Spatial variation of stress drop in the offshore source area of the 2024 Noto Peninsula earthquake

The 2024 Noto Peninsula earthquake (M7.6), which occurred in central Japan on January 1, is thought to have ruptured multiple fault segments with different strike and dip angles. Seismicity along these segments remained active for over one year after the mainshock. To understand the fault zone in terms of fluid and frictional properties, this study estimates the stress drop of small- to moderate-sized earthquakes. We used seismic waveform data from onshore and offshore stations for approximately one month, beginning in mid-January 2024. The estimated stress drop was nearly scale-independent and gradually decreased from southwest to northeast across the offshore source area. The possible causes of this lateral variation were fault orientation, seismic velocity, pore pressure, low initial stress and frictional properties. Among these, fault orientation, seismic velocity and low initial stress were considered unlikely, indicating that frictional properties, potentially influenced by pore pressure, were the likely cause. This study showed that the lateral variation in stress drop is not due to differences in focal depth. Within the 4–10 km depth range, earthquakes on the northeastern segments exhibited significantly smaller stress drops than those on the southern segments. We also found a statistically significant increase in stress drop with increasing depth between 2 and 10 km, whereas no clear depth dependence was observed at depths greater than 10 km. The results of this study may contribute to investigations of frictional properties and fluid effects in the offshore source area.