3:30 PM - 3:45 PM
[U16-01] The long-lasting earthquake swarm leading up to the 2024 M7.6 Noto-Hanto earthquake, Japan
An intense earthquake swarm has persisted for more than three years within a 20 km ×20 km area 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. Globally, it is rare that the long-lasting seismic swarm preceded such a large event. We have analyzed the continuous seismic waveforms to create a more precise earthquake catalog associated with this earthquake sequence. Based on this catalog, we have explored the spatial-temporal evolution of the seismicity before and after the M7.6 event. Note that Localized swarm-like seismicity 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. Swarm-like localized foreshocks indicate an involvement of fluid-driven slow slip transient during the foreshock stage and facilitate stress loading on the mainshock nucleation point.
Several previous studies using seismic and geodetic data suggest that the long-lasting earthquake swarm has been driven by upward fluid flow along pre-existing cracks/faults in the crust (e.g., Nishimura et al. 2023). Especially, Kato (2024, doi:1029/2023GL106444) recognized a rapid upward migration of the immediate aftershocks following the 2023 M6.5 and M5.9 events and implied fault-valve behavior that might be driven by upwelling of crustal fluids along the intensely fractured and permeable fault zones via the dynamic ruptures. If fluids could migrate along pre-existing faults, fault strength would be reduced by lubrication. In addition, the long-lasting intense seismicity and slow deformations detected by GNSS network have partially released the accumulated elastic stress in the swarm area, resulting in stress loading onto nearby fault segments. The strength of the faults gradually decreased, and the stress was partially released over three years, which may have triggered the latest M7.6 earthquake.
After the M7.6 rupture, the seismicity widespread over 150 km along the fault-strike. Although the northwest-dipping fault planes are recognized at the northeastern part of the aftershock zone, most aftershocks show the southeast-dipping fault planes. Along the several profiles where seismic stations are relatively dense, lithtric fault structure (stepper dip angle near the surface and shallower with increased depth.) cat be seen, indicating reactivation of pre-existing normal faults created during the opening of Japan Sea. Near the hypocenter of the M7.6 event, localized cluster activated after the M7.6 rupture, showing round-shaped epicenter distribution. Because this localized cluster shows swarm-like feature that tends to gradually become shallower over time, it is important to monitor the temporal evolution of the seismicity for a while.
Several previous studies using seismic and geodetic data suggest that the long-lasting earthquake swarm has been driven by upward fluid flow along pre-existing cracks/faults in the crust (e.g., Nishimura et al. 2023). Especially, Kato (2024, doi:1029/2023GL106444) recognized a rapid upward migration of the immediate aftershocks following the 2023 M6.5 and M5.9 events and implied fault-valve behavior that might be driven by upwelling of crustal fluids along the intensely fractured and permeable fault zones via the dynamic ruptures. If fluids could migrate along pre-existing faults, fault strength would be reduced by lubrication. In addition, the long-lasting intense seismicity and slow deformations detected by GNSS network have partially released the accumulated elastic stress in the swarm area, resulting in stress loading onto nearby fault segments. The strength of the faults gradually decreased, and the stress was partially released over three years, which may have triggered the latest M7.6 earthquake.
After the M7.6 rupture, the seismicity widespread over 150 km along the fault-strike. Although the northwest-dipping fault planes are recognized at the northeastern part of the aftershock zone, most aftershocks show the southeast-dipping fault planes. Along the several profiles where seismic stations are relatively dense, lithtric fault structure (stepper dip angle near the surface and shallower with increased depth.) cat be seen, indicating reactivation of pre-existing normal faults created during the opening of Japan Sea. Near the hypocenter of the M7.6 event, localized cluster activated after the M7.6 rupture, showing round-shaped epicenter distribution. Because this localized cluster shows swarm-like feature that tends to gradually become shallower over time, it is important to monitor the temporal evolution of the seismicity for a while.