Seismic refraction velocity structure in the vicinity of the Suruga Trough axial region revealed by OBS observations.

The Suruga Trough is located inside the Suruga Bay, in the central part of the Honshu island crust, formed by the subduction of the Philippine Sea Plate under the Eurasian Plate. This area coincides with the estimated focal region of the Tokai Earthquake. Air gun and MCS surveys were carried out by the R/V Shinyo-Maru of the Tokyo University of Marine Science and Technology (TUMSAT) to investigate the shallow structure of the Suruga Trough (to several km depth) in 2016 and 2017 (Tsuruga et al., 2017). In addition, the Tokai University complemented this survey with the installation of several pop-up Ocean Bottom Seismographs (OBSs), aiming to acquire the deeper crustal structure using the seismic refraction method. In this presentation we will report the results of the seismic refraction velocity structure on 4 survey lines estimated by the clear recordings of 7 OBSs. The granitic layer denoted by a velocity of 6.0km/sec in our model, was confirmed in survey line 01 on the Suruga Trough axial region but was not confirmed in survey line D located on the Eurasian Plate. Furthermore, the sedimentary layer appeared to be thickest in the East-West trending survey lines C and 02 across the Suruga Trough axial region. We understand that fact indicates the subduction status of the Philippine Sea Plate.