[J08-2] Imaging and interpreting lithospheric structures using seismic and geodetic approaches II

[J08-2-03] Structure of the incoming/subducting Pacific Plate in the central part of the Japan Trench: Results from repeated ocean bottom seismograph observations

^○Koichiro Obana¹, Gou Fujie¹, Tsutomu Takahashi¹, Takashi Tonegawa¹, Yojiro Yamamoto¹, Shuichi Kodaira¹, Masanao Shinohara² (1.Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan, 2.Earthquake Research Institute, University of Tokyo, Tokyo, Japan)

Oceanic plate subduction is an important process to transport the water from the Earth's surface into the mantle. Recent active seismic surveys present that systematic reduction of the seismic velocities within oceanic crust and the uppermost mantle of incoming oceanic plate with the approach toward the trench. These seismic velocity changes are considered as a result of the oceanic plate hydration/alteration in the trench-outer rise region. However the active seismic studies can resolve the structure at depths down to a few km below the oceanic Moho. After the 2011 Tohoku-Oki earthquake, seismicity within the incoming/subducting Pacific plate in the trench-outer rise region of the Japan Trench is very active. Ocean bottom seismograph (OBS) observations have been conducted repeatedly in the trench-outer rise region of the central part of the Japan Trench since 2011. The active seismicity would give a chance to obtain structure information of the oceanic lithosphere based on the passive earthquake observations. We investigated seismic velocity structure through the travel time tomography by using the passive OBS observation data consisting of 120 stations and more than 8000 events in total. The results show that the seismic velocities within the oceanic mantle seaward of the Japan Trench reduce toward the trench axis. The reduction of the P-wave velocities, which may relate to the hydration/alteration of the oceanic plate, extended down to a depth of about 20 km below the oceanic Moho. We also investigated anisotropy and Q structures by using the OBS data. We will discuss the structures of the incoming Pacific plate and its hydration/alteration in the trench-outer rise region by combining the results from these analyses.