A Decade of Global Navigation Satellite System/Acoustic Measurements of Back-Arc Spreading in the Southwestern Okinawa Trough

Horng-Yue Chen,Yoko Tu,Koto Mizuno,Takeru Kohmi,Kiyomichi Takemoto,Toshiaki Tsujii,Chin-Shang Ku,Hsin Tung,Ya-Ju Hsu,Cheng-Horng Lin,Masataka Ando,Ryoya Ikuta

doi:10.3389/feart.2021.601138

Abstract

Long-term seafloor geodetic measurements are important for constraining submarine crustal deformation near plate boundaries. Here we present an integrated analysis of a decade of GNSS/acoustic data collected at a site 60 km to the east of northeast Taiwan near the axis of the Okinawa Trough back-arc basin. We obtained a time-series of horizontal and vertical positions based on 18 measurements from 2009 to 2019. These data reveal a southeastward movement at a rate of 43 ± 5 mm/yr since 2012 with respect to the Yangtze Plate. The horizontal motion can be explained by the clockwise rotation of the Yonaguni Block and northern Central Range. In addition, the vertical displacement of the transponder array shows rapid subsidence of 22 ± 9 mm/yr from 2012 to 2019. The fast subsidence rate and negative free-air gravity anomaly in this region indicate that crustal thinning is compensated mainly by surface deformation rather than upward migration of the Moho. Taking into account the offset in 2012 owing to the replacement of the transponder array, the horizontal position time series of our site are best explained by two linear lines with a slope change in July 2013. The timing of the velocity change coincides broadly with a change in the nearby seismicity rate and dike intrusion 150 km away from the site. Our results highlight the potential of seafloor geodesy in assessing temporal changes in deformation near the spreading center of the Okinawa Trough, which cannot be one using data from onland GNSS stations.

Highlights

The Okinawa Trough is an active back-arc basin (Sibuet et al, 1987)
2 + μ2j j i za(t) zt 2 ⎫⎬ dt⎭ → min where Ti is the ith acoustic travel-time obtained at time ti (s), f (r0,ri, a) is the travel-time predicted by ray tracing from the position of the seafloor transponders r0j in the jth campaign and the onboard transducer ri, a(t) is the temporal varying coefficient of the speed of sound in seawater represented by superposition of cubic B-spline functions (Barnhill and Riesenfeld, 1974), and μj is the hyper parameter that balances the smoothness of a(t) with the travel-time residual
The seafloor transponder array has undergone southeastward movement at an average velocity of 43 mm/yr from 2012 to 2019, which is 18 mm smaller than the results of Chen et al (2018) for 2012–2016. This horizontal movement can be explained by clockwise block rotation of the Yonaguni Block and Central Range, which form the Yilan Plain, by back-arc rifting

Summary

Introduction

The Okinawa Trough is an active back-arc basin (Sibuet et al, 1987). The axis of the basin has its southwestern end in the Yilan Plain of Taiwan and extends to the Beppu–Shimabara rift zone in Japan. The spreading rate is fastest (60 mm/yr) around the Yaeyama rift in the southwestern Okinawa Trough (Nishimura et al, 2004; Figure 1). Episodic opening of the rift has been reported around the Yaeyama rift. Dike intrusion was detected north of Yonaguni Island in 2002 (Nakamura and Kinjo, 2018), beneath the Yilan Plain in 2005 (Lai et al, 2009), and north of Iriomote Island in 2013 (Ando et al, 2015). In contrast to the episodic rifting, onland Global Navigation

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