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[SGD01-P07] Long-term SSE along the Nankai trough detected by the GEONET after 2018
Keywords:long-term slow slip, Hyuga-nada Sea, Kii-channel
Introduction
In the Bungo Channel, long-term SSE has repeatedly occurred in a cycle of about 5 to 6 years. In central Shikoku, the occurrence of long-term SSE is estimated from the results of leveling in 1977-1980. The long-term SSE of Kii Channel occurred in 1996, 2000 and 2016. Under such circumstances, transitional crustal movements have occurred in northern Kyushu from around June 2018, and then transitional crustal movements have occurred around the Bungo Channel from around October 2018. Also, in central Shikoku, transient different from the crustal movements associated with the Bungo Channel SSE have occurred since early 2019 onward. Transient have occurred in southern Kyushu since July 2020 and continue until July 2021. In Kii Channel, transient have occurred since around April 2019 onward. In this study, we estimated the spatiotemporal changes in interplate slip in the Shikoku, Kyushu, and Kii Channel areas from the crustal movement data observed by GEONET.
Analytical method
From the position time series by the GEONET / F5 solution, the annual and semi-annual components were estimated by superimposing trigonometric functions and removed from the original position time series. The linear trend is removed from the time series from which the periodic component has been removed. In the Kyushu area, the linear trend of 2006-2009 in the Bungo Channel area and 2008-2011 in other areas is removed. In central Shikoku and the Kii Channel area, the linear trends for 2017-2018 have been removed. For the Kyushu region, the postseismic deformation of the 2011 Tohoku Earthquake and the 2016 Kumamoto Earthquake are removed based on the simulation results of viscoelastic relaxation (Suito 2017a; Suito 2017[J1] b). Using the east-west, north-south, and vertical position time series data obtained in this way, time-dependent inversion analysis was performed for the Kyushu / Shikoku region, the central Shikoku region, and the Kii Channel region. About 250 GEONET sites in the Nankai Trough area were used. The shape of the Philippine Sea plate compiled by Hirose et al. (2008) is represented by triangular elements in the analysis of the Kyushu / Shikoku region and the central Shikoku region and used for the analysis. The grid spacing is about 20-40km. The direction of slip on the plate interface was constrained to be the plate convergence direction. For the KiiChannel area, the shape of the Philippine Sea Plate compiled by Hirose et al. (2008) was represented by a spline phase, and the slip direction was constrained to the range from east to south. The grid spacing is about 20km.
Results and discussion
From around June 2018 to around autumn 2019, SSE occurred in the Bungo Channel in the northern Hyuga-nada. SSE occurred in central Shikoku at the same time as Bungo Channel SSE, and it continues as of December 2021. The Bungo Channel SSE was estimated to be Mw 6.9, and the central Shikoku SSE was estimated to be about Mw 6.3. The Bungo Channel SSE is in harmony with the 5-6 year cycle, and in central Shikoku, it occurred in 1977-1980 and 2013, but the repetition interval does not seem to be periodic. Long-term SSE occurred in the southern Hyuga-nada at the end of 2018, and long-term SSE occurred again in the southern part of Hyuga-nada from around June 2020. The southern Hyuga-nada SSE occurs in a cycle of about two years. Kii Channel SSE started around April 2019 and once converged around the summer of 2020, but continues as of December 2021. The repeat interval does not seem to be clear. After slip was estimated after the 2019 Tanegashima earthquake.
In the Bungo Channel, long-term SSE has repeatedly occurred in a cycle of about 5 to 6 years. In central Shikoku, the occurrence of long-term SSE is estimated from the results of leveling in 1977-1980. The long-term SSE of Kii Channel occurred in 1996, 2000 and 2016. Under such circumstances, transitional crustal movements have occurred in northern Kyushu from around June 2018, and then transitional crustal movements have occurred around the Bungo Channel from around October 2018. Also, in central Shikoku, transient different from the crustal movements associated with the Bungo Channel SSE have occurred since early 2019 onward. Transient have occurred in southern Kyushu since July 2020 and continue until July 2021. In Kii Channel, transient have occurred since around April 2019 onward. In this study, we estimated the spatiotemporal changes in interplate slip in the Shikoku, Kyushu, and Kii Channel areas from the crustal movement data observed by GEONET.
Analytical method
From the position time series by the GEONET / F5 solution, the annual and semi-annual components were estimated by superimposing trigonometric functions and removed from the original position time series. The linear trend is removed from the time series from which the periodic component has been removed. In the Kyushu area, the linear trend of 2006-2009 in the Bungo Channel area and 2008-2011 in other areas is removed. In central Shikoku and the Kii Channel area, the linear trends for 2017-2018 have been removed. For the Kyushu region, the postseismic deformation of the 2011 Tohoku Earthquake and the 2016 Kumamoto Earthquake are removed based on the simulation results of viscoelastic relaxation (Suito 2017a; Suito 2017[J1] b). Using the east-west, north-south, and vertical position time series data obtained in this way, time-dependent inversion analysis was performed for the Kyushu / Shikoku region, the central Shikoku region, and the Kii Channel region. About 250 GEONET sites in the Nankai Trough area were used. The shape of the Philippine Sea plate compiled by Hirose et al. (2008) is represented by triangular elements in the analysis of the Kyushu / Shikoku region and the central Shikoku region and used for the analysis. The grid spacing is about 20-40km. The direction of slip on the plate interface was constrained to be the plate convergence direction. For the KiiChannel area, the shape of the Philippine Sea Plate compiled by Hirose et al. (2008) was represented by a spline phase, and the slip direction was constrained to the range from east to south. The grid spacing is about 20km.
Results and discussion
From around June 2018 to around autumn 2019, SSE occurred in the Bungo Channel in the northern Hyuga-nada. SSE occurred in central Shikoku at the same time as Bungo Channel SSE, and it continues as of December 2021. The Bungo Channel SSE was estimated to be Mw 6.9, and the central Shikoku SSE was estimated to be about Mw 6.3. The Bungo Channel SSE is in harmony with the 5-6 year cycle, and in central Shikoku, it occurred in 1977-1980 and 2013, but the repetition interval does not seem to be periodic. Long-term SSE occurred in the southern Hyuga-nada at the end of 2018, and long-term SSE occurred again in the southern part of Hyuga-nada from around June 2020. The southern Hyuga-nada SSE occurs in a cycle of about two years. Kii Channel SSE started around April 2019 and once converged around the summer of 2020, but continues as of December 2021. The repeat interval does not seem to be clear. After slip was estimated after the 2019 Tanegashima earthquake.