2:00 PM - 2:15 PM
[SCG54-12] The Sofu seamount present in the source area of the October 9, 2023 earthquake and tsunami
★Invited Papers
Keywords:Sofu seamount, Submarine volcano, 8 October 2023 (UTC) tsunami, T-phase, Repeat bathymetric survey
In the area around Torishima Island in the Izu-Ogasawara Arc (Torishima Rift between Torishima and Sofugan), seismic activity has increased since October 2, 2023, and four earthquakes of magnitude M6.0 or greater had occurred by October 8 (according to Japan Meteorological Agency). Among them, a M6.5 normal fault earthquake on October 5 at 10:59 (JST) triggered a 0.2 m tsunami on Hachijo-Jima; the earthquakes on October 9 from 4:00 to 6:00 were unique in that no clear P- and S-waves were visible, only T-phases. A 0.7 m tsunami then reached the Hachijo-Jima. Since the tsunami was higher than estimated from the magnitude of the earthquake, it was pointed out that tsunami excitation may have been caused by a landslide, and that the generation of T-phases may have been caused by submarine volcanic activity.
The source of a series of T-phases observed on October 9, 2023, was analyzed by selecting waveform records from DONET, S-net, F-net, Pacific21 network, IMS, and MERMAID stations that azimuthally well encircle around the area. The result showed that T-phase sources were concentrated within a 10 km circle at the location of Sofu seamount in the west of the Sofugan.
JAMSTEC conducted a research cruise by the R/V "Kaimei" about one month after the earthquake and tsunami occurred. This cruise was realized by changing the schedule and adding a survey area to the previously scheduled survey around Nishinoshima, Fukutoku-Okanoba, and Miyake-jima. In the Sofu seamount area, we deployed ocean bottom seismographs (OBSs) and conducted a multibeam bathymetric survey.
The results of the bathymetric survey confirmed a caldera-like topography near the center of the Sofu seamount, where the T-phase sources were estimated. The inner diameter of the caldera is about 4.8 km, the depth of the caldera bottom is about 1300 m, and the average relative height of the caldera wall is about 100 m. On the north side of the caldera, there is a mountain with a relative height of about 400 m, which appears to be the central cone. The mountain top is depressed with an inner diameter of about 2 km, and the water depth at the bottom of the depression is about 1200 m.
Previously, the U.S. R/V Atlantis II conducted a bathymetric survey of the Sofu seamount in 1987. A comparison of the topography in 1987 and 2023 shows that the topography around the caldera has changed significantly. In 1987, the summit of the central cone had a convex topography. The water depth at the summit was about 800 m. There is about a 400 m difference from the concave bottom in 2023. The northern and northwestern flank of the central cone also shows a negative topographic change. On the other hand, the northeastern flank shows a positive change. The most of area within the caldera is rising, especially in the southwestern part of the caldera, rising about 60 m and forming a small knoll-like topography. The volume of the missing part in the caldera is about 360.96×106 m3 and that of the increased part is about 206.92×106 m3 calculated from the change in depth. This topographic change may suggest that sometime during the past 36 years from 1987 to 2023, explosive eruptions caused the collapse of the central cone, and the outflow of volcanic mudflow, some of which were deposited within the caldera.
Based on the above results, the Sofu seamount is highly likely an active submarine volcano. The source of the October 9 T-phases was likely volcanic activity at the Sofu seamount. The relation between the volcanic activity at the Sofu seamount and the tsunami generation is yet unresolved.
The source of a series of T-phases observed on October 9, 2023, was analyzed by selecting waveform records from DONET, S-net, F-net, Pacific21 network, IMS, and MERMAID stations that azimuthally well encircle around the area. The result showed that T-phase sources were concentrated within a 10 km circle at the location of Sofu seamount in the west of the Sofugan.
JAMSTEC conducted a research cruise by the R/V "Kaimei" about one month after the earthquake and tsunami occurred. This cruise was realized by changing the schedule and adding a survey area to the previously scheduled survey around Nishinoshima, Fukutoku-Okanoba, and Miyake-jima. In the Sofu seamount area, we deployed ocean bottom seismographs (OBSs) and conducted a multibeam bathymetric survey.
The results of the bathymetric survey confirmed a caldera-like topography near the center of the Sofu seamount, where the T-phase sources were estimated. The inner diameter of the caldera is about 4.8 km, the depth of the caldera bottom is about 1300 m, and the average relative height of the caldera wall is about 100 m. On the north side of the caldera, there is a mountain with a relative height of about 400 m, which appears to be the central cone. The mountain top is depressed with an inner diameter of about 2 km, and the water depth at the bottom of the depression is about 1200 m.
Previously, the U.S. R/V Atlantis II conducted a bathymetric survey of the Sofu seamount in 1987. A comparison of the topography in 1987 and 2023 shows that the topography around the caldera has changed significantly. In 1987, the summit of the central cone had a convex topography. The water depth at the summit was about 800 m. There is about a 400 m difference from the concave bottom in 2023. The northern and northwestern flank of the central cone also shows a negative topographic change. On the other hand, the northeastern flank shows a positive change. The most of area within the caldera is rising, especially in the southwestern part of the caldera, rising about 60 m and forming a small knoll-like topography. The volume of the missing part in the caldera is about 360.96×106 m3 and that of the increased part is about 206.92×106 m3 calculated from the change in depth. This topographic change may suggest that sometime during the past 36 years from 1987 to 2023, explosive eruptions caused the collapse of the central cone, and the outflow of volcanic mudflow, some of which were deposited within the caldera.
Based on the above results, the Sofu seamount is highly likely an active submarine volcano. The source of the October 9 T-phases was likely volcanic activity at the Sofu seamount. The relation between the volcanic activity at the Sofu seamount and the tsunami generation is yet unresolved.