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[HDS10-02] Initial sea surface displacement of non-seismic tsunami associated with the 2020 Alaska Sand Point earthquake
Keywords:Aleutian Trench, Tsunami inversion, Non-seismic tsunami
In the Aleutian Trench, the Pacific Plate is subducting beneath the North American Plate at a rate of about 65 mm/yr at an angle almost perpendicular to the trench axis. This plate subduction has repeatedly caused great interplate earthquakes, such as the 1938 M8.6, the 1946 M8.6, the 1964 M9.2. In the 1946 earthquake, off the coast of Unimak Island generated a huge tsunami. The maximum tsunami height at the coastlines was observed at approximately 40 meters in elevation. The tsunami was probably amplified by earthquake-induced submarine landslides. The area between the 1938 and 1946 rupture area had been recognized as a seismic gap because no significant earthquake happened in the 20th century. Around the seismic gap, three earthquakes greater than M7.6 occurred in 2020 and 2021. The second earthquake generated a tsunami larger than expected from its earthquake magnitude, implying the need for additional tsunami sources rather than a fault motion alone.
In this study, 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 (hereafter, 2020 Alaska Sand Point 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. Using the empirical formula between the magnitude of submarine landslide and sea surface disturbance, we found that the inverted initial sea surface displacement required a significant submarine landslide with a thickness of ~500 m, a length of ~8.0 km, a width of ~60 km and a volume of ~120 km³. As several papers indicated that the past tsunamis had been amplified by submarine landslides in the Aleutian Trench, the 2020 Alaska Sand Point earthquake would have also generated submarine landslides and the unexpected large tsunami.
In this study, 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 (hereafter, 2020 Alaska Sand Point 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. Using the empirical formula between the magnitude of submarine landslide and sea surface disturbance, we found that the inverted initial sea surface displacement required a significant submarine landslide with a thickness of ~500 m, a length of ~8.0 km, a width of ~60 km and a volume of ~120 km³. As several papers indicated that the past tsunamis had been amplified by submarine landslides in the Aleutian Trench, the 2020 Alaska Sand Point earthquake would have also generated submarine landslides and the unexpected large tsunami.