Potential megathrust co-seismic slip during the 2020 Sand Point, Alaska
strike-slip earthquake
Abstract
On October 2020, a Mw 7.6 earthquake struck to the south of the Shumagin
Islands in Alaska, nearly 3 months after the Mw 7.8 Simeonof megathrust
event. The initial models of the earthquake indicated a largely
strike-slip rupture; however, the observed tsunami was much larger and
widespread than expected for the focal mechanism. We investigate what
sea surface deformation is necessary to recreate the tsunami waveforms
using water-level inversion techniques. We find that the sea surface
deformation does not resemble that expected from a purely strike-slip
earthquake. We then carry out slip inversions with water level and
static GNSS data as input. We explore the likelihood of megathrust
co-seismic slip aiding tsunamigenesis. We propose that, concurrently
with strike-slip faulting, it is likely that a considerable slip
occurred on the megathrust westward and updip from the previous July
2020 event. We also propose that a smaller submarine landslide is likely
to have occurred in an area prone to them. The Sand Point earthquake
potentially released ~2 meters of accumulated slip in
the western Shumagin Gap, but likely did not slip updip of
~15 km depth.