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 M W 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 M W 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 M W 5.7 event and the initial rupture of the 2024 M W 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.