Slipping the Shumagin Gap: A Kinematic Rupture Model of the Mw 7.8 Simeonof Island, Alaska, Earthquake

Abstract:

On July 22, 2020, a Mw 7.8 earthquake nucleated directly to the east of Simeonof Island in Alaska, the southeasternmost island within the Shumagin Island chain. The earthquake likely ruptured the eastern part of the Shumagin gap, a relatively seismically quiescent region which over the last century is characterized by low geodetic coupling. In this study, we investigate the rupture kinematics of the 2020 Simeonof Island earthquake using a joint inversion of 9 high-rate Global Navigation Satellite System (GNSS) stations, 6 strong-motion accelerometer stations, and one DART buoy. We find that rupture was primarily focused between depths of 30-50 km, starting east of the Shumagin Islands and rupturing downdip towards the northwest, with little slip west of 160^oW, and peak slip of roughly 2 m. Our kinematic results match a profile of vertical deformation orthogonal to the strike along the Shumagin Islands, indicating a down-dip extent of rupture at 50 km and an up-dip limit between 30-40 km. Finally, we discuss the relationship of the Simeonof Island earthquake to the coupling model in the region and how it relates to the historical seismicity.