11:15 〜 11:45
[S22-04] [招待講演] GNSS観測によって明らかになった琉球列島の現在の地殻変動とスロースリップイベント
The Ryukyu Islands lie on a 1200-km-long island arc from Kyushu to Taiwan. Contemporary deformation of the Ryukyu Islands is characterized by subduction of the oceanic plate and active backarc spreading. The Philippine Sea plate subducts from the Ryukyu Trench beneath the islands. Numerous earthquakes and slow slip events (SSEs) occur along the subduction plate interface. Active backarc spreading along the Okinawa Trough also causes many shallow earthquakes in the island arc. The permanent GNSS observation network named GEONET was started in the 1990’s and it has greatly contributed our understanding on ongoing deformation due to these tectonic processes. Recently, our group has been constructed additional 15 GNSS stations in the islands, particularly the Yaeyama Islands, southernmost parts of the Ryukyu Islands. In this presentation, we review the previous geodetic studies on the contemporary deformation and SSEs as well as our new results of recent GNSS observations.
The secular GNSS velocity shows rapid backarc spreading along the Okinawa Trough and the spreading rate increases toward south (e.g., S. Nishimura, 2004; Nakamura, 2004; Watanabe and Tabei, 2004). Internal deformation of the island arc is rather small and modeled by rigid block rotation of three or four independent tectonic blocks. No significant contraction along the plate convergence was found, which suggests low coupling on the subduction plate interface. However, land geodetic data cannot resolve offshore coupling near the trench (e.g., Watanabe and Tabei, 2004) On the other hand, significant expansion along the arc was revealed by Nakamura (2004).
It is probable that SSEs as well as earthquakes accommodate a significant part of relative plate motion along the Ryukyu Trench. T. Nishimura (2014) conducted a systematic search of short-term SSEs using GNSS data and found 223 possible SSEs for 16 years. Detected SSEs distributes from the trench to the island arc with a depth range between 10 and 60 km shows highly heterogeneous along the trench. The most distinctive activity is bi-annual Mw~6.6 SSEs in the Yaeyama Islands (Heki and Kataoka, 2008; Tu and Heki, 2017; Kano et al., 2018). The cluster of SSEs locates southeast off southern Okinawajima, east off Kikaijima, and east of Tanegashima. Correlations between SSEs and other slow and regular earthquakes (e.g., low-frequency earthquakes, very-low-frequency earthquakes, and earthquake tremors) have been also found (Nakamura, 2015; Nakamura, 2017) but their relation is not simple and unlike the episodic tremor and slip events along Nankai Trough.
We estimated secular velocity at continuous stations including new stations from August 2017 to July 2020 in and around the Ryukyu Islands (Figure). This new velocity confirmed previous studies and gave us some additional insights on ongoing deformation. Expanded internal deformation in directions of both along and perpendicular to the trench are dominant in the Yaeyama Islands and Okinawajima but contraction along the relative plate motion is observed in a part of the Yaeyama Islands. Southeastward velocity of Io-torishima suggests volcanic inflation of the island.
Acknowledgment: GNSS RINEX data are provided by the Geospatial Information Authority, Japan Coast Guard, and International GNSS Service. This study is supported by JSPS KAKENHI Grant Number JP16H06474 in Scientific Research on Innovative Areas “Science of Slow Earthquakes”.
The secular GNSS velocity shows rapid backarc spreading along the Okinawa Trough and the spreading rate increases toward south (e.g., S. Nishimura, 2004; Nakamura, 2004; Watanabe and Tabei, 2004). Internal deformation of the island arc is rather small and modeled by rigid block rotation of three or four independent tectonic blocks. No significant contraction along the plate convergence was found, which suggests low coupling on the subduction plate interface. However, land geodetic data cannot resolve offshore coupling near the trench (e.g., Watanabe and Tabei, 2004) On the other hand, significant expansion along the arc was revealed by Nakamura (2004).
It is probable that SSEs as well as earthquakes accommodate a significant part of relative plate motion along the Ryukyu Trench. T. Nishimura (2014) conducted a systematic search of short-term SSEs using GNSS data and found 223 possible SSEs for 16 years. Detected SSEs distributes from the trench to the island arc with a depth range between 10 and 60 km shows highly heterogeneous along the trench. The most distinctive activity is bi-annual Mw~6.6 SSEs in the Yaeyama Islands (Heki and Kataoka, 2008; Tu and Heki, 2017; Kano et al., 2018). The cluster of SSEs locates southeast off southern Okinawajima, east off Kikaijima, and east of Tanegashima. Correlations between SSEs and other slow and regular earthquakes (e.g., low-frequency earthquakes, very-low-frequency earthquakes, and earthquake tremors) have been also found (Nakamura, 2015; Nakamura, 2017) but their relation is not simple and unlike the episodic tremor and slip events along Nankai Trough.
We estimated secular velocity at continuous stations including new stations from August 2017 to July 2020 in and around the Ryukyu Islands (Figure). This new velocity confirmed previous studies and gave us some additional insights on ongoing deformation. Expanded internal deformation in directions of both along and perpendicular to the trench are dominant in the Yaeyama Islands and Okinawajima but contraction along the relative plate motion is observed in a part of the Yaeyama Islands. Southeastward velocity of Io-torishima suggests volcanic inflation of the island.
Acknowledgment: GNSS RINEX data are provided by the Geospatial Information Authority, Japan Coast Guard, and International GNSS Service. This study is supported by JSPS KAKENHI Grant Number JP16H06474 in Scientific Research on Innovative Areas “Science of Slow Earthquakes”.