5:15 PM - 6:45 PM
[SSS08-P04] Recent seismic activity near the Kuril and Japan Trenches using the S-net (Seafloor observation network for earthquakes and tsunamis along the Japan Trench) data
Keywords:S-net, Japan Trench, Kuril Trench, hypocenter distribution
The Seafloor observation network for earthquakes and tsunamis along the Japan Trench (S-net) has been established in the Japan Trench and Kuril Islands Trench area. To investigate recent offshore seismic activity using S-net data, we obtained the hypocenter distribution of several seismically active areas that have occurred in the eight years since April 1, 2016. NonLinLoc (Lomax et al., 2000, 2014) was used as the hypocenter location code, and a correction based on sedimentary layer thicknesses obtained from multichannel seismic reflection profiles was applied to each S-net station (Nishizawa et al., 2022). The velocity structure model is a simple three-dimensional structure that takes into account the subducting Pacific plate, referring to the results of seismic explorations conducted in each region.
Due to the limitation of submarine cable installation, the number of S-net observation stations from Tokachi-oki to Nemuro-oki is not sufficient, especially on the continental shelf and landward slope where seismic activity is high. The velocity structure model used for the hypocenter location is not easy to set because the shallow structure changes significantly especially around the Kushiro submarine canyon. Therefore, it is difficult to determine the hypocenters precisely. Although the obtained hypocenter distribution depends on the setting of the velocity structure model, the results show low seismic activities in the slip area of the 2003 Tokachi-oki Earthquake (Mj 8.0) and near the trench axis, and an image of seismic activity along the subducting plate boundary in the Tokachi-oki area.
Seismic activity near the trench axis of the Sanriku-oki region is not uniform. In the aftershock area of the 1994 Sanriku-Oki earthquake (Mj 7.6) at 40.5°N, seismic activity was detected in the shallow area at the plate boundary near the trench axis. On the other hand, no seismic activity is detected in the area between the trench axis and a plate boundary depth of 10 km near 39.5°N, which is the aftershock area of the 2011 off Iwate Prefecture earthquake (Mj 7.4). An Mj 6.0 (Mw 5.8: F-net MT) earthquake occurred at the plate boundary on Aug. 25, 2023, about 60 km landward from the trench axis near 39.5°N, and somewhat active seismicity was observed in the vicinity of the source region. The hypocenter relocation shows that the seismic activity that started on Aug. 25 extended along the plate boundary about 40 km and the hypocenter moved eastward with time but did not reach the trench axis, and subsided in about 10 days.
A seismically active zone with a length of about 80 km exists off the east coast of Chiba Prefecture from Choshi to the southeast toward the triple junction of the trench. In this area, the tip of the subduction of the Philippine Sea (PHS) plate is located above the subducting Pacific plate, and complex seismic activity has been observed. Slow slip events have been repeatedly observed in the west and east of the seismically active zones, and active seismic activity, including six earthquakes of Mw 4 or greater (maximum Mj 5.4), has been observed since Dec. 21, 2023. The depths of the hypocenters obtained varied widely but corresponded to the estimated location of the upper surface of the PHS plate in this region. Until Jan. 10, 2024, 223 events with MHi-net 2.0 or greater occurred intermittently, concentrated within an area of about 20 km, although they appeared to move slightly to the east.
Due to the limitation of submarine cable installation, the number of S-net observation stations from Tokachi-oki to Nemuro-oki is not sufficient, especially on the continental shelf and landward slope where seismic activity is high. The velocity structure model used for the hypocenter location is not easy to set because the shallow structure changes significantly especially around the Kushiro submarine canyon. Therefore, it is difficult to determine the hypocenters precisely. Although the obtained hypocenter distribution depends on the setting of the velocity structure model, the results show low seismic activities in the slip area of the 2003 Tokachi-oki Earthquake (Mj 8.0) and near the trench axis, and an image of seismic activity along the subducting plate boundary in the Tokachi-oki area.
Seismic activity near the trench axis of the Sanriku-oki region is not uniform. In the aftershock area of the 1994 Sanriku-Oki earthquake (Mj 7.6) at 40.5°N, seismic activity was detected in the shallow area at the plate boundary near the trench axis. On the other hand, no seismic activity is detected in the area between the trench axis and a plate boundary depth of 10 km near 39.5°N, which is the aftershock area of the 2011 off Iwate Prefecture earthquake (Mj 7.4). An Mj 6.0 (Mw 5.8: F-net MT) earthquake occurred at the plate boundary on Aug. 25, 2023, about 60 km landward from the trench axis near 39.5°N, and somewhat active seismicity was observed in the vicinity of the source region. The hypocenter relocation shows that the seismic activity that started on Aug. 25 extended along the plate boundary about 40 km and the hypocenter moved eastward with time but did not reach the trench axis, and subsided in about 10 days.
A seismically active zone with a length of about 80 km exists off the east coast of Chiba Prefecture from Choshi to the southeast toward the triple junction of the trench. In this area, the tip of the subduction of the Philippine Sea (PHS) plate is located above the subducting Pacific plate, and complex seismic activity has been observed. Slow slip events have been repeatedly observed in the west and east of the seismically active zones, and active seismic activity, including six earthquakes of Mw 4 or greater (maximum Mj 5.4), has been observed since Dec. 21, 2023. The depths of the hypocenters obtained varied widely but corresponded to the estimated location of the upper surface of the PHS plate in this region. Until Jan. 10, 2024, 223 events with MHi-net 2.0 or greater occurred intermittently, concentrated within an area of about 20 km, although they appeared to move slightly to the east.