17:15 〜 18:45
[U15-P54] 2024年M7.6能登半島地震震源域の3次元構造探査: 流体の地震発生への影響について
キーワード：地震波トモグラフィー、流体、令和6年能登半島地震、スラブ脱水
The 2024 M7.6 Noto-Hanto earthquake caused extensive damage to the local society, with a total of 1,545 deaths and injuries and 1,134,990 residential damages. Its mainshock epicenter is located in the northern Noto Peninsula, where seismic swarm has been active since 2020. Many previous tomographic studies have revealed low-velocity (low-V) anomalies in the lower crust and the uppermost mantle beneath the Noto peninsula, which are interpreted as fluids ascending from the upper mantle. The resistivity structure and high helium isotope ratios (3He/4He) support this interpretation. The intrusion of the fluids into crustal faults can reduce the shear strength of the fault zone and induce the large earthquakes. Thus, investigation of the origin and distribution of crustal fluids by determining high-resolution velocity tomography is important to clarify the mechanism of earthquake generation. Here we perform seismic tomography to study the detailed 3-D velocity structure of the crust and upper mantle using a large number of arrival-time data of aftershocks of the 2024 Noto-Hanto earthquake.
We use the data recorded at 791 permanent seismic stations installed in central Japan. A total of 10,024 local events are selected from the Japan Metrological Agency (JMA) catalog, which generated 616,771 P-wave and 443,111 S-wave arrival times. Among these events, 106 are aftershocks of the 2024 Noto-Hanto earthquake. The tomographic method of Zhao et al. (1992) is applied to invert P and S wave travel-time residuals for 3-D Vp and Vs models beneath central Japan down to a depth of ~300 km. Our results reveal the Philippine Sea (PHS) slab as a dipping high-velocity zone of ~40 km thick extending down to a depth of 150 km. A significant low-V anomaly extending to the Noto Peninsula is visible in the uppermost mantle above the PHS slab. Low-V anomalies also appear in the upper mantle above the subducting Pacific slab and seem to connect with the low-V anomaly above the PHS slab. In the east-west vertical cross-sections through the Noto Peninsula, a low-V anomaly parallel to the Pacific slab is continuously imaged from ~300 km depth in the back arc region to the crust below active volcanoes. This low-V anomaly appears to diverge at a high-velocity anomaly located at ~50 km depth beneath the Toyama Bay and extend to the Noto Peninsula. In the upper crust beneath the Noto Peninsula, low-V anomalies exist around the source zones of the 2007 Noto-Hanto earthquake (M6.9) and the 2024 Noto-Hanto earthquake. These results suggest that fluids derived from the slab dehydration and hot upwelling in the mantle wedge are supplied to the crust beneath the Noto peninsula and influenced the occurrence of the large crustal earthquakes.
We use the data recorded at 791 permanent seismic stations installed in central Japan. A total of 10,024 local events are selected from the Japan Metrological Agency (JMA) catalog, which generated 616,771 P-wave and 443,111 S-wave arrival times. Among these events, 106 are aftershocks of the 2024 Noto-Hanto earthquake. The tomographic method of Zhao et al. (1992) is applied to invert P and S wave travel-time residuals for 3-D Vp and Vs models beneath central Japan down to a depth of ~300 km. Our results reveal the Philippine Sea (PHS) slab as a dipping high-velocity zone of ~40 km thick extending down to a depth of 150 km. A significant low-V anomaly extending to the Noto Peninsula is visible in the uppermost mantle above the PHS slab. Low-V anomalies also appear in the upper mantle above the subducting Pacific slab and seem to connect with the low-V anomaly above the PHS slab. In the east-west vertical cross-sections through the Noto Peninsula, a low-V anomaly parallel to the Pacific slab is continuously imaged from ~300 km depth in the back arc region to the crust below active volcanoes. This low-V anomaly appears to diverge at a high-velocity anomaly located at ~50 km depth beneath the Toyama Bay and extend to the Noto Peninsula. In the upper crust beneath the Noto Peninsula, low-V anomalies exist around the source zones of the 2007 Noto-Hanto earthquake (M6.9) and the 2024 Noto-Hanto earthquake. These results suggest that fluids derived from the slab dehydration and hot upwelling in the mantle wedge are supplied to the crust beneath the Noto peninsula and influenced the occurrence of the large crustal earthquakes.