura, Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan and Shuichi Kodaira, IFREE JAMSTEC, Yokohama, Japan
Large earthquakes and their related tsunami impact submarine geological processes that can form specific submarine event deposits. Most of widely distributed event beds are typically fine-grained turbidites. Examples from the Nankai Trough and the Beppu Bay suggest that a terminal isolated depression is area with the highest potential for the deposition of fine-grained turbidites. The 2004 off the Kii Peninsula earthquakes caused a fine-grained turbidite deposited in a small basin on the Nankai accretionary prism slope, and formed a thick acoustically transparent layer. Off Sanriku thickness of the 2011 Tohoku-oki earthquake and tsunami event beds was usually a few-several cm, but was highest in the Japan Trench with several tens cm. The Japan Trench floor is the deepest and terminal basin. The terminal basin is a potential area for marine paleoseismology. Preservation potential is a factor for usage of submarine event beds as paleoseimsological tool. Repeated sampling of surface sediments from off Sanriku after the 2011 event suggests that the preservation potential of fine-grained event bed is low in the strong benthos activity zone. Sedimentation rate is another factor on the preservation potential. Quick and high sediment accumulation after the event bed deposition diminishes the pressure for the destruction of the event beds by benthos activity. Thus, a small basin covered by the bottom water with low dissolved oxygen concentration, and occurred under the high primary productivity areas is a good example of basins with high preservation potential of event beds. High resolution seismic profiles in the central Japan Trench indicate that occurrence of the well-stratified strata filling the trench and subducting graben depressions. These strata have high potential for seismo-turbidite deposition. Thus, small basins on the mid-slope terrace and the trench floor of the central Japan Trench are most preferable and potential candidates for submarine paleoseismology.