3:00 PM - 3:15 PM
[SCG44-06] Strain accumulation and release processes associated with the occurrence of SSEs in the Japanese Islands
Keywords:GNSS, Slow Slip Event, Tokai region, Boso-Oki, Bungo Channel, Strain accumulation and release
1. Introduction
In the deeper extension of the plate interface of the seismogenic zone of the impending megathrust earthquake in the Nankai subduction zone, long-term slow slip events (L-SSEs) such as the 2000-2005 and Tokai L-SSE and the 2009-2011 and 2018-2019 Bungo Channel L-SSEs have occurred repeatedly. Additionally, short-term SSEs (S-SSEs) have occurred in Boso-Oki repeatedly every several years. The purpose of this study is to verify the relationship between the strain accumulation and release processes associated with the occurrence of these SSEs quantitatively, using GNSS time series data.
2. Method
The steady-state interseismic displacement field was obtained, by analyzing GNSS time series data, and the strains (principal strain, dilatation, and maximum shear strain) accumulated from the reference date to just before the occurrence of a SSE were calculated. Similarly, the displacement and strain fields associated with the same SSE were obtained from the displacements of onset and end dates of the occurrence of the SSE. The spatial distributions and amounts of the strains accumulated prior to the onset of the SSEs and the strain changes associated with the SSEs were then compared, and the relationship between them was investigated.
In this study, the Tokai L-SSE (July 1, 2000 to June 30, 2005), the Boso-Oki SSE (August 1, 2007 to August 29, 2007), and the Bungo Channel L-SSEs (January 1, 2010 to March 14, 2011 and January 1, 2018 to December 31, 2019) were investigated. The occurrence periods for these SSEs were 5.0, 0.079, 1.2, and 2.0 years, respectively. The daily coordinates of GEONET (F5 solution) were used for the analysis; the GNSS time series data include common mode errors, steps due to antenna exchange, coseismic steps, and annual and semi-annual variations. In this study, the displacements were calculated, by removing the common mode error and the steps due to antenna exchange from the displacement data before the onset of a SSE, and by removing all of them from the displacement data during the occurrence of a SSE. The analysis period before the occurrence of the SSE was taken from the end of the previous SSE of each SSE to just before the occurrence of the SSE, except for the Tokai L-SSE. As for the Tokai L-SSE, setting up of GNSS stations in the Tokai district started on 21 March 1996, and we were not able to obtain GNSS time series data after the end of the previous L-SSE. Therefore,1 January 1997 was set as the start date of the strain accumulation before the occurrence of the L-SSE. Then, the strain field was calculated from the displacement field, using the method proposed by Shen et al. (1996).
3. Results
In the Tokai L-SSE, the accumulation and release of dilatation were identified on the east side of Lake Hamana. In terms of the amount of accumulation and release, the amount of release was larger than that of accumulation. However, this may be due to the short analysis period before the onset of the L-SSE.
In the Boso-Oki S-SSE and the two Bungo Channel L-SSEs, the regions of large dilatation accumulation almost coincided with those of large dilatation release. The amount of dilatation accumulation was larger than the amount of dilatation release.
These results indicate that for each SSE, a strong inverse correlation was identified in the dilatation field where the spatial distribution of large dilatation accumulation before the occurrence of the SSE coincided with that of large dilatation release during the occurrence of the SSE.
In the deeper extension of the plate interface of the seismogenic zone of the impending megathrust earthquake in the Nankai subduction zone, long-term slow slip events (L-SSEs) such as the 2000-2005 and Tokai L-SSE and the 2009-2011 and 2018-2019 Bungo Channel L-SSEs have occurred repeatedly. Additionally, short-term SSEs (S-SSEs) have occurred in Boso-Oki repeatedly every several years. The purpose of this study is to verify the relationship between the strain accumulation and release processes associated with the occurrence of these SSEs quantitatively, using GNSS time series data.
2. Method
The steady-state interseismic displacement field was obtained, by analyzing GNSS time series data, and the strains (principal strain, dilatation, and maximum shear strain) accumulated from the reference date to just before the occurrence of a SSE were calculated. Similarly, the displacement and strain fields associated with the same SSE were obtained from the displacements of onset and end dates of the occurrence of the SSE. The spatial distributions and amounts of the strains accumulated prior to the onset of the SSEs and the strain changes associated with the SSEs were then compared, and the relationship between them was investigated.
In this study, the Tokai L-SSE (July 1, 2000 to June 30, 2005), the Boso-Oki SSE (August 1, 2007 to August 29, 2007), and the Bungo Channel L-SSEs (January 1, 2010 to March 14, 2011 and January 1, 2018 to December 31, 2019) were investigated. The occurrence periods for these SSEs were 5.0, 0.079, 1.2, and 2.0 years, respectively. The daily coordinates of GEONET (F5 solution) were used for the analysis; the GNSS time series data include common mode errors, steps due to antenna exchange, coseismic steps, and annual and semi-annual variations. In this study, the displacements were calculated, by removing the common mode error and the steps due to antenna exchange from the displacement data before the onset of a SSE, and by removing all of them from the displacement data during the occurrence of a SSE. The analysis period before the occurrence of the SSE was taken from the end of the previous SSE of each SSE to just before the occurrence of the SSE, except for the Tokai L-SSE. As for the Tokai L-SSE, setting up of GNSS stations in the Tokai district started on 21 March 1996, and we were not able to obtain GNSS time series data after the end of the previous L-SSE. Therefore,1 January 1997 was set as the start date of the strain accumulation before the occurrence of the L-SSE. Then, the strain field was calculated from the displacement field, using the method proposed by Shen et al. (1996).
3. Results
In the Tokai L-SSE, the accumulation and release of dilatation were identified on the east side of Lake Hamana. In terms of the amount of accumulation and release, the amount of release was larger than that of accumulation. However, this may be due to the short analysis period before the onset of the L-SSE.
In the Boso-Oki S-SSE and the two Bungo Channel L-SSEs, the regions of large dilatation accumulation almost coincided with those of large dilatation release. The amount of dilatation accumulation was larger than the amount of dilatation release.
These results indicate that for each SSE, a strong inverse correlation was identified in the dilatation field where the spatial distribution of large dilatation accumulation before the occurrence of the SSE coincided with that of large dilatation release during the occurrence of the SSE.