Characteristics of the source process of the 2024 M7.6 Noto Peninsula earthquake revealed from array back-projection analysis in both low- and high-frequency bands

In the vicinity of Suzu City, the northeastern-most city of the Noto Peninsula, earthquake swarms accompanied by crustal deformation attributed to upwelling fluid from deep underground have been active since December 2020. The largest earthquake of this activity, with a magnitude of 7.6, occurred on January 1, 2024. The aftershock distributions indicated that multiple faults ruptured over a range of 150 km. Here, we aimed to reveal temporal and spatial variation of seismic wave radiation and discuss on source process of the M7.6 event. The source process of this event was estimated using the array back-projection method. This method facilitates the estimation of the source of wave packets in the waveform recorded by an array of observation sites. In addition to local governments’ densely distributed seismic intensity observation sites, we used strong ground motion data recorded by the NIED K-NET and KiK-net. The analysis was performed by assuming three fault planes, as suggested in previous studies. Velocity waveforms in two frequency bands (0.05–2.0 Hz and 0.5–5.0 Hz) were used in the analysis, and we estimated the areas of strong radiation intensity in each frequency band, representing the source of seismic waves. In the analysis of the low-frequency band, we estimated strong radiation intensity areas around the rupture initiation point and a shallow part of the northern Noto Peninsula, corresponding to areas of large fault slips that caused the uplift of the coastline. Conversely, there was no strong radiation from the fault setup off the northeast coast of the Noto Peninsula in the low-frequency band, suggesting the absence of significant slip. Analysis of the high-frequency band revealed distributions of strong radiation intensities complementary to those in the low-frequency band. The source of the subevent, which occurred approximately 20 s after the rupture initiation, was determined to be near the northern coast of the Noto Peninsula.