'Double Puzzle' at the Shumagin seismic gap, Alaska Peninsula: intraslab strike-slip faulting loaded by lateral variations in megathrust fault friction

Along the Aleutian megathrust, the Shumagin seismic gap (162 degrees W-158.5 degrees W) had not hosted a large megathrust earthquake during the observational period 1946-2020. Geodetic evidence suggests a prominent trench-parallel transition from strong to weak kinematic coupling in this segment, indicating varying frictional properties of the megathrust. In 2020 July and October, the occurrence of two large, but dissimilar earthquakes in this seismic gap (the M-w 7.8 Simeonof Island thrust event on 2020 July 22 followed by the M-w 7.6 Sand Point intraslab strike-slip event on 2020 October 19) presented a unique opportunity to examine the interaction between stresses on the megathrust and within the downgoing slab. We use geodetic and geophysical evidence to derive a more accurate kinematic coupling model of the megathrust in this area and show that the M-w 7.6 Sand Point earthquake within the downgoing Pacific slab, likely occurred as a result of both trench-perpendicular interseismic shear stress caused by variable kinematic coupling of the megathrust and coseismic stress changes resulting from the Simeonof Island event. Furthermore, we show that the location of the strike-slip event coincides with along-strike change in the megathrust gravitational anomaly and flexural bending of the downgoing plate, suggesting a long-term interaction between megathrust frictional properties and the structure of the plate interface and downgoing slab.