Upper Plate Structure and Megathrust Properties in the Shumagin Gap Near the July 2020 M7.8 Simeonof Event.

Subduction zone architecture and properties are thought to control megathrust slip behavior, but few constraints on crustal structure and megathrust properties are available at sufficient resolution and depth, hindering understanding of linkages between structure and behavior. Here we present a P‐wave seismic velocity model based on wide‐angle seismic data integrated with collocated reflection imaging in the weakly coupled Shumagin Gap in the Alaska subduction zone, where a M7.8 occurred in July 2020. We show that this earthquake occurred near and below the Moho of the overriding plate where the megathrust is characterized by a 3‐ to 5‐km‐thick reflection band interpreted to represent tectonic mixing. The rheological heterogeneity of the plate boundary near and below the Moho could account for abundant interplate seismicity, repeated M7.x events, and patchiness of the 2020 rupture. Velocity variations in the overriding continental crust imply changes in rigidity that could further influence megathrust slip. Plain Language Summary: Slip on subduction zone plate boundary faults (megathrusts) produces large and destructive earthquakes, but many questions remain about what controls the size and character of these events. To address this question, we used seismic imaging data to determine subduction zone configuration and properties offshore Alaska in the area of a magnitude 7.8 earthquake that occurred in July 2020. We constrain the thickness of the crust overlying the megathrust, variations in strength within the crust, and changes in the total width and complexity of the fault zone. The M7.8 earthquake occurred on a part of the fault overlain by both continental crust and upper mantle, where imaging shows the fault zone may be particularly heterogeneous, which could explain the patchiness of slip during this event. Variations in strength of the overriding plate may also influence both shallow and deep fault slip in this subduction zone. Key Points: The megathrust intersects the continental Moho in the Shumagin Gap at ∼33 kmMegathrust heterogeneity near and below continental Moho could explain patchy slip in 2020 M7.8 event and abundant megathrust seismicityChanges in seismic velocity in the overriding crust imply changes in rigidity that could influence megathrust slip behavior [ABSTRACT FROM AUTHOR]