ALBERT

Description: While 12 years have already passed since the 2011 Tohoku-oki earthquake (M9.0), postseismic deformation of the earthquake is still ongoing. In order to detect the postseismic deformation of the Tohoku-oki earthquake, we have been performing repeating observations at seafloor sites for the GNSS-Acoustic (GNSS-A) geodetic observation technique since 2012 using both manned research vessels and an unmanned surface vehicle, called Wave Glider (WG), as a sea surface platform. We developed and tested a system to conduct the GNSS-A observations using the WG in 2019 and have continuously operated the system twice a year since 2020, visiting many (around 15) observation sites. Based on the observation data collected using research vessels and the WG, we estimated the displacement time series of the seafloor transponder array of each site. The spatio-temporal variations in seafloor crustal deformation after the 2011 Tohoku-oki earthquake indicate that the viscoelastic relaxation is the primary cause of the westward motion around the main rupture area of the M9 event, while the afterslip on the shallow plate interface played an important role in producing eastward displacements in the region south to the main rupture area. With respect to the region north to the main rupture area, no distinct spatial deformation pattern has been observed. This implies that multiple factors, i.e., viscoelastic relaxation, afterslip, interplate locking, and episodic slow slip events, are responsible for the complicated spatial deformation in this region. We will also report the current conditions of the equipment for GNSS-A observations in the presentation.