Overview of the seafloor geodetic observation conducted by the Japan Coast Guard using the GNSS-Acoustic ranging combination technique

The Japanese Islands lie along subduction zones where the Pacific Plate and the Philippine Sea Plate subduct, making the islands one of the most seismically active zones in the world. Japan, prone to earthquake disasters throughout its history, has constructed numerous seismic and geodetic observation networks to elucidate the mechanism of catastrophic megathrust earthquakes that occur along the plate boundary near the subduction zones.

The Hydrographic and Oceanographic Department of the Japan Coast Guard (JCG) is one of the government branches that conduct geodetic observations to advance megathrust earthquake research. JCG conducts seafloor geodetic observation using the GNSS-Acoustic ranging combination technique (GNSS-A). GNSS-A enables us to measure the global coordinates of a seafloor reference point in precision of centimeters by simultaneously conducting GNSS observation of a sea surface platform (i.e., survey vessel, buoy, autonomous vehicle…) and trilateration of seafloor benchmarks using acoustic ranging. As of now, JCG regularly conducts GNSS-A observations at 27 seafloor sites along the Japan Trench and the Nankai Trough, named the Seafloor Geodetic Observation Array (SGO-A).

JCG has been conducting GNSS-A seafloor geodetic observation since 2000, and numerous technological advancements have been made in the past 20 years, significantly improving the observation frequency and positioning precision. The observation system currently operated by the JCG using survey vessels enables us to measure 3-4 times per year per seafloor site (Ishikawa et al. 2020, Front. Earth Sci.). Recently, we have developed an open-source GNSS-A analysis software named “GARPOS”, which simultaneously estimates sound speed perturbation and seafloor benchmark positions using empirical Bayesian inversion (Watanabe et al. 2020, Front. Earth Sci.).

Our regular observation at the SGO-A sites along the Japan Trench has revealed co- and postseismic processes of the 2011 Tohoku-oki Earthquake (Watanabe et al. 2021, EPS). Along the Nankai Trough, we have elucidated heterogeneous interplate coupling (Yokota et al. 2016, Nature) and shallow slow slip events (Yokota and Ishikawa 2020, Sci. Adv.). In this presentation, we review our observation and analysis methods, tectonic phenomena revealed from our observation, and the latest observation results at our SGO-A sites.