Offshore northern Ishigaki-Jima Island, in the southern Okinawa Trough,
offers outstanding opportunities to explore the rifting stage of a
backarc system. We report the results of integrated marine geological
and geophysical surveys with high-density survey lines in this area. We
identify a graben bounded by normal faults and extending approximately
59 km in an ENE-WSW direction off-axis of the southern Okinawa Trough.
Submarine volcanoes with active hydrothermalism and associated intrusive
structures lie in the graben. Magnetic anomaly and seismicity data in
and around the graben suggest the presence of relatively shallow magma
acting as a heat source. All features identified in and around the
graben suggest active rifting in the southern Okinawa Trough.