Subseafloor tectonic phenomena along the Japan Trench and the Nankai Trough revealed from recent GNSS-A observation at Japan Coast Guard’s SGO-A sites

The Japan Coast Guard (JCG) has been performing seafloor geodetic observation using the Global Navigation Satellite System-Acoustic ranging combination technique (GNSS-A), to reveal the subseafloor tectonic phenomena occurring at the subduction zones along the Japanese archipelago. JCG currently deploys 27 seafloor reference points named the Seafloor Geodetic Observation Array (SGO-A) along the Japan Trench and the Nankai Trough. Decadal GNSS-A observation at the SGO-A sites along the Japan Trench has revealed complex postseismic movements after the 2011 Tohoku-oki Earthquake, indicating that afterslips may have occurred at the northern and southern ends of the coseismic slip, and that viscoelastic relaxation is continuing beneath the main rupture zone and also near the trench axis offshore of Fukushima. From the observations at the sites along the Nankai Trough, we have detected transient signals that may be caused by shallow slow slip events occurring near the trough axis. We are expecting further advancement in the observation frequency and positioning accuracy of GNSS-A to detect smaller, short-term subseafloor tectonic phenomena, to contribute to the study of megathrust earthquakes.