Two-Staged Rupture of the 19 October 2020 Mw 7.6 Strike-Slip Earthquake Illuminated the Boundary of Coupling Variation in the Shumagin Islands, Alaska

Abstract

Abstract On 19 October 2020, an Mw 7.6 earthquake occurred within the Shumagin Islands, Alaska, which is the largest strike-slip earthquake occurred in the shallow subducted plates with abundant seismic observations. Here, we relocated the earthquake sequence, implemented back-projection analyses, and finite-fault inversion to investigate the source processes of the mainshock, and calculated mainshock focal mechanisms using the polarities of P waves and W-phase inversion, respectively. Our results show that the faulting of the mainshock can be divided into two segments with the initial rupture along a steep plane (strike = 15°, dip = 81°) and propagation southeastward along a more shallowly dipping plane (strike = 344°, dip = 48°). The inferred strikes of the mainshock faults are similar to the orientations of preexisting structures in the source region, likely indicating that the 2020 Mw 7.6 earthquake ruptured along the preexisting plate fabric in the downgoing plate. The fabrics are located at the boundary with significant variations of the plate coupling, indicating that these structures within the subducting plate may affect the interplate coupling or as a result of the varying interplate coupling in subduction zones.