[J06-P] Poster

[J06-P-09] Low-frequency earthquake distribution covered with undrained layer of the overlying plate along Tokai plate boundary of the Nankai subduction zone

^○Sadaomi Suzuki¹, Makoto Okubo², Kazutoshi Imanishi³, Junichi Nakajima⁴ (1.Tono Research Institute of Earthquake Science (TRIES), Association for the Development of Earthquake Prediction (ADEP), Mizunami 509-6132, Japan, 2.Kochi Earthquake Observatory, Kochi University, Kochi 780-8073, Japan, 3.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8567, Japan, 4.Department of Earth and Planetary Sciences, School of Science, Tokyo Institute of Technology, Tokyo 152-8551, Japan)

We have developed a novel method that uses a 3D array to detect the P and S waves of deep low-frequency earthquakes (LFEs) along the Nankai subduction zone. We tried to find their P and S wave pairs by using a 3D array (6 km x 4 km area) with 14 seismic stations in the Tokai area including ones with deep (600 m at the deepest) borehole seismographs. We observed remarkable LFE activity over November 10-30, 2010 and identified P and S phases of LFEs with high quality result by using a semblance method. Referring to those identified P and S phases we manually picked the arrival times of not only both P and S waves of the 3D array stations but also S waves and rarely P waves of Hi-net stations. Using the arrival times we relocated precise hypocenters of the 15 LFEs. Those hypocenters distribute in the depth range from 26 km to 34 km approximately along the plate boundary inclining in depth from 30 km to 32 km. We compared the distribution of LFEs with the observed P-wave (dVp) and S-wave (dVs) velocity perturbation cross-sections along the subduction zone. Most of the LFEs occurred in the areas with dVp and dVs lower than about -3% along the plate boundary. And in the overlying plate just above the whole of LFE distribution there is a thin (several km thickness) layer with dVp and dVs higher than about 3%. We interpret that the higher velocity layer corresponds to an undrained layer keeping high pore-fluid pressures for LFE activity along the plate boundary.