[S22] AM-2

[S22-06] 2024年能登半島地震（Mw 7.5）に伴う地震時地震波速度変化

Paris Nicolas¹、*伊東 優治²、Brenguier Florent³、Wang Qing-Yu³、Higueret Quentin³、盛 一笑⁴、髙木 涼太⁵、内田 直希⁵、岡田 知己⁵、平原 聡⁵、木村 洲徳⁵、酒井 慎一² (1. グルノーブルアルプ大学環境地球科学研究所、2. 東京大学地震研究所、3. グルノーブルアルプ大学地球科学研究所、4. 中国科学技術大学地球和空間科学学院、5. 東北大学大学院理学研究科地震・噴火予知研究観測センター)

On January 1st, 2024, a Mw 7.5 earthquake struck the northeastern tip of the Noto Peninsula in central Japan, which has experienced a prolonged seismic swarm since late 2020. Previous studies suggest that this swarm and its temporal migration are driven by fluid diffusion from the lower crust, possibly originating from the mantle. This fluid intrusion decreases effective pressure, facilitating fault reactivation and seismic activity. Earthquakes can alter crustal material properties through coseismic dynamic damage, static stress changes, and fluid migration. Since seismic wave velocities are sensitive to these crustal properties, monitoring their temporal changes provides insights into the damage levels and recovery processes during seismic cycles. The sensitivity of seismic velocities to shaking can also reveal areas of high crustal fluid pressure, as a decrease in effective pressure increases seismic susceptibility. Thus, measurements of seismic velocity changes associated with the 2024 Noto mainshock may provide important insights into crustal properties and fluid pressure distribution in the swarm area. In this study, we use ambient noise seismic interferometry to analyze seismic velocity perturbations induced by the Mw 7.5 Noto Peninsula earthquake. We analyze data from 42 permanent Hi-net seismic stations and 4 temporary stations operated by Tohoku University and the University of Tokyo in the regions that experienced moderate to strong seismic shaking (Sakai et al., https://doi.org/10.5281/zenodo.6767362, 2022; Okada et al., https://doi.org/10.1186/s40623-024-01974-0, 2024a; Okada et al., https://doi.org/10.5281/zenodo.10939231, 2024b). Our investigation focuses on the frequency band between 0.3 and 1.1 Hz. In this frequency band, we identify a stable seismic noise source likely linked to wind-driven swell activity near the Sea of Japan coast. We construct daily autocorrelations and cross-correlations of nearby stations and measure travel-time shifts (dt) of early coda waves as a function of time-lag (t) between reference and altered correlations to derive the coseismic relative seismic velocity changes. Our results indicate an average reduction of approximately 0.4% in surface wave velocities across the Noto Peninsula, peaking at 0.6%, similar to observations following the 2008 Mw 7.2 Iwate-Miyagi Nairiku earthquake. Notably, the location of the maximum velocity drop varies with frequency: lower frequencies peak in the northeastern tip, while higher frequencies peak in the western corner of the peninsula. We discuss the origins of these velocity drops by comparing them with Peak Ground Velocity, Peak Ground Acceleration, and coseismic static stress changes. Within the peninsula, the velocity reduction correlates strongly with both seismic shaking and static stress changes. This suggests that both static and dynamic stress changes influence the coseismic velocity drop, complicating the quantitative assessment of their respective contributions. Conversely, no significant anomalies in the coseismic velocity perturbations were observed in the northeastern area associated with the seismic swarm. This implies a lack of pressurized fluids at shallow depths (~2 km), where our measurements are most sensitive. Outside the peninsula, velocity perturbations appear primarily driven by dynamic stress changes, as static stress changes in these areas are negligible.