[S-CG46] Uncovering stress accumulation and fault strengthening of megathrust earthquakes

[SCG46-10] Constraining Megathrust Coupling in the Shumagin Gap Region of the Aleutian Subduction Zone

★Invited Papers

*Matthew W Herman¹, Kevin P Furlong² (1.California State University Bakersfield, 2.Pennsylvania State University)

Keywords:Megathrust Coupling, Locking Model, Shumagin Gap, Seafloor Geodesy

A sequence of recent large earthquakes in the Aleutian subduction zone near the Shumagin Islands provides constraints on the nature of plate interface shear stress and slip deficit accumulation and release near this seismic gap. We have developed a suite of finite element subduction zone locking models to compare coupling patterns with observations of these earthquakes (the July 2020 M 7.8 Simeonof megathrust event, the October 2020 M 7.6 Sand Point intra-slab strike-slip event, the July 2021 M 8.2 Chignik megathrust event, and the July 2023 M 7.2 megathrust event). The pattern of megathrust coupling that satisfies seismic, geodetic, and geologic datasets includes an along-strike transition from high coupling on the plate interface east of the Shumagin Gap to low coupling in the Shumagin Gap. The combination of earthquake observations and model results also provides new insights towards other key questions about coupling in this region: Is the shallow megathrust (near the trench) coupled, and what is the associated earthquake/tsunami potential? And can we reconcile observations of very low inter-earthquake slip deficit accumulation with the occurrence of seismicity – including a large tsunamigenic earthquake in 1946 – along the western edge of the Shumagin Gap? Post-Chignik earthquake seafloor GNSS observations are compatible with low coupling on the shallow megathrust; this area accumulates slip deficit because of nearby locked zones and participates in co-seismic and/or post-seismic slip when earthquakes rupture adjacent locked zones. Our modeling also indicates that relatively small areas of the shallow megathrust west of the Shumagin Gap can be locked and still produce a minimal onshore signal. With no surrounding locked zones restricting slip in this region, these locked zones could consistently produce up to M ~7.5 earthquakes, although reaching M ~8.0 with such small locked zones is challenging. Our results indicate the broader potential for integrated earthquake cycle observations and physics-based locking models to reveal shallow subduction megathrust shear stress and slip evolution.