5:15 PM - 6:30 PM
[SCG50-P04] GNSS observation around the shear zone in southern Kyushu, southwest Japan (5)
Keywords:shear zone in southern Kyushu, left-lateral strike slip fault, dense GNSS observation, the 2016 Kumamoto earthquake, post-seismic crustal deformation
GNSS observation (Ref.1; Ref.2) illustrates an active left-lateral shear zone with high shear strain rate (>10-7/yr) in southern Kyushu (hereafter SKSZ), where M5-6 earthquakes have occurred frequently (Ref.3). However, the surface topography does not have the traces of obvious active faults. In order to explore the crustal deformation in more detail, we established 10 GNSS sites across the SKSZ and started observation from March 2016. From the observation up to November 17, 2019, the slip rate and locking depth were estimated to be 13 mm/yr and 22 km (Ref.4) using a vertical strike-slip fault model (Ref.5). On the other hand, block fault modeling using GNSS data (e.g., Ref.6) shows the slip rate of 7-8 mm/yr at the SKSZ and our estimated locking depth seems to be slightly deeper than the average thickness of the earth’s crust. To solve the discrepancies, we added another year of GNSS data and examined other deformation factors such as after slip of the 2016 Kumamoto earthquake (hereafter KEQ2016) and re-estimated the slip rate and locking depth of the SKSZ.
Daily site coordinates at our sites from April 19, 2016 to November 30, 2020 were estimated using Bernese GNSS Software (ver.5.2). Subsequently, we corrected those by removing the following factors: 1) common mode bias, 2) viscoelastic deformation due to the KEQ2016, 3) volcanic deformation at Kirishima and Sakurajima volcanoes, 4) slow slip event in the Bungo Channel, 5) coseismic displacement of a M6.3 earthquake in the Hyuganada on May 10, 2019, 6) subduction of the Philippine Sea plate, 7) after slip of the KEQ2016, and 8) annual and semi-annual variations in coordinates time series. After the processes, site velocities relative to the Amur plate were estimated. Additionally, we also applied the Ref.5 fault model to the site velocities of a target area. The area was assumed to be a width of 25 km in the EW direction and a length of 90 km in the NS direction from the tentative center of the SKSZ (E130.5°, N32.0°), and its strike was assumed to be N104.8°E (Ref.4). However, we obtain only the solutions that the locking depth exceeded average crustal thickness in the case of single strike-slip zone. Therefore, we introduced the case of double strike-slip zones. As a result, we found that the first and second deformations occur at 13.83±6.75 km north and 40.98±7.23 km south from the tentative center of the SKSZ with left-lateral slip rates of 10.06±4.13 mm/yr and 6.43±4.24 mm/yr and locking depths of 8.13±11.26 km and 5.23±13.36 km.
For the first deformation, our slip rate is consistent with that derived from block fault modeling and our locking depth is close to the hypocentral depths of the 1997 Northwestern Kagoshima earthquakes. For the second deformation, it is not known for the geodetic observation and our dense GNSS observation detects it for the first time. From the seismological study, the hypocenters with left-lateral strike-slip mechanisms lead from Kagoshima city to Koshikijima (Ref.7), and its strike and slip sense agree with our observation. This suggests that aseismic slip at a deep part of the earth’s crust contributes to the strain accumulation and at least two deformation centers with left-lateral strike-slip are in the SKSZ.
This study was carried out by a contract with the Agency of Natural Resources and Energy, part of the Ministry of Economy, Trade and Industry of Japan as part of its R&D program supporting development of technology for geological disposal of high-level radioactive waste.
Reference
1) Wallace et al. (2009): Geology, 37(2), 143-146, 2009.
2) Nishimura et al. (2018): Geosphere, 14(2), 535-551, 2018.
3) Takayama and Yoshida (2007): J. Geophys. Res., 112, B06413, 2007.
4) Watanabe et al. (2020): JpGU-AGU Joint Meeting 2020, SSS14-P23, 2020.
5) Savage and Burford (1973): J. Geophys. Res., 78, 832-845, 1973.
6) Kimura et al. (2019): J. Geophys. Res., 124, 6140-6164, 2019.
7) Kakuta and Goto (2002): J. Seismol. Soc. Japan, 55, 317-336, 2002.
Daily site coordinates at our sites from April 19, 2016 to November 30, 2020 were estimated using Bernese GNSS Software (ver.5.2). Subsequently, we corrected those by removing the following factors: 1) common mode bias, 2) viscoelastic deformation due to the KEQ2016, 3) volcanic deformation at Kirishima and Sakurajima volcanoes, 4) slow slip event in the Bungo Channel, 5) coseismic displacement of a M6.3 earthquake in the Hyuganada on May 10, 2019, 6) subduction of the Philippine Sea plate, 7) after slip of the KEQ2016, and 8) annual and semi-annual variations in coordinates time series. After the processes, site velocities relative to the Amur plate were estimated. Additionally, we also applied the Ref.5 fault model to the site velocities of a target area. The area was assumed to be a width of 25 km in the EW direction and a length of 90 km in the NS direction from the tentative center of the SKSZ (E130.5°, N32.0°), and its strike was assumed to be N104.8°E (Ref.4). However, we obtain only the solutions that the locking depth exceeded average crustal thickness in the case of single strike-slip zone. Therefore, we introduced the case of double strike-slip zones. As a result, we found that the first and second deformations occur at 13.83±6.75 km north and 40.98±7.23 km south from the tentative center of the SKSZ with left-lateral slip rates of 10.06±4.13 mm/yr and 6.43±4.24 mm/yr and locking depths of 8.13±11.26 km and 5.23±13.36 km.
For the first deformation, our slip rate is consistent with that derived from block fault modeling and our locking depth is close to the hypocentral depths of the 1997 Northwestern Kagoshima earthquakes. For the second deformation, it is not known for the geodetic observation and our dense GNSS observation detects it for the first time. From the seismological study, the hypocenters with left-lateral strike-slip mechanisms lead from Kagoshima city to Koshikijima (Ref.7), and its strike and slip sense agree with our observation. This suggests that aseismic slip at a deep part of the earth’s crust contributes to the strain accumulation and at least two deformation centers with left-lateral strike-slip are in the SKSZ.
This study was carried out by a contract with the Agency of Natural Resources and Energy, part of the Ministry of Economy, Trade and Industry of Japan as part of its R&D program supporting development of technology for geological disposal of high-level radioactive waste.
Reference
1) Wallace et al. (2009): Geology, 37(2), 143-146, 2009.
2) Nishimura et al. (2018): Geosphere, 14(2), 535-551, 2018.
3) Takayama and Yoshida (2007): J. Geophys. Res., 112, B06413, 2007.
4) Watanabe et al. (2020): JpGU-AGU Joint Meeting 2020, SSS14-P23, 2020.
5) Savage and Burford (1973): J. Geophys. Res., 78, 832-845, 1973.
6) Kimura et al. (2019): J. Geophys. Res., 124, 6140-6164, 2019.
7) Kakuta and Goto (2002): J. Seismol. Soc. Japan, 55, 317-336, 2002.