Fig. 1

a Regional tectonic map of the Okinawa Trough. Black color corresponds to areas above the sea level. Dark gray shaded areas and light gray shaded areas indicate seafloors shallower than 700 and 4000 m, respectively, highlighting the topographic lows corresponding to the Okinawa Trough back-arc basin. The solid black line is the plate boundary between the Philippine Sea plate and the Eurasian plate (Bird 2003). The red rectangle indicates the location of the study area (b, c). b Bathymetric map of the southern Okinawa Trough showing submarine volcanoes (dashed orange lines; Watanabe et al. 1995; Arai et al. 2017; Misawa et al. 2020), hydrothermal sites (red stars; Beaulieu and Szafranski 2020) and magmatic intrusions (white triangles; Arai et al. 2017). Thin black lines indicate the seismic reflection profiles of YA01 and YA05 (Arai et al. 2017). Thick brown lines indicate the locations of seismic reflection images shown in Figs. 1d, 4. c Seismic activity during the period of January 2001 to August 2019. Earthquakes with magnitude greater than 2 and whose depth is shallower than 40 km are shown by gray dots. Earthquakes with a magnitude greater than 5 are highlighted by larger white circles. The black circle is the epicenter of a huge earthquake in 1938 (M7.2). The earthquake data are taken from the JMA catalog. Yellow squares are seismic stations used for the JMA catalog in the study area. White curves are faults, most of which are recognized as normal faults with east–west strike (The Headquarters for Earthquake Research Promotion 2016). d Seismic reflection image along the YA05 profile (Arai et al. 2017) showing an intrusive body beneath the Yaeyama Rift and a potential magmatic body (marked by a reflector with negative polarity)