Fast and slow intraplate ruptures during the 19 October 2020 magnitude 7.6 Shumagin earthquake

Strong tsunami excitation from slow rupture of shallow subduction zone faults is recognized as a key concern for tsunami hazard assessment. Three months after the 22 July 2020 magnitude 7.8 thrust earthquake struck the plate boundary below the Shumagin Islands, Alaska, a magnitude 7.6 aftershock ruptured with complex intraplate faulting. Despite the smaller size and predominantly strike-slip faulting mechanism inferred from seismic waves for the aftershock, it generated much larger tsunami waves than the mainshock. Here we show through detailed analysis of seismic, geodetic, and tsunami observations of the aftershock that the event implicated unprecedented source complexity, involving weakly tsunamigenic fast rupture of two intraplate faults located below and most likely above the plate boundary, along with induced strongly tsunamigenic slow thrust slip on a third fault near the shelf break likely striking nearly perpendicular to the trench. The thrust slip took over 5 min, giving no clear expression in seismic or geodetic observations while producing the sizeable far-field tsunami. The 19 Oct 2020 M-W 7.6 Shumagin earthquake involved unprecedented source complexity with two fast ruptures straddling the megathrust and strong tsunami excitation from a long-duration upper plate thrust rupture undetected by seismic and geodetic data.

Automated summary

Three months after a magnitude 7.8 thrust earthquake struck the Shumagin Islands, a magnitude 7.6 aftershock occurred, generating larger tsunami waves than the mainshock. Detailed analysis of seismic, geodetic, and tsunami observations revealed an unprecedented source complexity, involving weakly tsunamigenic fast rupture of two intraplate faults and strongly tsunamigenic slow thrust slip on a third fault near the shelf break.