Fig. 1

a Tectonic map in and around the Okinawa Trough (OT). Sky-blue dashed lines indicate westward extension of the Tokara Valley and Kerama Gap. NOT northern Okinawa Trough, MOT middle Okinawa Trough, SOT southern Okinawa Trough, TV Tokara Valley, KG Kerama Gap, MS Miyako Saddle, IS Ishigaki Saddle, MJ Miyako-jima, IJ Ishigaki-jima, LOFZ Luzon-Okinawa fracture zone, BMFZ Botan–Miyako fracture zone, OH Oki–Hateruma Basin. Red dotted line shows the volcanic front. b Scheme of the tectonic provinces in the East China Sea summarized by Inoue (2007). 1, Fukien-Reinan Massif; 2, Goto Uplift Zone; 3, Senkaku Uplift Zone; 4, Ryukyu Arc; 5, Ryukyu (Nansei-Shoto) Trench; A, Goto Basin; B, Tungai Basin; C, Senkaku Basin; D, Okinawa Trough; E, Ryukyu forearc basin; F, Amami Basin (Kizaki, 1985); A + B + C, East China Sea Basin (Taiwan Basin), G, Basin under Tungai Slope (Kimura 1990). c Location of the seismic lines. Red, green, and sky-blue lines are the across-trough, along-trough, and along-rift/rift-parallel survey lines, respectively. Blue lines indicate positions of previous surveys. Iwasaki et al. (1990), Nakahigashi et al. (2004), Nagumo et al. (1986), Hirata et al. (1991), Lee et al. (1980), Arai et al. (2017), Klingehoefer et al. (2009). Violet asterisks show hydrothermal vents (http://vents-data.interridge.org/ventfields?order=name_3&sort=asc;)