4:30 PM - 4:45 PM
[HDS11-15] The Sofugan tsunami in October 2023: Accelerating submarine caldera uplift
On 8 October 2023 UTC, unexpected tsunamis with maximum amplitudes of > ~0.6 m were observed around Japan without any major tsunamigenic earthquake. The Japan Meteorological Agency attributed the tsunami to an M~5 earthquake at 20:25 near Sofugan, ~80 km south of Torishima Island in the Izu-Bonin islands. Associated with this tsunami, 14 successive minor earthquakes (mb = 4.3–5.7) were identified between 19:53 and 21:26 by the U.S. Geological Survey (USGS). Sandanbata et al. (2024) analyzed the ocean-bottom pressure gauges of the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET) (Kaneda et al. 2015; Kawaguchi et al. 2015) to discuss the the temporal history of this tsunami generation process, and they suggested the repetitive generation of the tsunamis during the series of the events. To further reveal and identify this unusual tsunami generation mechanism, it is required to quantify the location and the amount of seafloor deformation in detail. In this study, we estimated the horizontal locations and temporal history of the tsunami source, using tsunami data from the DONET and S-net (the Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench, Aoi et al. 2020) deployed > ~600 km away from the source region.
We first estimated the distributions of the sea-surface vertical displacement due to the repetitive events using the tsunami source inversion approach. In contrast to the ordinary inversion analyses that assume the tsunami was excited by a single event, we took into account of the repetitive tsunami generation over ~1.5 hours by assuming the multiple source functions in the time domain, which initiated at the origin times of the 14 earthquakes determined by USGS. Our results showed that each event was located at almost the same location, cumulatively constituting a significant uplift. Near the uplift area a bathymetric feature of a submarine volcanic caldera was confirmed by the seafloor bathymetry survey by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC).
We further examine the amount and temporal history of the seafloor uplift, assuming the seafloor uplift with the same spatial distribution repetitively took place over the submarine caldera. We found that the seafloor uplift amount of each event gradually increased in time and the total uplift was larger than ~3 m. Similar accelerating processes of repetitive events with increasing event magnitudes as well as decreasing inter-event times have been reported for the activity of the volcanic caldera (e.g., Michon et al. 2009; Wang et al. 2023), suggesting that the seafloor uplift due to each event was brought by the volcanic activity of this submarine caldera.
In the volcanic activity of the calderas, a trapdoor faulting, or the sudden slip of the intra-caldera ring fault caused by overpressurization of its underlying magma reservoir, can cause the sudden uplift (Sandanbata et al. 2022; Zheng et al. 2022). It has been often reported that the submarine trapdoor faulting excites significant tsunamis with larger amplitudes than that expected from their seismic magnitudes (e.g., Fukao et al. 2018; Sandanbata et al. 2022). The large uplift estimated by our analysis is consistent with these features of the trapdoor faulting mechanism, which may suggest the successive occurrence of the trapdoor faultings at the caldera. If the trapdoor faulting mechanism is the cause of this seafloor uplift, the repetitive and accelerating occurrence of the tsunamigenic multiple events within a few hours seems to be unique, whereas the past events consisted of only a single event.
We first estimated the distributions of the sea-surface vertical displacement due to the repetitive events using the tsunami source inversion approach. In contrast to the ordinary inversion analyses that assume the tsunami was excited by a single event, we took into account of the repetitive tsunami generation over ~1.5 hours by assuming the multiple source functions in the time domain, which initiated at the origin times of the 14 earthquakes determined by USGS. Our results showed that each event was located at almost the same location, cumulatively constituting a significant uplift. Near the uplift area a bathymetric feature of a submarine volcanic caldera was confirmed by the seafloor bathymetry survey by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC).
We further examine the amount and temporal history of the seafloor uplift, assuming the seafloor uplift with the same spatial distribution repetitively took place over the submarine caldera. We found that the seafloor uplift amount of each event gradually increased in time and the total uplift was larger than ~3 m. Similar accelerating processes of repetitive events with increasing event magnitudes as well as decreasing inter-event times have been reported for the activity of the volcanic caldera (e.g., Michon et al. 2009; Wang et al. 2023), suggesting that the seafloor uplift due to each event was brought by the volcanic activity of this submarine caldera.
In the volcanic activity of the calderas, a trapdoor faulting, or the sudden slip of the intra-caldera ring fault caused by overpressurization of its underlying magma reservoir, can cause the sudden uplift (Sandanbata et al. 2022; Zheng et al. 2022). It has been often reported that the submarine trapdoor faulting excites significant tsunamis with larger amplitudes than that expected from their seismic magnitudes (e.g., Fukao et al. 2018; Sandanbata et al. 2022). The large uplift estimated by our analysis is consistent with these features of the trapdoor faulting mechanism, which may suggest the successive occurrence of the trapdoor faultings at the caldera. If the trapdoor faulting mechanism is the cause of this seafloor uplift, the repetitive and accelerating occurrence of the tsunamigenic multiple events within a few hours seems to be unique, whereas the past events consisted of only a single event.