14:15 〜 14:30
[SCG54-13] Earthquake activities near the Sofu Seamount with possible relations to Tsunamis observed in October 2023
Seismic activity near the Tori-shima Island has been active since October 2, 2023 (JST), and a M6.5 normal-faulting earthquake occurred on October 5. Associated with this earthquake, a tsunami with a height of 0.3 m was observed at Hachijo-jima Island. In addition, several earthquakes occurred on October 9. While these earthquakes were between mb 4.3 and 5.4, tsunamis larger than those expected from seismic magnitudes were observed, including a tsunami with a height of 0.6 m at Hachijo-jima Island. P- and S-wave arrivals of the earthquakes on October 9 were not clear, but these events have clear T-phases. Source locations of the T-phases were estimated near the Sofu Seamount, west of the Sofu-gan volcano, from acoustic and seismic observations.
Hypocenters by USGS and GCMT solutions of a series of the seismic activity in October 2023 have been located in and around the Tori-shima rift, which is a back-arc rift west of the volcanic front of Izu-Ogasawara arc between Tori-shima and Sofu-gan. Normal-faults and volcanic intrusions were imaged by previous crustal structure surveys near the Tori-shima rift, but relationships between the seismic activities in October 2023, normal-faults around Tori-shima rift, and Sofu Seamount are not clear.
We have conducted earthquake observations using ocean bottom seismographs (OBSs) to obtain the accurate hypocenter locations near the Sofu Seamount and to investigate the relationships between seismic activities and tsunami generations on October 2023. We deployed three short-period OBSs (SPOBSs) and three broadband OBSs (BBOBSs) in November 2023, during a research cruse by R/V Kaimei of JAMSTEC. These SPOBSs were recovered in early December 2023 and additional six SPOBSs and six BBOBSs were deployed. For the three SPOBSs recovered in December 2023, seismic events were detected visually from the continuous record of the SPOBSs and P- and S-wave arrivals are picked manually. The hypocenter locations were estimated by using a 1-D velocity structure based on previous crustal structure surveys near the Sofu Seamount.
We determined hypocenters of approximately 450 earthquakes with both P- and S-wave arrivals picked at all three OBSs during the period from November 13 to December 3, 2024. These hypocenters show a linear distribution extended to NNW direction from Sofu Seamount. The seismic activities extended outside the SPOBS network used in this analysis, but the activity area extended about 50 km in the Tori-shima rift. On the other hand, the earthquakes with both P- and S-wave arrivals picked at all three OBSs were not determined beneath the Sofu Seamount, where significant topographic changes have been observed although the timing is unknown. More dense observations are ongoing near the Sofu Seamount, but we will discuss the relationships between seismic activities in October 2023, Tori-shima rift, and Sofu Seamount
Hypocenters by USGS and GCMT solutions of a series of the seismic activity in October 2023 have been located in and around the Tori-shima rift, which is a back-arc rift west of the volcanic front of Izu-Ogasawara arc between Tori-shima and Sofu-gan. Normal-faults and volcanic intrusions were imaged by previous crustal structure surveys near the Tori-shima rift, but relationships between the seismic activities in October 2023, normal-faults around Tori-shima rift, and Sofu Seamount are not clear.
We have conducted earthquake observations using ocean bottom seismographs (OBSs) to obtain the accurate hypocenter locations near the Sofu Seamount and to investigate the relationships between seismic activities and tsunami generations on October 2023. We deployed three short-period OBSs (SPOBSs) and three broadband OBSs (BBOBSs) in November 2023, during a research cruse by R/V Kaimei of JAMSTEC. These SPOBSs were recovered in early December 2023 and additional six SPOBSs and six BBOBSs were deployed. For the three SPOBSs recovered in December 2023, seismic events were detected visually from the continuous record of the SPOBSs and P- and S-wave arrivals are picked manually. The hypocenter locations were estimated by using a 1-D velocity structure based on previous crustal structure surveys near the Sofu Seamount.
We determined hypocenters of approximately 450 earthquakes with both P- and S-wave arrivals picked at all three OBSs during the period from November 13 to December 3, 2024. These hypocenters show a linear distribution extended to NNW direction from Sofu Seamount. The seismic activities extended outside the SPOBS network used in this analysis, but the activity area extended about 50 km in the Tori-shima rift. On the other hand, the earthquakes with both P- and S-wave arrivals picked at all three OBSs were not determined beneath the Sofu Seamount, where significant topographic changes have been observed although the timing is unknown. More dense observations are ongoing near the Sofu Seamount, but we will discuss the relationships between seismic activities in October 2023, Tori-shima rift, and Sofu Seamount