Slipping the Shumagin Gap: A Kinematic Coseismic and Early Afterslip Model of the Mw 7.8 Simeonof Island, Alaska, Earthquake
Brendan W. Crowell, Diego Melgar
Abstract
Abstract In July 2020, a Mw 7.8 earthquake initiated directly to the east of Simeonof Island offshore of the Alaska Peninsula. The earthquake ruptured the eastern part of the Shumagin Gap, a region devoid of large earthquakes over the last century and characterized by low geodetic coupling. Here, we investigate the rupture kinematics of the earthquake using a joint inversion of high‐rate GNSS and strong‐motion data. We find that the rupture was focused between depths of 30–45 km, starting east of the Shumagin Islands and rupturing downdip towards the northwest, with little slip west of 160°W. Early postseismic observations indicate that the entirety of the Shumagin Gap at depths between 40–60 km ruptured with aseismic afterslip and aftershocks. Historically, this earthquake resembles the Shumagin Islands earthquake of 1917, indicating that a possible rupture asperity exists to explain low interseismic coupling and repeating ~M7.5–8 earthquakes.