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

The Noto Peninsula, extending northward into the Sea of Japan, features a narrow, elongated shape, complex coastal topography, and numerous active faults along its coastline. Since December 2020, intense earthquake swarms accompanied by crustal deformation have occurred in the northeastern peninsula, likely caused by fluid upwelling from deep underground. The largest event, a Magnitude 7.6 earthquake, struck on January 1, 2024, with aftershock distributions indicating multiple faults ruptured over approximately 150 km. This study aimed to clarify the temporal and spatial variation in seismic wave radiation and investigate the source process of the M7.6 event using the back-projection method. This method estimates the origin of wave packets recorded by a seismic array. In Japan, seismic networks operated by local governments often include densely distributed stations to evaluate seismic intensity. We used these dense sites as a seismic array complemented by strong ground motion data from NIED K-NET and KiK-net. The analysis assumed three fault planes, based on previous studies. Velocity waveforms in two frequency bands (0.05–2.0 Hz and 0.5–5.0 Hz) were used to estimate areas of strong radiation intensity, representing the sources of seismic waves. In the low-frequency band, strong radiation intensity was observed near the rupture initiation point and in shallow regions of the northern Noto Peninsula, corresponding to large fault slips that caused the uplift of the coastline. In contrast, no strong radiation intensity was detected off the northeast coast of the Noto Peninsula in the low-frequency band, suggesting the absence of a significant slip. High-frequency analysis revealed distributions of strong radiation intensities complementary to those in the low-frequency band. A subevent occurring around 20 s after the rupture initiation was found to originate near the northern coast of the Noto Peninsula.

Graphical Abstract