Diverse Rupture Behaviors of M5 Earthquakes Reveal Heterogeneous Fluid Effects in Noto Peninsula, Central Japan

Abstract

The 15-s-long weak initial rupture of the 2024 MW 7.5 Noto earthquake overlapped with a fluid-rich region of a preceding earthquake swarm and was accompanied by enhanced high-frequency seismic radiation. To understand the radiation and related source processes, we investigate rupture behaviors of four nearby M5+ events. We find that the 5 May 2023 MW 5.7 event exhibits similar characteristic radiation, resulting in a relatively low source spectral decay rate. Apparent moment-rate functions and dynamic rupture simulations, constrained from near-source waveform data, consistently suggest a northeastward rupture with multiple asperities. Such rupture heterogeneities under a fluid-rich condition can explain the weak, long seismic radiation but with enhanced high-frequency signals in the MW 5.7 event and the initial rupture of the 2024 MW 7.5 Noto earthquake. The multi-asperity model also holds implications for other observations, including the depth dependence of high-frequency radiation and the low spectral falloff rates observed for low-frequency earthquakes.