[S-CG57] New perspectives of subduction zone earthquake dynamics through experiments across-scales

[SCG57-01] A challenge to develop GNSS buoy system for high-functional tsunami monitoring and continuous observation of ocean-bottom crustal movements – a review and future problems -

*Teruyuki Kato¹, Yukihiro Terada², Keiichi Tadokoro³, Akira Futamura⁴ (1.Institute of Regional Development, Taisho University, 2.National Institute of Technology, Kochi College, 3.Graduate School of Environmental Studies, Nagoya University, 4.National Institute of Technology, Yuge College)

Keywords:GNSS buoy, disaster mitigation, tsunami, ocean bottom crustal movements

We have developed a new GNSS buoy system for tsunami early detection and continuous monitoring of sea bottom crustal movements. The presentation reviews this five-year project conducted by a JSPS Kakenhi grant. Based on the lessons learnt from the case of the 2011 Tohoku-Oki tsunami, we decided to put the buoy much farther from the coast. For this purpose, we employed two new technologies. First, we introduced Precise Point Positioning with Ambiguity Resolution (PPP-AR) algorithm for the estimation of the buoy coordinates without using baseline mode analysis. Second, we used a satellite data transmission for sending precise orbits and clocks for PPP-AR analysis on the buoy and also sending back the obtained precise position of the buoy to land, together with other ancillary data taken on the buoy. We used the commercially available Thuraya satellite for data transmission. In order to estimate the position of the transponder array at the sea bottom, we used the GNSS-acoustic system and developed a new algorithm for the simultaneous estimation of the coordinates of the transponder array at the sea bottom and the velocity structure of the sea water. We borrowed a buoy that is used for fishery by Kochi Prefecture. The buoy is placed about 40km south of Cape Ashizuri, Japan. The experiment continued for more than four years and obtained significant results for the future applications. PPP-AR enabled the positioning of the buoy with a few centimeters’ accuracy as far as the signal is fixed. Together with Point Variance Detection (PVD) analysis for a shorter period of wind waves, we were able to monitor long and short periods of waves separately in real time mode. The precipitable water vapor (PWV) and total electron content (TEC) data obtained from GNSS data were evaluated for their accuracy if they can be used for their own research field. TEC data showed that they were accurate enough for monitoring ionospheric disturbances. PWV data seems also valuable for monitoring water vapor variations over the ocean and they could be used in the objective analysis for weather prediction. Moreover, an experiment was conducted to get the data for improving the satellite data transmission rate due to tilting of buoy. Based on the data, a new algorithm using IoT was proposed for improving the efficiency of data transmission from many buoys to the dedicated satellite for a buoy array. On the other hand, a number of problems occurred during the experiment; for example, frequent power failures occurred mainly due to power shortages due to insufficient solar panels or unknown reasons, though some of them were suspicious of problems in satellite transmission system. For the GNSS-acoustic system, separation of parameters between array coordinates and acoustic velocity structures were found difficult unless buoy swings widely enough around the transponder array area. Examinations and overcoming of these problems will make it possible for a long-term GNSS buoy operation at far offshore and it will bring us a new powerful tool not only for disaster mitigation but also for applications in the various fields of earth science.