Variations in Reflection Signature and Slip Behavior of the Subduction Interface Offshore Alaska Peninsula From 152º to 161º W

Abstract

We present seismic reflection images of the plate interface up to depth of ~65 km across a ~130,000 km 2 area offshore the Alaska Peninsula. These images capture systematic along-strike and downdip variations in the thickness of the plate interface reflection package, which we interpret in terms of megathrust properties and slip nature. A relatively thin reflection package (<2 km) characterizes the megathrust at depths <24-34 km, marking an area of localized brittle deformation that is likeliest to host the largest seismic asperities. The plate boundary reflection band thickness transitions from 2-5 km over depths of ~24-41 km, suggesting a thickening zone of cumulative deformation and greater heterogeneity corresponding to conditionally stable megathrust behavior. At depths >35-41 km, the reflection package maintains a thickness of 5-6 km and marks the predominantly aseismically slipping megathrust areas of which only the shallower part can host earthquake slip. Historic and recent megathrust earthquakes in the SW Kodiak Asperity and Semidi Segment generally occur within the thin and/or transitional reflection bands. In contrast, the 2020 Simeonof and earlier M7.x earthquakes in the Shumagin Gap are estimated to also rupture the shallowest section of megathrust characterized by the thick reflection band (~35-41 km depth). Although the shallow part of the plate boundary ( in the Semidi and Shumagin segments did not rupture in recent 2020 M7.8 Simeonof and 2021 M8.2 Chignik earthquakes, the seismic properties of the megathrust suggest it may be capable of earthquake slip in the future, as may have occurred in 1788.