- 1Institut des Géosciences de l'Environnement, Université Grenoble Alpes, Saint-Martin-d'Hères, France (nicolas.paris3@univ-grenoble-alpes.fr)
- 2Earthquake Research Institute, The University of Tokyo, Tokyo, Japan
- 3Institut des Sciences de la Terre, Université Grenoble Alpes, Saint-Martin-d'Hères, France
- 4Institut Terre & Environnement de Strasbourg, Université de Strasbourg, Strasbourg, France
- 5Laboratory of Seismology and Physics of the Earth's Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, China
- 6Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai, Japan
The 2024 MW 7.5 Noto earthquake in Japan was preceded by an intense seismic swarm, likely driven by upward fluid migration. Crustal seismic velocities are influenced by external perturbations from earthquakes, with the presence of pressurized fluids in the crust amplifying these induced velocity changes. Consequently, we characterize subsurface fluids by examining the coseismic velocity changes associated with the 2024 mainshock. We combined data from permanent Hi-net and temporary seismic stations. The temporary stations were deployed by Tohoku University and Earthquake Research Institute, the University of Tokyo (1, 2, 3).
Our analysis reveals significant coseismic velocity drops, averaging ~0.5% in the near field (i.e., the Noto Peninsula) and reaching 0.6–0.8% near the coseismic slip peaks. These observed velocity drops correlate strongly with modeled velocity drops by coseismic static stress changes. Peak Ground Velocity (PGV) and Peak Ground Acceleration (PGA), which are proxies for dynamic stress changes, are also strongly correlated with the observed velocity drops. However, disentangling the contributions of static and dynamic stress changes to the observed velocity drops remains challenging due to their similar spatial patterns. In the far field (i.e., outside the Noto Peninsula), the coseismic velocity drops are on average ~0.1%, predominantly attributed to dynamic stress changes, as static stress changes are negligible at greater distances.
While the addition of temporary stations significantly enhances resolution in the pre-mainshock swarm zone, no significant coseismic velocity drop anomalies were detected in the shallow crust down to ~2.5 km depths. This suggests that the volume of pressurized fluids in the shallow crust is not anomalously large, implying that the fluid migration preceding the mainshock is likely confined to greater depths.
1: Sakai et al., https://doi.org/10.5281/zenodo.6767362, 2022
2: Okada et al., https://doi.org/10.1186/s40623-024-01974-0, 2024a
3: Okada et al., https://doi.org/10.5281/zenodo.10939231, 2024b
How to cite: Paris, N., Itoh, Y., Brenguier, F., Wang, Q.-Y., Sheng, Y., Okada, T., Uchida, N., Higueret, Q., Takagi, R., Sakai, S., Hirahara, S., and Kimura, S.: Coseismic crustal seismic velocity changes associated with the 2024 MW 7.5 Noto earthquake, Japan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11265, https://doi.org/10.5194/egusphere-egu25-11265, 2025.
