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[HDS11-14] The Sofugan tsunami in October 2023: Wave amplification by repetitive tsunami generation
Keywords:Tsunami, Izu-Bonin Arc, T-phase, Tsunami earthquakes
On October 8, 2023 (UTC), unanticipated tsunamis with a maximum wave height of 60 cm were observed in the Izu-Bonin Islands and along the coasts spanning from the south to the west of Japan. Since no major earthquake was observed in the region, the tsunamis were not forecasted. It was only after the tsunami started to be recorded by a tsunami-meter on Hachijojima Island that the tsunami advisory was issued by the Japan Meteorological Agency (JMA). According to the U.S. Geological Survey earthquake catalog, tens of moderate-sized seismic events only with body-wave magnitudes mb 4–5 took place in a region 30–40 km to the west of Sofugan volcano (note that the epicenters were in the “near-Torishima” region, according to the JMA’s classification). Yet, the observed tsunami waves were far larger than those expected empirically for typical tectonic earthquakes with similar sizes, indicating the peculiar source process of the near-Torishima tsunami.
The tsunami waves were recorded at ocean bottom pressure (OBP) gauges of Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET) off the southwestern coast of Japan. The OBP records show that smaller waves arrived at ~20:40 (UTC) and the largest waves with an amplitude of ∼20 mm were recorded late after ∼22:10, exhibiting long-lasting tsunamis for hours with late arrivals of the largest waves. To capture the features in detail, we apply a waveform stacking method to the OBP records of nearby stations. The stacked OBP waveform demonstrates that the long-lasting tsunami signals were composed of multiple tsunami wave trains with strong dispersive characters, suggesting tsunami waves were repeatedly produced.
We then examine the temporal history of the tsunami generation process by estimating its source time function (STF) using a waveform deconvolution method. Under a hypothesis that multiple source events occurred at similar locations but at different timings, and that each single event produced tsunami waveforms with the same shapes but different amplitudes, we deconvolve the stacked OBP waveform with an empirical Green’s function retrieved from an initial part of the waveform. Consequently, we obtain a STF composed of 23 source events, and find that the superposition of multiple tsunami waveforms due to >10 events that repeated for ~1.5 hours from 19:53 to 21:26 can reproduce major parts of the stacked OBP waveform. The tsunami-generating source events took place almost at the same timings with the 14 moderate-sized seismic events near Sofugan volcano, and repetitively radiated strong T-phases that were recorded by an ocean bottom seismometer of DONET, strongly suggesting their common origin. In the sequence, the source events later in time overall had a larger magnitude and took place with a shorter inter-event time. The inter-event times of the later large events, 200–300 s, were similar to the dominant wave periods of the observed tsunami waves. Because of the match between the inter-event time and the wave period, the phases of multiple tsunami waveforms from the later events overlapped with each other, resulting in the amplification of wave heights of the late-arrival waves.
This study has shown the tsunami wave amplification mechanism resulting from repetitive tsunami generation occurring more than 10 times within hours for the first time. The coincidences of tsunamis, seismic waves, and intense oceanic acoustic waves, suggest that underwater volcanic processes or landslides near Sofugan volcano are the most likely candidates for the source mechanism. However, additional investigations are essential to conclusively determine of the mechanism. The near-Torishima tsunami underscores the challenges in forecasting tsunamis in the Izu-Bonin region. Although the region hosts many active volcanic islands and submarine volcanoes, there is currently no offshore tsunami observation system. This highlights the urgent need to enhance preparedness for tsunami hazards in the region.
[Reference]
Sandanbata, O., Satake, K., Takemura, S., Watada, S., Maeda, T., & Kubota, T. (2024). Enigmatic tsunami waves amplified by repetitive source events near Sofugan volcano, Japan. Geophysical Research Letters, 51, e2023GL106949. https://doi.org/10.1029/2023GL106949
The tsunami waves were recorded at ocean bottom pressure (OBP) gauges of Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET) off the southwestern coast of Japan. The OBP records show that smaller waves arrived at ~20:40 (UTC) and the largest waves with an amplitude of ∼20 mm were recorded late after ∼22:10, exhibiting long-lasting tsunamis for hours with late arrivals of the largest waves. To capture the features in detail, we apply a waveform stacking method to the OBP records of nearby stations. The stacked OBP waveform demonstrates that the long-lasting tsunami signals were composed of multiple tsunami wave trains with strong dispersive characters, suggesting tsunami waves were repeatedly produced.
We then examine the temporal history of the tsunami generation process by estimating its source time function (STF) using a waveform deconvolution method. Under a hypothesis that multiple source events occurred at similar locations but at different timings, and that each single event produced tsunami waveforms with the same shapes but different amplitudes, we deconvolve the stacked OBP waveform with an empirical Green’s function retrieved from an initial part of the waveform. Consequently, we obtain a STF composed of 23 source events, and find that the superposition of multiple tsunami waveforms due to >10 events that repeated for ~1.5 hours from 19:53 to 21:26 can reproduce major parts of the stacked OBP waveform. The tsunami-generating source events took place almost at the same timings with the 14 moderate-sized seismic events near Sofugan volcano, and repetitively radiated strong T-phases that were recorded by an ocean bottom seismometer of DONET, strongly suggesting their common origin. In the sequence, the source events later in time overall had a larger magnitude and took place with a shorter inter-event time. The inter-event times of the later large events, 200–300 s, were similar to the dominant wave periods of the observed tsunami waves. Because of the match between the inter-event time and the wave period, the phases of multiple tsunami waveforms from the later events overlapped with each other, resulting in the amplification of wave heights of the late-arrival waves.
This study has shown the tsunami wave amplification mechanism resulting from repetitive tsunami generation occurring more than 10 times within hours for the first time. The coincidences of tsunamis, seismic waves, and intense oceanic acoustic waves, suggest that underwater volcanic processes or landslides near Sofugan volcano are the most likely candidates for the source mechanism. However, additional investigations are essential to conclusively determine of the mechanism. The near-Torishima tsunami underscores the challenges in forecasting tsunamis in the Izu-Bonin region. Although the region hosts many active volcanic islands and submarine volcanoes, there is currently no offshore tsunami observation system. This highlights the urgent need to enhance preparedness for tsunami hazards in the region.
[Reference]
Sandanbata, O., Satake, K., Takemura, S., Watada, S., Maeda, T., & Kubota, T. (2024). Enigmatic tsunami waves amplified by repetitive source events near Sofugan volcano, Japan. Geophysical Research Letters, 51, e2023GL106949. https://doi.org/10.1029/2023GL106949