Successive tsunamigenic events near the Sofu Seamount inferred from high-frequency teleseismic P and regional T waves

An unexpected major tsunami from the region near the Sofu Seamount was observed on 8 October 2023. The Sofu Seamount is located approximately 600 km from the coast of Japan. Due to far epicentral distances and the successive occurrence of seismic events, the conventional seismic analysis to reveal the accompanying seismic sequence cannot work well. We investigated high-frequency teleseismic P and regional T waves from the accompanying seismic sequence during the tsunamigenic events near the Sofu Seamount. Envelope shapes of teleseismic P and regional T waves were similar, indicating that T-wave envelopes also reflected source properties of seismic sequence. During seismic events near the Sofu Seamount, observed regional envelopes were characterized by weak body waves and large amplitude T waves with durations of 39-68 s. According to numerical simulations of seismic wave propagation using a realistic topography model, characteristics of T waves exhibit weak slope-angle and strong source-depth dependencies. Strong T waves with durations less than 60 s only appeared in results with sources at depths ≤ 0.5 km below the seafloor. We concluded that high-frequency radiation of the accompanying seismic sequence during the tsunamigenic events near the Sofu Seamount possibly occurred at shallower depths just below the seafloor. If seismic and tsunami sources coincide, shallower source depths might cause tsunamigenic uplifts. The observed peak seafloor uplifts and T-wave amplitudes during tsunamigenic events were scaled. This result suggests the possibility of tsunami forecasting based on T-wave amplitudes from submarine volcanoes.