The July 2020 Simeonof Earthquake: Coseismic Slip and the Megathrust Slip Budget

On 22 July 2020, the Mw 7.8 Simeonof megathrust earthquake struck offshore of the Alaska Peninsula. This was the largest event since 1917 in the Shumagin seismic gap, a region of transitional plate interface coupling from highly coupled to the east to creeping to the west. In this study, we examine the rupture process of the 2020 Simeonof earthquake using a combination of static GNSS displacements, InSAR displacements, high-rate GNSS waveforms and tele-seismic waveforms (Xiao et al., submitted). Due to the discontinuous nature of the deformation field, we use InSAR data unwrapped for each individual island and tie the displacement field either to GNSS observations or keep these floating, i.e. we estimate an ambiguity parameter during the inversion. Our results show that the highest slip was centered below Big and Little Koniuji Islands and occurred between 20 s and 50 s after the rupture initiation. We find that InSAR observations, especially floating data on near-field islands, provide essential constraints for the slip inversion, and require lower slip across the less coupled region in the western part of the Shumagin Islands. Comparing two alternative plate coupling models demonstrates that the 1917-present slip budget was largely balanced by the 2020 Simeonof earthquake at those depths from 30 to 40 km. However, both coupling models suggest substantial slip deficit in shallow near trench regions, indicating that a significant earthquake can still occur there. We also investigate postseismic deformation over the year following the earthquake, using continuous site data and data from campaign measurements in July 2021.