17:15 〜 19:15
[SCG52-P01] Monitoring of plate coupling along the Nankai Trough using seafloor observation networks: A case of the 2024 Hyuga-nada Earthquake
キーワード：長期孔内観測システム、DONET、南海トラフ臨時情報
The Nankai Trough, which is thought to be a region where a large subduction earthquake will occur in the near future, is thought to be made up of four segments: the Tokai, Tonankai, Nankai and Hyuga-nada segments. According to records of historical earthquakes, in some cases, such as the 1707 Hōei earthquake, all segments are destroyed at once, while in others, such as the 1854 Ansei earthquake, the Tonankai segment is destroyed first, followed by the Nankai segment about 30 hours later, or, as in the case of the 1944 Showa Tonankai earthquake and the 1946 Showa Nankai earthquake, there is a time difference of about two years. For this reason, real-time monitoring of the stress field at the segment boundary is expected to provide very important information for objectively evaluating the possibility of a multiple event with time delay triggered by partial rupturing.
Recently, the Nankai Trough Earthquake Extra Information was issued on the occasion of the Hyuga-nada Earthquake on August 8, 2024 (Mw 7.1, thrust-type at low angle along dip direction) which satisfies interplate earthquake and JMA magnitude greater than 7 as partial rupturing. From January in 2024, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has operated Long-Term Borehole Monitoring System (LTBMS) beneath the seafloor off Kii channels. This is a new version from the LTBMS off Kumano-nada installed above the source region of the 1944 Tonankai earthquake. These LTBMS are connected to the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET), which enables to monitor the seafloor crustal deformation in real time. Just after the occurrence of the Hyuga-nada Earthquake, JAMSTEC investigated the possibility of the significant seafloor crustal deformation and seismic activity by monitoring the data of pore pressure in LTBMS and broadband seismogram in DONET.
As a result, no significant volumetric strain changes attributable to seafloor crustal deformation were observed in the Kii channel, which shows that the Hyuga-nada Earthquake did not have a significant effect on the Nankai segment. On the other hand, in the Kumano-nada, we confirmed that contraction began on the downdip region from around the end of July, and that it turned to dilation from around August 5, ending around the middle of the month. Although the change in pore pressure was caused by shallow SSE in the Kumano-nada, it occurred before the Hyuga-nada Earthquake, and the rate of change did not change before or after the Hyuga-nada Earthquake. These results suggest that there was no direct relationship between the SSE and the Hyuga-nada Earthquake.
In conclusion, we believe that a proper understanding of the process of inter-segment tectonics and being able to monitor it in real time is essential for grasping the risk of occurrence of megathrust earthquakes in advance, including the Nankai Trough Earthquake Extra Information.
Recently, the Nankai Trough Earthquake Extra Information was issued on the occasion of the Hyuga-nada Earthquake on August 8, 2024 (Mw 7.1, thrust-type at low angle along dip direction) which satisfies interplate earthquake and JMA magnitude greater than 7 as partial rupturing. From January in 2024, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has operated Long-Term Borehole Monitoring System (LTBMS) beneath the seafloor off Kii channels. This is a new version from the LTBMS off Kumano-nada installed above the source region of the 1944 Tonankai earthquake. These LTBMS are connected to the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET), which enables to monitor the seafloor crustal deformation in real time. Just after the occurrence of the Hyuga-nada Earthquake, JAMSTEC investigated the possibility of the significant seafloor crustal deformation and seismic activity by monitoring the data of pore pressure in LTBMS and broadband seismogram in DONET.
As a result, no significant volumetric strain changes attributable to seafloor crustal deformation were observed in the Kii channel, which shows that the Hyuga-nada Earthquake did not have a significant effect on the Nankai segment. On the other hand, in the Kumano-nada, we confirmed that contraction began on the downdip region from around the end of July, and that it turned to dilation from around August 5, ending around the middle of the month. Although the change in pore pressure was caused by shallow SSE in the Kumano-nada, it occurred before the Hyuga-nada Earthquake, and the rate of change did not change before or after the Hyuga-nada Earthquake. These results suggest that there was no direct relationship between the SSE and the Hyuga-nada Earthquake.
In conclusion, we believe that a proper understanding of the process of inter-segment tectonics and being able to monitor it in real time is essential for grasping the risk of occurrence of megathrust earthquakes in advance, including the Nankai Trough Earthquake Extra Information.