Enigmatic Tsunami Waves Amplified by Repetitive Source Events Near Sofugan Volcano, Japan

• Published in: Geophysical research letters 2024-01, Vol.51 (2), p.n/a
• Main Authors: Sandanbata, Osamu, Satake, Kenji, Takemura, Shunsuke, Watada, Shingo, Maeda, Takuto, Kubota, Tatsuya
• Format: Article
• Language: English
• Subjects: Acoustic waves; Bottom pressure; Earthquakes; Gauges; Islands; Ocean bottom; Ocean bottom seismometers; Ocean floor; Oceans; Pressure gages; Pressure gauges; Seismic activity; Seismic waves; Seismometers; Sequencing; Sound waves; tsunami; tsunami earthquakes; Tsunamis; T‐phase; volcanic tsunami; Volcanoes; Wave amplification; Wave height; Wave packets; Wave period; Wave trains; Waveforms
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL106949

On 8 October 2023, mysterious tsunamis with a maximum wave height of 60 cm were observed in Izu Islands and southwestern Japan, although only seismic events with body‐wave magnitudes mb 4–5 have been documented to the west of Sofugan volcano. To investigate the source process, we analyze tsunami waveforms recorded by an array network of ocean bottom pressure gauges. Stacked waveforms of pressure gauge records suggest recurrent arrivals of multiple wave trains. Deconvolution of the stacked waveforms by tsunami waveforms from an earlier event revealed over 10 source events that intermittently generated tsunamis for ∼1.5 hr. The temporal history of this sequence corresponds to the origin times of T‐phases estimated by an ocean bottom seismometer and of the seismic swarm, implying a common origin. Larger events later in the sequence occurred at intervals comparable to the tsunami wave period, causing amplification of later phases of the tsunami waves. Plain Language Summary On 8 October 2023, mysterious tsunamis hit Izu Islands and southwestern Japan, reaching up to 60 cm in height, although only small‐to‐moderate seismic events were reported in the region. To resolve how the mysterious tsunami waves were generated, we analyze the waves recorded by a tsunami observation network off the southwestern coast of Japan. We find that the tsunami waves were intermittently produced by repetitive source events for approximately 1.5 hr, and the wave amplification happened because the inter‐event times matched the wave periods. These abnormal submarine events excited significant oceanic acoustic waves, as well as measurable tsunamis, which provides valuable information to further study what took place in the ocean. Key Points On 8 October 2023 (UTC), enigmatic tsunamis up to ∼60 cm were recorded along broad Japanese coasts without large earthquakes Analysis of stacked tsunami waveform data shows >10 events intermittently caused tsunamis for ∼1.5 hr with T‐phase and seismic excitation Larger events later in the sequence occurred with intervals similar to the wave period, amplifying the tsunami waves in the later phase