Updip Fluid Flow in the Crust of the Northeastern Noto Peninsula, Japan, Triggered the 2023 Mw 6.2 Suzu Earthquake During Swarm Activity

• Published in: Geophysical research letters 2023-11, Vol.50 (21), p.n/a
• Main Authors: Yoshida, Keisuke, Uchida, Naoki, Matsumoto, Yoshiaki, Orimo, Masaki, Okada, Tomomi, Hirahara, Satoshi, Kimura, Shuutoku, Hino, Ryota
• Format: Article
• Language: English
• Subjects: Aftershocks; aseismic slip; earthquake migration; Earthquakes; Fault lines; fluid; Fluid flow; large earthquake; Rupture; Seismic activity; subduction zone; swarm; Velocity
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL106023

An Mw 6.2 earthquake occurred in Suzu, northeastern Noto Peninsula, Japan, on 5 May 2023, followed by many aftershocks. Before this mainshock‐aftershock sequence, an intense earthquake swarm lasted in the vicinity for 2.5 years. Here, we estimated the rupture process of the Mw 6.2 mainshock and relocated >20,000 surrounding small earthquakes. The results show that systematic upward migration occurred via a complex network of faults in the preceding swarm period and that the mainshock rupture was initiated near the shallow end of the swarm earthquakes. The mainshock rupture propagated farther updip, followed by many aftershocks in the shallow extension. Upward fluid movement likely caused systematic upward earthquake migration from a depth of 18–5 km. The present results indicate the importance of monitoring swarm events since large (M > 6) and dangerous earthquakes can occur during such swarms. Plain Language Summary Growing evidence suggests that fluid movements at depth can cause earthquake swarms in the overriding plates of subduction zones. However, there is less evidence of fluid involvement in the occurrence of large (M > 6) earthquakes, and it has been reported that the generation environments of large earthquakes and swarm earthquakes tend to be different. During intense swarm activity in the northeastern Noto Peninsula, Japan, starting at the end of 2020, an M6.2 earthquake occurred on 5 May 2023, followed by many aftershocks. To clarify the occurrence mechanism of the M6.2 mainshock, we relocated >20,000 small earthquakes and estimated the rupture process. The results showed a systematic upward migration of earthquakes in the preceding swarm period on the deep side of the mainshock fault (depth from 20 to 12 km). The mainshock rupture initiated near the shallow end of the swarm activity and propagated updip (depth from 12 to 9 km). Many aftershocks followed in shallower part of the same fault (depth from 10 to 5 km). Upward fluid migration likely caused the observed systematic upward earthquake migration throughout the sequence. The results indicate that earthquakes as large as M > 6 can be triggered by fluid migration during enhanced seismicity in swarms. Key Points An Mw 6.2 event initiated near the shallow end of a fault where the preceding earthquake swarm showed systematic upward migration The Mw 6.2 rupture propagated farther updip, followed by many shallow aftershocks on the fault Upward fluid migration can trigger earthquakes as large as