Intraslab Deformation in the 30 November 2018 Anchorage, Alaska, MW 7.1 Earthquake

Abstract

Anchorage, Alaska, was strongly shaken on 30 November 2018 by an MW 7.1 earthquake that ruptured within the underthrust Pacific plate at depths of from 45 to 65 km. Ground failures occurred in saturated lowlands filled with sediments, producing notable road damage, but there was limited structural damage in Anchorage, only ~12 km south of the epicenter. The earthquake has a normal faulting geometry with a shallowly dipping east‐west tension axis indicating intraslab deformation, likely between the underthrust Yakutat terrane and adjacent Pacific seafloor. Separate and joint inversions of teleseismic P and SH waves, regional strong ground motions, and GPS static displacements provide a weak preference for a westward steeply dipping rupture plane with up to 2 m of slip distributed over a single slip patch with dimensions of 20 × 20 km. The ~12 s long rupture expanded northward. Aftershocks occur at shallower depths than the mainshock slip zone. Plain Language Summary The earthquake that struck on 30 November 2018, causing damage in Anchorage, Alaska, involved a fault rupture within the Pacific plate, which is sinking into the mantle beneath Alaska along the convergence zone between the Pacific and North American plates. Anchorage was seriously damaged during the great 1964 Alaska earthquake, which had a magnitude of 9.2 and resulted from sudden sliding on the shallow plate boundary; far less damage was produced by the 2018 event, which had a magnitude of 7.1 and involved deeper deformation of the underthrust slab. There is a lateral change in the dip of the sinking plate with the thick, relatively buoyant oceanic plateau called the Yakutat terrane having shallow dip to the east of the earthquake while normal thickness oceanic crust dips more steeply to the west. The 2018 event was located in the central region of the slab distortion. Intraslab events of this type present significant earthquake hazard, but it is difficult to determine their likelihood of occurrence.