Fine-scale fault zone structure controlled the nucleation of the 2024 Noto Peninsula earthquake

Nicolas DeSalvio, Wenyuan Fan, Yihe Huang, Matthias Morzfeld, & Ryo Okuwaki

In Preparation July 8, 2026, SCEC Contribution #15081

On January 1st, 2024 a Mw7.5 earthquake struck Japan's Noto Peninsula following a 3-year foreshock sequence with fluid-driven seismic swarms. The location of nucleation within such fluid-rich fault zones remains poorly understood. Here we jointly analyze compact seismicity bursts and high-resolution in-situ Vp/Vs ratios across the sequence. We find that the fault zone is a compartmentalized system of fluid-rich and fluid-poor domains, rather than a single weakened volume. Seismicity bursts concentrate in the drier domains and are consistent with transient aseismic slip, distinct from the fluid-driven seismicity in wetter domains. The mainshock nucleated at a structurally distinct, dry patch rather than where swarm activity concentrated. This indicates that the nucleation site is controlled by pre-existing fault structure and decoupled from the preparation. The decoupling cautions against inferring the nucleation site from precursory seismicity and highlights the importance of distinguishing fluid-saturated and dry fault rock across fluid-rich settings.

Citation
DeSalvio, N., Fan, W., Huang, Y., Morzfeld, M., & Okuwaki, R. (2026). Fine-scale fault zone structure controlled the nucleation of the 2024 Noto Peninsula earthquake. TBD, (in preparation).