Overview of Japan Coast Guard’s GNSS-A seafloor geodetic observation and data management

The Japanese Archipelago lies along subduction zones, where megathrust earthquakes are known to occur repeatedly (e.g., Mw 9.0 2011 Tohoku-oki Earthquake). The source regions of such megathrust earthquakes mainly lie beneath the seafloor, making it necessary to monitor the crustal deformation not only on land but also on the seafloor. The Japan Coast Guard (JCG) has been conducting seafloor geodetic observation using the GNSS-Acoustic ranging combination technique (GNSS-A) since the early 2000s. By combining GNSS and underwater acoustic ranging, GNSS-A measures the absolute position of a seafloor benchmark in the precision of centimeters, which provides us fruitful information on the crustal deformation under ocean. JCG currently deploys 27 seafloor sites along the Japan Trench and the Nankai Trough named the Seafloor Geodetic Observation Array (SGO-A). JCG and the Univ. Tokyo group has been working on open data of GNSS-A. In the past, we have published the position time series data (Yokota et al. 2018), and the GARPOS software (Watanabe et al. 2020) which is used in our current routine analysis. Recently, we have been discussing on a standardized data format for GNSS-A in the task force of the Inter-Commission Committee on Marine Geodesy (ICCM) in the International Association of Geodesy. Currently, we are releasing the latest position time series data of each SGO-A site, and the GNSS-A observation dataset in the GARPOS data format (https://www1.kaiho.mlit.go.jp/chikaku/kaitei/sgs/datalist_e.html).