2:30 PM - 2:45 PM
[SVC36-04] Earthquake activity in the Torishima Rift and Sofu Seamount related to the October 2023 tsunamis observed in Japan
Seismic activity including several M6-class earthquakes became active near Torishima and Sofugan islands in the Izu-Ogasawara arc of Japan in early October 2023. According to the USGS and GCMT catalogues, three M6-class earthquakes occurred on October 3, 5, and 6. The tsunamis were observed following the normal-faulting earthquakes on October 5 and 6. In addition, on October 8 (UT), tsunamis were observed following smaller earthquakes with distinct T-phases. Analysis of T-phases and tsunami waveforms, and bathymetry surveys suggested that the tsunamis on October 8 are likely related to volcanic activity of the Sofu seamount located west of Sofugan Island. On the other hand, it remains unclear where the entire activity occurred and how it related to the tsunamis on October 5 and 6.
We conducted earthquake observations using ocean bottom seismographs (OBSs) near the Sofu Seamount from November to December 2023. The hypocenter distribution revealed from the OBS observations using three short-period OBSs exhibits a linear trend extending about 50 km in NNW direction from the Sofu Seamount to the Torishima Rift, which is a back-arc rift located between Torishima and Sofugan Islands. The limited number of the OBSs and large separation between them make difficult to constrain hypocenter depths. However, the earthquakes in the Torishima Rift likely occurred at depths of 15 to 20 km or even shallower at a depth of approximately 5 km based on the residual changes with hypocenter depth. The possible shallow hypocenters suggest that the normal-fault might extend close to the seafloor.
We calculated tsunami waveforms to confirm the relationship between the hypocenter distributions obtained from the OBS observations and the tsunamis in early October 2023. We assumed normal-fault parameters with reference to the CMT solutions by the USGS and GCMT for the Mw 6.1 earthquake on October 5. The JMA reported a tsunami with a maximum height of 0.2 m at Yaene, Hachijo-jima Island associated with this earthquake. The fault location is assumed to be in the middle of the Torishima Rift at the northern flank of the Sofu seamount with reference to the hypocenter distributions revealed from the OBS observations. Even though the tsunami height data at Yaene, Hachijo-jima Island were provide at 0.1 m step, the calculated tsunami waveform exhibits a good agreement with the moving-averaged record. This imply that the early October earthquakes and tsunamis were related to the normal-faults in the middle of the Torishima Rift.
Multi-channel seismic (MCS) surveys have shown the presence of shallow normal faults and intrusions extending to the seafloor in the back-arc area near the Torishima Rift. Furthermore, the hypocenters revealed from the OBS observations coincide with extrusive lave in the Torishima Rift, which was identified from sidescan imagery. The earthquakes and tsunamis of early October 2023 are likely related not only to the volcanic activity at the Sofu Seamount, which has been pointed out to be associated with the tsunamis on October 8, but also to the magmatic activity in the Torishima Rift.
We conducted earthquake observations using ocean bottom seismographs (OBSs) near the Sofu Seamount from November to December 2023. The hypocenter distribution revealed from the OBS observations using three short-period OBSs exhibits a linear trend extending about 50 km in NNW direction from the Sofu Seamount to the Torishima Rift, which is a back-arc rift located between Torishima and Sofugan Islands. The limited number of the OBSs and large separation between them make difficult to constrain hypocenter depths. However, the earthquakes in the Torishima Rift likely occurred at depths of 15 to 20 km or even shallower at a depth of approximately 5 km based on the residual changes with hypocenter depth. The possible shallow hypocenters suggest that the normal-fault might extend close to the seafloor.
We calculated tsunami waveforms to confirm the relationship between the hypocenter distributions obtained from the OBS observations and the tsunamis in early October 2023. We assumed normal-fault parameters with reference to the CMT solutions by the USGS and GCMT for the Mw 6.1 earthquake on October 5. The JMA reported a tsunami with a maximum height of 0.2 m at Yaene, Hachijo-jima Island associated with this earthquake. The fault location is assumed to be in the middle of the Torishima Rift at the northern flank of the Sofu seamount with reference to the hypocenter distributions revealed from the OBS observations. Even though the tsunami height data at Yaene, Hachijo-jima Island were provide at 0.1 m step, the calculated tsunami waveform exhibits a good agreement with the moving-averaged record. This imply that the early October earthquakes and tsunamis were related to the normal-faults in the middle of the Torishima Rift.
Multi-channel seismic (MCS) surveys have shown the presence of shallow normal faults and intrusions extending to the seafloor in the back-arc area near the Torishima Rift. Furthermore, the hypocenters revealed from the OBS observations coincide with extrusive lave in the Torishima Rift, which was identified from sidescan imagery. The earthquakes and tsunamis of early October 2023 are likely related not only to the volcanic activity at the Sofu Seamount, which has been pointed out to be associated with the tsunamis on October 8, but also to the magmatic activity in the Torishima Rift.