Accelerating Seafloor Uplift of Submarine Caldera near Sofugan Volcano, Japan, Resolved by Distant Tsunami Recordings

On 8 October 2023 UTC, significant tsunamis were observed around Japan without any major tsunamigenic earthquake, associated with a series of 14 successive minor earthquakes (mb = 4.5–5.4) near Sofugan in the Izu-Bonin islands. To examine the cause of this tsunami, we estimated the horizontal locations of the tsunami source and temporal history of the seafloor displacement, using the tsunami data recorded by the ocean-bottom pressure gauges > ~600 km away. Our results showed the main tsunami source was an uplift located at a caldera-like bathymetric feature near Sofugan, suggesting the involvement of caldera activity in the tsunami generation. The total seafloor uplift was larger than ~3 m, and the uplift amount of each event gradually increased over time, reflecting an accelerating occurrence of multiple sudden caldera uplifts within only a few hours.