October 8, 2023 Tsunami: Izu Islands, Japan

Ref #13616:

The first tsunami advisories were issued for the Izu and Ogasawara Islands. Tremors had occurred near the Izu Islands about an hour earlier, but their magnitudes were not known. It has, however, been confirmed that no quakes took place with an intensity of 1 or higher on the Japanese seismic scale of zero to 7. Advisories were issued for increasingly wide areas after tsunami waves were observed. An official noted at the news conference that the JMA was unable to accurately identify the epicenter or magnitude of any of the tremors because they took place in areas with few observation points. He said they could have been caused by volcanic activity, a volcanic eruption, a large-scale underwater crustal deformation or a landslide.

Ref #13614: An earthquake with a preliminary magnitude of 5.0 occurred in the Izu Islands, Japan region at 2026 UTC on Sunday October 8 2023. Based on all available data a tsunami was generated at approximately the time and location of this earthquake.

Ref #13699:

This tsunami was at first thought to be caused by an earthquake 75 mins before the warning, at 20:25 UTC (5:25 JST), whose moment magnitude (Mw) was 4.7 as estimated by the [USGS]. Events of this magnitude typically have coseismic deformation < 1 cm, which is far too small to cause hazardous tsunamis. This suggests that the tsunami might not have been associated with the earthquake and was possibly caused by a non-seismic source […] we speculate that the tsunami and seismic swarm were caused by the intermittent volcanic eruptions, whose vent opened on the western bulge of the Sofu-gan volcano and generated the sea surface uplift; and the eruption ended at 21:21 UTC. It is consistent with that the earthquakes that generated T waves stopped at 21:21 UTC. In addition, although the exact details of the source are unknown, on October 20, 11 days after this event, a pumice raft with a length of 80 km was observed northwest of the Sofu-gan volcano by the Japan Coast Guard (Japan Coast Guard, 2023). The last recorded eruption of the Sofu-gan volcano was in 1975 (Geological Survey of Japan, 2013). The tsunami and swarm-like seismic event analyzed in this paper may be possibly associated with the new eruption.

Ref #13733:

In late October 2023, Japan Coast Guard (2023) found drifted materials in the sea around Torishima and Sofugan volcanoes, parts of which were analyzed and confirmed as volcanic pumices […] Mizutani and Melgar (2023) analyzed parts of the tsunami waveforms, suggesting its possible origin of submarine volcanic activity. In early November, Japan Agency for Marine-Earth Science and Technology (2023) surveyed the seafloor near the source region and preliminarily reported the existence of a submarine caldera ∼20 km to the west of Sofugan volcano. Hence, we speculate the tsunami sequence is associated with volcanic unrest, most likely near the submarine caldera […] submarine trapdoor faulting events are tsunamigenic but exhibit atypically inefficient seismic excitation (Sandanbata et al., 2021, 2022). […] the trapdoor faulting mechanism at the newly found caldera near Sofugan volcano may be a plausible candidate for the October 2023 tsunamis.

Ref #14219:

Later observations suggested that the peculiar earthquakes causing the tsunamis on October 9 were associated with a submarine eruption […] Considering that no significant morphological changes were identified other than at Sofu Seamount in our mapped area, the explosive eruption and landslides at Sofu Seamount are likely the sources of tsunami generation.

[The eruption's resulting] landslides removed a total of 141 × 106 m3 of materials. […] The timing of the eruption can be constrained to October 9, 2023 when T-phase swarm and tsunamis were observed to have originated near Sofu Seamount after the 7 day-long earthquake swarm during October 2–8.