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Abstract

Two large earthquakes occurred in the Alaska-Aleutian subduction zone in July 2020 (Mw 7.8) and July 2021 (Mw 8.2), generating tsunamis characterized by considerably longer periods than that typically expected from their moment magnitudes. The 2020 earthquake resulted in approximately 40–90 min tsunami periods (Mulia et al., 2022, GRL; Heidarzadeh and Mulia, 2021, Ocean Eng.). Similarly, the 2021 event exhibited long-period tsunamis of >50 min (Mulia et al., 2022, SRL). For comparison, the April 2014 Illapel, Chile, earthquake (Mw 8.2) and the November 2016 Kaikoura, New Zealand, earthquake (Mw 7.8) produced tsunamis with dominant periods ranging from 15 to 21 min (Heidarzadeh et al., 2019, Ocean Eng.). To reveal the underlying cause for such anomalous occurrences, we conducted an inversion analysis using tsunami and geodetic data. Our inversion results indicated the up-dip extent of both earthquakes confined at a depth of ~20 km of the plate interface, which corresponds to the shelf break on the surface. Therefore, the coseismic surface displacement predominantly took place at shallow water depths of ~200 m within the broad continental shelf extending ~120 km offshore. Consequently, it is responsible for the long-period tsunami waves as the water depth is inversely proportional to the period. This geophysical setting is uniquely attributed to the Alaska-Aleutian subduction zone, which is rarely found in other major subduction systems. References: https://doi.org/10.1029/2021GL094937; https://doi.org/10.1785/0220210359; https://doi.org/10.1016/j.oceaneng.2021.109243.