[U-15] The 2024 Noto Peninsula Earthquake (1:J)

[U15-P13] Relationship between the preceding earthquake swarm and the 2024 Mw7.5 Noto Peninsula earthquake inferred from a detailed three-dimensional S-wave velocity structure

*Ryota Takagi¹, Keisuke Yoshida¹, Tomomi Okada¹ (1.Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University)

Keywords:Noto Peninsula earthquake, Earthquake swarm, Seismic velocity structure, Dense seismic observation

An intense earthquake swarm lasting ~3 years in the Noto Peninsula ultimately led to the 2024 Mw 7.5 Noto Peninsula earthquake. The spatiotemporal distribution of swarm earthquakes and associated crustal deformation suggest that the earthquake swarm is driven by crustal fluid. However, their connection to the generation of the Mw7.5 earthquake remains unknown, which is important for accessing the temporal evolution of earthquake swarms occurring in various regions. Here, we investigate a detailed S-wave velocity structure based on dense seismic observation conducted in October–November 2023 to understand the relationship between the earthquake swarm and the 2024 Mw7.5 Noto Peninsula earthquake. We used 32-day continuous records from 12 seismic nodes (SmartSolo IGU-BD3C-5) and 10 permanent short-period stations. The natural frequency of the seismic node is 0.2 Hz, and the instrument noise is comparable to the Hi-net short-period sensor. We applied ambient noise surface wave tomography to the continuous data and estimated a three-dimensional S-wave velocity structure down to ~25 km depth. The results show a prominent high-velocity body beneath the Noto Peninsula. The high-velocity body is located beneath the ring-shaped swarm cluster implying magmatic structures, and continues to surface igneous rocks at the west of the swarm area. Since volcanic activity was active during the expansion of the Sea of Japan 15 to 30 million years ago, this high-velocity body may represent solidified magma. Whereas the preceding swarm earthquakes occurred while avoiding the high-velocity body, the initial significant rupture of the Mw 7.5 earthquake occurred within it. The spatial correlation between the high-velocity body and the earthquakes and rupture suggests that the solidified ancient magma initially acted as an impermeable barrier for the swarm earthquakes and eventually acted as the seismic asperity of the Mw 7.5 earthquake. This study highlights that permeability heterogeneity and distribution of seismic asperity play significant roles in swarm evolutions and the generation of subsequent large earthquakes, and ancient magma can produce such subsurface heterogeneity.