Intraslab Deformation in the 30 November 2018 Anchorage, Alaska, M_W 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.

Structural Heterogeneity in Source Zones of the 2018 Anchorage Intraslab Earthquake and the 1964 Alaska Megathrust Earthquake

“…C. Liu et al () suggested that the causative normal fault of the Anchorage earthquake is nearly N‐S oriented and dips westward with an angle of 65°. Southeast of this earthquake, focal mechanisms of most intraslab events during 2007 to 2008 in the Kenai Peninsula show also normal faulting, and their tensional axes are rotated dominantly in the E‐W direction rather than simply the slab dip direction (J. Li et al, ).…”

“…The USGS‐NEIC focal mechanism of the 2018 Anchorage earthquake shows that it was a normal‐faulting event occurring at 46.7‐km depth within the subducting Pacific slab (Figure ). The preferred fault model of C. Liu et al () shows that the causative fault strikes nearly N‐S and dips 65° westward. Previous focal mechanism studies (J. Li et al, ; Lu et al, ) revealed that intraslab earthquakes in this area are dominantly normal‐faulting type with an E‐W T‐axis and thus suggested that they are related to slab bending.…”

Coseismic Rupture Geometry and Slip Rupture Process During the 2018 Mw 7.1 Anchorage, South‐Central Alaska Earthquake: Intraplate Normal Faulting by Slab Tear Constrained by Geodetic and Teleseismic Data

“…In the second step, we only invert the slip distribution by fixing the fault geometry derived from step one. Integrating both teleseismic and geodetic data, we solve the slip following a kinematic inversion finite‐fault strategy (Ji et al, ) with simulated annealing method (Sen & Stoffa, ), and we weight them equally (Liu et al, ).…”

“…Integrating both teleseismic and geodetic data, we solve the slip following a kinematic inversion finitefault strategy (Ji et al, 2002) with simulated annealing method (Sen & Stoffa, 1991), and we weight them equally (Liu et al, 2019).…”

“…In addition, geodetic inversion of fault slip can be oversmoothed (particularly for small, deep events) such that they are too smooth to explain the teleseismic data (Pritchard et al, ). Therefore, the complimentary nature of geodetic and teleseismic data has been simultaneously considered in most recent publications (e.g., Liu et al, , ; Yue, ), resulting in tight constraint for the slip distribution. However, given that the teleseismic data and geodetic data have different sensitivities to coseismic rupture properties (Pritchard et al, , ), a mature strategy to determine the reliable focal mechanism is fault geometry inverted by uniform model with geodetic data first, then the spatial distribution of fault slip tightly constrained by integrating teleseismic and geodetic data (Ding et al, ).…”