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[SSS12-07] Widespread aftershocks of the 2024 Noto Peninsula Earthquake, Japan, and long-term influences on nearby active faults
Keywords:Noto Peninsula Earthquake, active fault, Coulomb stress change, seismicity rate change
In the 2024 Noto Peninsula Earthquake (M7.6), a ~150 km section of known offshore active faults, from offshore northern Noto Peninsula to west of Sado Island (Inoue & Okamura, 2010), ruptured in a linked sequence. The source fault extended beyond the F-43 and F-42 faults (MLIT, 2014) to the southern tip of Amamimisaki, far over the 2007 Noto Peninsula Earthquake (M6.9) source region. Following the mainshock, widespread aftershocks occurred, including an M6.6 event on November 26, 2024, offshore west of Ishikawa Prefecture, with JMA intensity 5- in Wajima and Shika. This activity likely resulted from stress transfer. We here explore seismic responses to coseismic stress changes over the first post-event year. We then assess whether stress loading on major active faults promoted nucleation of large earthquakes and if stress shadow regions, initially undetected, emerged over time.
We calculated the static Coulomb Failure Stress Change (ΔCFF) caused by the 2024 Noto Peninsula Earthquake using the source fault model of Toda (2024) (Fig. 1). While many source models have been already proposed, we adopted a fault geometry extending ENE from Suzu to west of Wajima and curving westward along the coastline to Amamimisaki, as its southwestern shape significantly affects stress changes on nearby active faults. The receiver faults considered include offshore and onshore active faults evaluated by the Headquarters for Earthquake Research Promotion (2024). The ΔCFF distribution is shown in Fig. 1a, with values at the center of ~5 km × ~5 km sub-patches, assuming an apparent friction coefficient μ’ of 0.4. The Bijosan, Amamimisaki, Hakui-oki-nishi, Hakui-oki-higashi, and Ouchigata fault zones exhibit stress increases exceeding 1 bar (0.1 MPa). The M6.6 earthquake on November 26, 2024, occurred on the Hakui-oki-nishi fault, consistent with the increased ΔCFF. The Morimoto-Togashi, Tonami-heiya, and Kurehayama fault zones also exceed the 0.1 bar threshold for potential seismic influence (Stein, 1999), while reverse faults around Toyama Bay generally show negative ΔCFF.
To analyze post-mainshock seismic response, we mapped seismicity rate changes over one year after the mainshock, comparing to the two years before the mainshock (Fig. 1b). Using JMA unified hypocenter catalog (provisional), we calculated rate changes on a 0.05° grid with a 10 km moving cylinder. Seismicity rate changes over noise level were observed within ~100 km of the source fault. Notably, off-fault aftershocks increased near the Hakui-oki fault, leading up to the M6.6 earthquake on November 26. The Ouchigata fault also showed sustained high activity for over a year. The Morimoto-Togashi and Atotsugawa fault zones exhibited moderate increases, while seismicity rate in southeastern Toyama Bay (near M4.7) briefly jumped but returning to pre-mainshock levels now. Although ΔCFF was mostly negative for reverse faults in Toyama Bay, the M4.7 event on July 11, 2024, was a strike-slip event (NIED, 2024), where ΔCFF was positive on both nodal planes (Fig. 1b), suggesting that post-mainshock activity in this area was dominated by strike-slip fault systems. No significant activity change was detected around the Nadachi-Takada-heiya fault zone, located 70–100 km from the source. As in previous studies, the 0.1 bar (10 kPa) ΔCFF threshold is also seen in observed seismicity rate changes in this event. Additionally, beneath the seismically active Ouchigata fault zone, a NW-SE hypocenter cross-section reveals a cluster suggesting a ~45°southeast-dipping fault plane. Given the prolonged increase in seismicity along the active faults around the 2024 source fault, these findings warrant disaster prevention considerations.
We calculated the static Coulomb Failure Stress Change (ΔCFF) caused by the 2024 Noto Peninsula Earthquake using the source fault model of Toda (2024) (Fig. 1). While many source models have been already proposed, we adopted a fault geometry extending ENE from Suzu to west of Wajima and curving westward along the coastline to Amamimisaki, as its southwestern shape significantly affects stress changes on nearby active faults. The receiver faults considered include offshore and onshore active faults evaluated by the Headquarters for Earthquake Research Promotion (2024). The ΔCFF distribution is shown in Fig. 1a, with values at the center of ~5 km × ~5 km sub-patches, assuming an apparent friction coefficient μ’ of 0.4. The Bijosan, Amamimisaki, Hakui-oki-nishi, Hakui-oki-higashi, and Ouchigata fault zones exhibit stress increases exceeding 1 bar (0.1 MPa). The M6.6 earthquake on November 26, 2024, occurred on the Hakui-oki-nishi fault, consistent with the increased ΔCFF. The Morimoto-Togashi, Tonami-heiya, and Kurehayama fault zones also exceed the 0.1 bar threshold for potential seismic influence (Stein, 1999), while reverse faults around Toyama Bay generally show negative ΔCFF.
To analyze post-mainshock seismic response, we mapped seismicity rate changes over one year after the mainshock, comparing to the two years before the mainshock (Fig. 1b). Using JMA unified hypocenter catalog (provisional), we calculated rate changes on a 0.05° grid with a 10 km moving cylinder. Seismicity rate changes over noise level were observed within ~100 km of the source fault. Notably, off-fault aftershocks increased near the Hakui-oki fault, leading up to the M6.6 earthquake on November 26. The Ouchigata fault also showed sustained high activity for over a year. The Morimoto-Togashi and Atotsugawa fault zones exhibited moderate increases, while seismicity rate in southeastern Toyama Bay (near M4.7) briefly jumped but returning to pre-mainshock levels now. Although ΔCFF was mostly negative for reverse faults in Toyama Bay, the M4.7 event on July 11, 2024, was a strike-slip event (NIED, 2024), where ΔCFF was positive on both nodal planes (Fig. 1b), suggesting that post-mainshock activity in this area was dominated by strike-slip fault systems. No significant activity change was detected around the Nadachi-Takada-heiya fault zone, located 70–100 km from the source. As in previous studies, the 0.1 bar (10 kPa) ΔCFF threshold is also seen in observed seismicity rate changes in this event. Additionally, beneath the seismically active Ouchigata fault zone, a NW-SE hypocenter cross-section reveals a cluster suggesting a ~45°southeast-dipping fault plane. Given the prolonged increase in seismicity along the active faults around the 2024 source fault, these findings warrant disaster prevention considerations.