[S-CG45] Ocean Floor Geoscience

[SCG45-08] Structure and seismicity in the southern part of the Okinawa Trough and their implications for rifting and volcanic processes

*Ryuta Arai¹, Ayanori Misawa², Makoto Otsubo², Saki Ishino², Masataka Kinoshita³, KH-21-3 Shipboard Scientists (1.Japan Agency for Marine-Earth Science and Technology, 2.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 3.Earthquake Research Institute, The University of Tokyo)

Keywords:Okinawa Trough, Back-arc rifting, Submarine volcano

The Okinawa Trough is an active rift zone behind the Ryukyu subduction zone and provides a great research field to study the deformation and evolution of the lithosphere in a back-arc setting. The southern part of the Okinawa Trough exhibits the highest rifting rate and the deepest seafloor depth (>2,000 m) in the basin. Consistently, recent seismic surveys show that significant crustal stretching occurs beneath the rift axis marked by the linear bathymetric depression (Yaeyama Rift) and suggest that the southern part of the Okinawa Trough is thought to be at a transitional stage from continental rifting to seafloor spreading (e.g., Arai et al., 2017). Another important feature here is that off-axis volcanic features are in close proximity to on-axis magmatic intrusions. From the viewpoint of seismicity, large earthquakes (with a magnitude greater than 5) frequently occur around the rift axes. These earthquakes mostly accompany rapid increase in seismicity rate (swarm activity), implying sudden changes in stress regime probably induced by large-scale dike intrusion. Although these tectonic and volcanic processes are expected to closely interact each other, their relative roles in controlling the overall back-arc rifting process are poorly understood.
Aiming at better understanding the tectonic and volcanic processes associated with the continental back-arc rifting, we conducted integrated marine geophysical and geological surveys in the southern part of the Okinawa Trough using the research vessel Hakuhomaru in January 2021 (KH-21-3). During the cruise, we acquired high-resolution multichannel seismic reflection data along the 5 rift-parallel (SW-NE direction) lines and 4 rift-perpendicular (NW-SE direction lines using two GI guns with a total volume of 710 cu.in. and a 1200-m-long, 48-channel streamer cable. The study area covers the Yaeyama Rift and the potential submarine volcano located further south, providing three-dimensional structural constraints on both on-axis and off-axis magmatic intrusions at shallow depths. In this presentation, we discuss the structure and rifting processes in the southern part of the Okinawa Trough based on the newly obtained seismic reflection images and seismic activity.