Abstract
The July 2020 Mw7.8 Alaska tsunamigenic earthquake was a fresh call for potential large tsunamis associated with the Aleutian subduction zone. The second largest ever-recorded earthquake worldwide (Mw 9.2) occurred in this zone in 1964 indicating its massive earthquake and tsunami risk. Our analysis of the July 2020 tsunami revealed that it involves very long period waves (51–64 min) which is unusual for an Mw7.8 earthquake. The tsunami coastal amplitude was small (~0.5 m) which is much smaller than that usually expected from a tsunamigenic earthquake of this size. Here, through numerical simulations and spectral analyses, we explain the ultra-long period and small amplitude waves of the tsunami. Our analysis using an analytical equation showed that the ultra-long period of the tsunami (51–64 min) can be reproduced using the shallow water depth around the source region (100–200 m) and the length of the coseismic deformation area (~100 km). By comparing the coastal amplitude of this event with four other similar-size and similar-mechanism earthquakes (thrust Mw7.8), we attribute the relatively small coastal amplitude of this tsunami to the deep focal depth of the earthquake (28 km) and the extremely shallow water around the source region.
Highlights
Offshore Alaska was the site of a large Mw7.8 earthquake on July 22, 2020 which was followed by a small tsunami with a coastal runup of around half a meter (Fig. 1)
A tsunami warning was issued by the US Tsunami Warning System following the earthquake; the warning was called off after confirming that the tsunami height was minimal based on the tsunami records on offshore Deep-ocean Assessment and Reporting of Tsunamis (DART) devices
The tsunami generated by July 2020 Mw7.8 thrust earthquake was unusual in two ways: (i) the period of the tsunami was very long (51–64 min) and (ii) its coastal amplitude was small (~0.5 m)
Summary
Offshore Alaska was the site of a large Mw7.8 earthquake on July 22, 2020 which was followed by a small tsunami with a coastal runup of around half a meter (Fig. 1). There are two unusual signatures in the sea level data of the 2020 tsunami, which motivated this research: (i) the tsunami has a very long period of ~60 min (Fig. 2) which is unusual for a tsunami from an Mw7.8 earthquake;. A similar-magnitude (Mw7.8) and similar-mechanism (thrust) earthquake at the depth of 15.1 km in Kaikoura, New Zealand generated a tsunami with period of 15–20 min and coastal runup of up to 7 m (Power et al, 2017; Heidarzadeh et al, 2019). The innovation of this study is that, for the first time, we apply analytical equations and numerical simulations to explain the unusual tsunami waves generated by an Mw7.8 earthquake
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