3:45 PM - 4:00 PM
[SCG50-08] Lack of inter-plate coupling along the southern end of the Ryukyu trench deduced from GNSS-A geodetic measurements
★Invited Papers
Keywords:GNSS/A geodesy, Seafloor crustal deformation, Ryukyu trench, Inter-plate coupling
We have been conducting GNSS-A geodetic measurement campaigns in the Hateruma Basin along the southwestern end of the Ryukyu trench with seafloor site OHTM. In 2023, we newly installed a seafloor site OMYK off Miyakojima Island and started new series of measurement campaigns. Both benchmarks show no landward motion, which suggests lack of inter-plate coupling to generate huge mega-thrust earthquakes.
The Yaeyama Islands, located at the southwestern edge of the Ryukyu trench, are known to have been devastated by the Yaeyama Tsunami that occurred on April 24, 1771. Nakamura (2009) suggested that the source of the Yaeyama tsunami may have been a Mw 8.0 tsunami earthquake that occurred at the shallow plate boundary at the southwest edge of the Ryukyu trench. Ando et al. (2018) also reported from a trench survey that four tsunamis including the 1771 Yaeyama Tsunami have hit the east coast of Ishigaki Island in the past 2000 years. The regularly repeating tsunamis remind us of mega-thrust earthquakes as their source.
We installed the seafloor site OHTM in the Hateruma forearc basin, about 40 km off the south coast of Hateruma Island in 2014, to test the presence of inter-plate coupling that may have generated these tsunamis. The OHTM is located at a water depth of about 3300 m, that is composed of three transponders arranged in an equilateral triangle with a circumscribed circle diameter of approximately 1/√2 of the water depth. After the observation until 2022, the OHTM was found to be moving toward the trench with respect to the Yangtza plate at about 49.7±7.3mm/yr which is almost same as that of the Ryukyu arc (50 mm/yr), suggesting that there is no coupling under the Hateruma basin.
We installed a new seafloor site OMYK off Miyakojima Island in 2023, 150 km east along the Ryukyu trench from OHTM. In contrast to the Hateruma Basin of OHTM which has a negative free-air gravity anomaly and topography suggesting long-term subsidence, the OMYK was installed at a depth of 880 m on the Miyako deep-sea terrace which has a positive free-air gravity anomaly. Two measurement campaigns, one at the time of installation of the seafloor site and the other one year later, showed that OMYK moves by 78 ± 24 mm/yr velocity in a south-southeast direction relative to the Yangtza plate. Although this is a preliminary result, it suggests that there is no inter-plate coupling also beneath the Miyako deep-sea terrace. Further long-term repeated observations are needed to obtain accurate estimates of the coupling rate.
In our presentation, we will introduce the installation of a towed buoy system for GNSS-A measurements and its contribution to improving the accuracy of the seafloor positioning.
<Reference>
Nakamura (2009). GRL, 36, L19307. DOI 10.1029/2009GL039730
Ando et al. (2018). Tectonophysics, 722, 265-276
The Yaeyama Islands, located at the southwestern edge of the Ryukyu trench, are known to have been devastated by the Yaeyama Tsunami that occurred on April 24, 1771. Nakamura (2009) suggested that the source of the Yaeyama tsunami may have been a Mw 8.0 tsunami earthquake that occurred at the shallow plate boundary at the southwest edge of the Ryukyu trench. Ando et al. (2018) also reported from a trench survey that four tsunamis including the 1771 Yaeyama Tsunami have hit the east coast of Ishigaki Island in the past 2000 years. The regularly repeating tsunamis remind us of mega-thrust earthquakes as their source.
We installed the seafloor site OHTM in the Hateruma forearc basin, about 40 km off the south coast of Hateruma Island in 2014, to test the presence of inter-plate coupling that may have generated these tsunamis. The OHTM is located at a water depth of about 3300 m, that is composed of three transponders arranged in an equilateral triangle with a circumscribed circle diameter of approximately 1/√2 of the water depth. After the observation until 2022, the OHTM was found to be moving toward the trench with respect to the Yangtza plate at about 49.7±7.3mm/yr which is almost same as that of the Ryukyu arc (50 mm/yr), suggesting that there is no coupling under the Hateruma basin.
We installed a new seafloor site OMYK off Miyakojima Island in 2023, 150 km east along the Ryukyu trench from OHTM. In contrast to the Hateruma Basin of OHTM which has a negative free-air gravity anomaly and topography suggesting long-term subsidence, the OMYK was installed at a depth of 880 m on the Miyako deep-sea terrace which has a positive free-air gravity anomaly. Two measurement campaigns, one at the time of installation of the seafloor site and the other one year later, showed that OMYK moves by 78 ± 24 mm/yr velocity in a south-southeast direction relative to the Yangtza plate. Although this is a preliminary result, it suggests that there is no inter-plate coupling also beneath the Miyako deep-sea terrace. Further long-term repeated observations are needed to obtain accurate estimates of the coupling rate.
In our presentation, we will introduce the installation of a towed buoy system for GNSS-A measurements and its contribution to improving the accuracy of the seafloor positioning.
<Reference>
Nakamura (2009). GRL, 36, L19307. DOI 10.1029/2009GL039730
Ando et al. (2018). Tectonophysics, 722, 265-276