Triggered Shallow Megathrust Slip following the 2020 Mw7.6 Sand Point (Alaska) Strike-Slip Earthquake Explains its Tsunami

Abstract

On October 19, 2020, the Mw7.6 Sand Point earthquake occurred near the Shumagin Islands in Alaska. Moment tensor solutions suggest oblique strike-slip faulting, but the earthquake was accompanied by an uncharacteristically large tsunami. Prior models explain this with co-seismic megathrust slip release, or shallow very slow thrust slip in the overriding plate. Here, we analyze the most complete land-based data set available for this earthquake including teleseismic and high-rate GNSS waveforms, static GNSS and InSAR deformation, and relocated aftershock distributions. We test two models: a single strike-slip fault model, and a two-fault model consisting of strike-slip and thrust segments. While both satisfy the observations, neither model reproduces the tsunami well. To locate the tsunami source, we performed a systematic test of thrust-slip models. This suggests that delayed, very long period concentrated aseismic slip on the shallow megathrust caused the tsunami without generating measurable surface displacements. This occurred minutes after the mainshock, at a location with mainshock-induced Coulomb stress increase, suggesting a triggered tsunami earthquake. The partitioning of strike-slip and triggered megathrust slip offers opportunities to better understand tsunami earthquakes, and—if prevalent for megathrust earthquakes—might impact earthquake and tsunami hazards.