Enigmatic tsunami waves due to repetitive volcanic processes near Sofu Seamount, Izu–Bonin Arc

On 8 October 2023, enigmatic tsunamis with maximum wave heights of ~60 cm were observed in the Izu Islands and southwestern Japan, although only m_b 4.3–5.4 seismic events were reported near Sofu Seamount in the Izu–Bonin arc in the USGS catalog. To investigate the source process, we analyze tsunami waveforms recorded by ocean-bottom pressure gauges of the DONET array off southwestern Japan. Stacked waveforms reveal recurrent arrivals of multiple wave trains. Deconvolution using a waveform segment of the tsunami from an early isolated event identifies at least 14 successive events that intermittently generated tsunamis over ~1.5 h. Their timings closely coincide with individual events of the seismic swarm and strong seawater acoustic waves (T waves) recorded by ocean-bottom seismometers, indicating a common source. Larger events later in the sequence occurred at intervals comparable to the tsunami period, amplifying later wave phases. Our tsunami waveform analyses summarized above, reported in Sandanbata et al. (2024, GRL), indicate a shallow, repetitive, and atypical non-tectonic tsunami source processes, consistent with volcanic activity. Subsequent independent studies have provided additional constraints supporting a volcanic origin. Recent bathymetric surveys revealed evidence of a submarine eruption near Sofu Seamount (Minami and Tani, 2024, Mar. Geol.). In addition, Kubota et al. (2024, GRL) constrained tsunami source locations to the vicinity of the seamount based on independent tsunami waveform analyses, while Takemura et al. (2024, JGR: Solid Earth) inferred shallow source depths using high-frequency seismic body and T waves. We propose that repetitive volcanic unrest, potentially involving submarine eruptions, caldera deformation, and/or flank collapses, generated the enigmatic tsunamis, although the exact mechanisms remain unresolved.