GNSS-A observation along the Nankai Trough by Japan Coast Guards Seafloor Geodetic Observation Array (SGO-A)

The Japan islands are located at a subduction zone where the oceanic plates subduct beneath the overriding continental plates. Along the subduction zone, megathrust earthquakes followed by devastating tsunamis have repeatedly occurred. For the elucidation of the mechanism of such earthquakes, numerous research institutes and government branches in Japan have been conducting seismological and geodetic observations. The Japan Coast Guard (JCG) has been conducting seafloor geodetic observation since the early 2000s using the Global Navigation Satellite System-Acoustic ranging combination technique (GNSS-A). GNSS-A enables us to precisely measure the global coordinates of a seafloor site by combining GNSS observation conducted on a sea surface platform and acoustic ranging between the on-board transducer and the mirror transponders installed at the seafloor site. Currently, JCG regularly conducts GNSS-A observation at 27 seafloor sites along the Japan Trench and Nankai Trough named the Seafloor Geodetic Observation Array (SGO-A) using survey vessels. SGO-A has revealed numerous subseafloor tectonic phenomena such as the co- and postseismic processes of the 2011 Tohoku Earthquake (Sato et al. 2011; Watanabe et al. 2014; Watanabe et al. 2021), interplate coupling (Yokota et al. 2016) and slow slip events (Yokota and Ishikawa, 2020) in the offshore region of the Nankai Trough. In this presentation, we present the latest results of the GNSS-A observations at the 19 SGO-A sites in the Nankai Trough region. We also present the slip deficit rates estimated by geodetic inversion using the method of Yabuki and Matsuura (1992), and discuss on the conditions of the plate boundary in the Nankai Trough region.