Optimizing a Model of Coseismic Rupture for the 22 July 2020 MW 7.8 Simeonof Earthquake by Exploiting Acute Sensitivity of Tsunami Excitation Across the Shelf Break

The Shumagin seismic gap along the Alaska Peninsula experienced a major, M-W 7.8, interplate thrust earthquake on 22 July 2020. Several available finite-fault inversions indicate patchy slip of up to 4 m at 8-48 km depth. There are differences among the models in peak slip and absolute placement of slip on the plate boundary, resulting from differences in data distributions, model parameterizations, and inversion algorithms. Two representative slip models obtained from inversions of large seismic and geodetic data sets produce very different tsunami predictions at tide gauges and deep-water pressure sensors (DART stations), despite having only secondary differences in slip distribution. This is found to be the result of the acute sensitivity of the tsunami excitation for rupture below the continental shelf in proximity to an abrupt shelf break. Iteratively perturbing seismic and geodetic inversions by constraining fault model extent along dip and strike, we obtain an optimal rupture model compatible with teleseismic P and SH waves, regional three-component broadband and strong-motion seismic recordings, hr-GNSS time series and static offsets, as well as tsunami recordings at DART stations and regional and remote tide gauges. Slip is tightly bounded between 25 and 40 km depth, the up-dip limit of slip in the earthquake is resolved to be well-inland of the shelf break, and the rupture extent along strike is well-constrained. The coseismic slip increased Coulomb stress on the shallow plate boundary extending to the trench, but the frictional behavior of the megathrust below the continental slope remains uncertain.

Automated summary

The Shumagin seismic gap along the Alaska Peninsula experienced a major interplate thrust earthquake, and there are differences in the slip distribution and tsunami predictions among the models. The sensitivity of tsunami excitation near the abrupt shelf break results in significant differences in the tsunami predictions, despite minor differences in slip distribution. By iteratively perturbing seismic and geodetic inversions, an optimal rupture model is obtained that is compatible with various data recordings.