Successive Tsunamigenic Events Near Sofu Seamount Inferred From High‐Frequency Teleseismic P and Regional T Waves.

An unexpected major tsunami from the region near Sofu Seamount was observed on 8 October 2023. 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 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 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 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. Plain Language Summary: On 8 October 2023, an earthquake sequence, which included 15 moderate‐size (magnitudes of 4.3–5) earthquakes, was observed in the region near Sofu Seamount. When a moderate‐size (M < 6) earthquake occurs in offshore regions, we generally observe no tsunami signals. However, during this earthquake sequence, an unexpected major tsunami was observed along the coast of Japan. To investigate tsunami sources near Sofu Seamount, we analyzed observed T waves at ocean‐bottom seismometers near the coast of southwest Japan. T waves are ocean acoustic waves and can effectively propagate at distances larger than several thousand kilometers. Observed T waves were significant compared with P waves, propagating through the subsurface. We also analyzed simulation results of high‐frequency seismic wave propagation from Sofu Seamount to the Japanese coast. Comparing simulation results with observed T waves allows us to constrain the depth of moderate‐size earthquakes. Because efficient T‐wave generation only appears in shallower sources, moderate‐size earthquakes should be located at depths ≤0.5 km below the seafloor, and such shallower seismic/deformation phenomena might cause observable tsunamis even for magnitudes ≤5. Key Points: We studied high‐frequency teleseismic P and regional T waves during tsunamigenic events near Sofu SeamountAccording to envelope shapes, accompanying seismic events during tsunamigenic events should be at depths ≤0.5 km below the seafloorShallow sources might cause tsunamigenic seafloor uplifts even for smaller seismic magnitudes [ABSTRACT FROM AUTHOR]