3D shear wave velocity structure in the southwestern Okinawa Trough

The southwestern Okinawa Trough is an active back-arc basin, extending and rifting within the continental lithosphere. Studying the southwestern Okinawa Trough is important for understanding the processes involved in the opening and development of a back-arc basin. At various times between 2010 and 2018, the Institute of Earth Sciences, Academia Sinica, Taiwan deployed 34 Ocean Bottom Seismometers (OBSs) over a region ∼ 0.2°×0.3° in longitude/latitude on the southwest edge of Okinawa Trough offshore northeast Taiwan. Ambient noise recorded on vertical velocity and pressure sensors is used to retrieve Rayleigh/Scholte waves to study the subseafloor shear wave velocity structure. A reference phase velocity dispersion curve is forward-modeled, according to a shear velocity model from an adjacent site determined by Kuo et al. (2015). Phase velocity dispersion curves are measured from the retrieved Rayleigh/Scholte waves and unwrapped according to which branch of the modeled phase velocities they appear closest to. Applying a 3D Monte Carlo inversion, shear wave velocities are estimated to ∼ 4 km depth using the obtained frequency-dependent phase velocities. A low velocity in the basin centre indicates the back-arc rifting is active at the southwest edge of Okinawa Trough. A comparison to the sites studied by Kuo et al. (2015) in the east of our study region suggests that back-arc rifting and extension may taper from east to west towards Taiwan.