Fig.1 : Study sea area. Air gun shooting was conducted on the thick black lines. The open circles indicate locations of ocean-bottom seismographs (OBSs). Inset left figure is bathymetry of entire Izu-Ogasawara-Mariana arc. IOA, SB, MA, MT, WMR, PVB, KPR indicate Izu-Ogasawara arc,Shikoku basin,Mariana arc,Mariana Trough,West Mariana Ridge,Parece Vela Basin,Kyshu-Palau Ridge,respectively.