[S22] AM-2

[S22-01] A long-persisting seismic swarm and the subsequent nucleation of the 2024 M7.6 Noto earthquake

*Aitaro KATO¹, Shigeki NAKAGAWA¹, Eiji KURASHIMO¹, Shinichi SAKAI¹, Kentaro EMOTO², Satoshi MATSUMOTO² (1. Earthquake Research Institute, the University of Tokyo, 2. Institute of Seismology and Volcanology, Kyushu University)

An intense earthquake swarm has persisted for over three years beneath the northeastern tip of the Noto Peninsula, central Japan, since November 2020. On January 1st, 2024, an M7.6 earthquake rupture nucleated within the swarm area and propagated bilaterally toward ENE and WSW directions along multiple faults over 100 km. Globally, it is rare that a long-lasting seismic swarm preceded such a large event. To explore how seismicity evolved in space and time domains, we analyzed the continuous seismic waveforms from 11 months prior to the M7.6 rupture, recorded by a dense seismic network deployed above the swarm area, by applying a machine-learning phase picker and template-matching technique. Immediately after the 2023 M6.5 event, seismic activity spiked but gradually decreased over time, followed by a sharp decline three months later. But the seismicity remained at high rate in areas where the M7.6 rupture nucleated, in contrast to the gradual decay in the surrounding areas. Note that a highly localized swarm started about 1.5 hours before the M7.6 rupture at the deeper part of the southeast-dipping fault that hosted the 2023 M6.5 event. After the M5.5 event 4 minutes before the mainshock rupture, the foreshock area expanded along strike and dip directions. The swarm-like foreshock sequence indicates the involvement of fluid-driven slow slip transient and facilitates stress loading on the mainshock nucleation point. Interestingly, the foreshock sequence took place by the reactivation of a weak fault structure that had been ruptured about eight months earlier. The combination of the swarm seismicity and the subsequence expansion of the foreshock area was similar to one observed before the 2011 M9 Tohoku-Oki earthquake, although the spatial and temporal scales are different. These results highlight the importance of earthquake swarms along weak faults as a triggering of intense foreshock activity.