Japan Coast Guard's GNSS-A seafloor geodetic observation: analysis scheme and development of data format

The GNSS-Acoustic ranging combination technique (GNSS-A) is a method to precisely measure the absolute position of the seafloor benchmark in the centimeter level by combining GNSS and acoustic ranging. The Hydrographic and Oceanographic Department of the Japan Coast Guard (JCG) has been conducting GNSS-A observation at 27 seafloor sites along the Japan Trench and the Nankai Trough named the Seafloor Geodetic Observation Array (SGO-A), to reveal the geophysical processes related to megathrusts that occur at the subduction zones along the Japanese archipelago. From the observations at the SGO-A sites along the Japan Trench, we have discovered the co- and postseismic processes of the 2011 Tohoku-oki Earthquake (Sato et al. 2011; Watanabe et al. 2021). Heterogeneous interplate coupling (Yokota et al. 2016) and shallow slow slip events (Yokota and Ishikawa 2020) have been revealed by observation along the Nankai Trough.

Currently, JCG conducts routine analysis using the open-source GNSS-A analysis software GARPOS (Watanabe et al. 2020). We are aiming an “open” GNSS-A, and the SGO-A coordinate time series obtained from the routine analysis are published online. In this presentation, we review our observation and analysis methods, and discuss on the latest observation results obtained at the SGO-A sites. Additionally, we also introduce our GNSS-A data format, which we have been discussing in the seafloor geodesy data standardization task force of the Inter-Commission Committee on Marine Geodesy (ICCM) in the International Association of Geodesy.