GNSS-Acoustic observation using the Wave Glider to detect the seafloor crustal deformation off the Pacific coast of Northeastern Japan

Observations of seafloor crustal deformation are important to investigate the interplate locking states in subduction zones where mega-thrust earthquakes recur. The GNSS-Acoustic (GNSS-A) observation technique was developed to detect seafloor crustal deformation, and the results related to crustal deformation associated with large earthquakes around the Japanese Islands are remarkable. However, GNSS-A surveys require very large financial and human costs, because a research vessel as a sea-surface platform is necessary to perform the observation campaigns. To make it possible to conduct frequent observations, we have been developing automatic GNSS-A data acquisition system using an unmanned surface vehicle, the Wave Glider (WG). The WG was equipped with the GNSS-A observation instruments, namely two GNSS antennas, an acoustic transducer, a microelectromechanical system gyroscope, and associated control and logging units. We additionally installed the Thuraya satellite communication system to control the GNSS-A observations in 2020. The first observations using this system were performed at a seafloor station off Aomori Prefecture in July 2019, and we confirmed that data acquisition and autonomous activation of the seafloor stations were successfully executed by controlling the power supply to the payload via satellite communication with the WG [Iinuma et al., 2021, Front. Earth Sci.]. Following the initial success, four more campaigns have already been performed in June to July, 2020 (37 days at 14 sites), in October to November, 2020 (23 days at 8 sites), in November to December, 2020 (12 days at 3 sites), and in April to May, 2021 (48 days at 17 sites). And we are now preparing another campaign in this October to November. The results of the data processing indicate temporal and spatial variation of the postseismic deformation associated with the 2011 Tohoku-oki earthquake. We will introduce the outline of the observations using the WG and the results that may include the forthcoming campaign in the meeting.