The deep Shumagin gap filled: Kinematic rupture model and slip budget analysis of the 2020 Mw 7.8 Simeonof earthquake constrained by GNSS, global seismic waveforms, and floating InSAR

Abstract

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 enigmatic area known as the Shumagin seismic gap, a region of transitional plate interface coupling from highly coupled to the east to creeping to the west. Hence, this event provides a rare chance to understand rupture mechanics on such heterogeneously coupled faults. In this study, we examine the rupture process of the 2020 Simeonof earthquake with a combination of static GNSS displacements, InSAR displacements, high-rate GNSS waveforms and tele-seismic waveforms. Due to the discontinuous nature of the deformation field, we use InSAR data for individual islands 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 demonstrated that the rupture process of this event is unidirectional, initiating at the hypocenter and propagating westward for about 130 s with an average rupture velocity of ∼1.9 km/s. The highest slip was centered below the Koniuji Islands and occurred between 20 s to 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, building confidence in the slipping of the less coupled region. Comparison with an existing and an alternative plate coupling model demonstrated that the remaining slip budget is unlikely to be able to generate a large event at depths from 30 to 40 km, as the Shumagin seismic gap has been mostly filled by the 2020 Simeonof earthquake at those depths. However, both coupling models suggest a substantial slip deficit in shallow near trench regions, suggesting that a significant earthquake can still occur. Seafloor geodetic observations are required to further constrain the near-trench plate coupling.