Abstract

Large earthquakes occurred sequentially around a seismic gap of the Aleutian Trench, with a magnitude of 7.8 in July 2020, 7.6 in October 2020, and 8.2 in July 2021. The second earthquake generated a tsunami larger than expected based on its magnitude, implying the need for identifying additional tsunami sources rather than the sole use of fault motion. Therefore, we performed tsunami waveform inversions to estimate an initial sea surface displacement unrelated to the fault motion for the tsunami following the October 2020 earthquake. The inversion that assumed simultaneous generation of the non-seismic and seismic tsunamis did not yield a realistic initial sea surface displacement. The estimated initial sea surface displacement, assuming a 5-min delay of the non-seismic tsunami, showed a simple bipolar pattern with subsidence on the landward side and uplift on the trench side. Application of the empirical formula on the magnitude of submarine landslide and sea surface disturbance revealed that the inverted initial sea surface displacement required a significant submarine landslide with a thickness of ~ 500 m, length of ~ 8.0 km, width of ~ 60 km, and volume of ~ 120 km3. As several studies have indicated that past tsunamis were amplified by submarine landslides in the Aleutian Trench, the October 2020 earthquake likely generated submarine landslides and the unexpected large tsunami.