Early Postseismic Deformation of the 29 July 2021 Mw8.2 Chignik
Earthquake Provides New Constraints on the Downdip Coseismic Slip
Abstract
On 29 July 2021, an Mw 8.2 megathrust earthquake struck the Alaska
Peninsula. Quantifying the coseismic slip and the afterslip that
followed this earthquake provides us the opportunity to clarify the
megathrust slip budget and the earthquake hazard potential there.
However, the estimated coseismic slip distribution inversion result is
strongly affected by assumptions made in the inversion. The spatial
pattern of stress-driven afterslip is mainly controlled by the coseismic
slip distribution, so that it can provide new information about the
coseismic slip distribution and is useful to assess the assumptions made
in the coseismic inversion. The orientation and relative magnitudes of
postseismic displacements at sites on the Alaska Peninsula require that
the afterslip be concentrated ~130km from the trench. As
a result, coseismic slip models including slip at that distance or less
to shore, predict postseismic deformation that systematically misfits
the observations. A narrower coseismic rupture plane with an abrupt
downward termination of slip provides a much better fit to the observed
postseismic signal than models where the slip tapers gently with depth.
We considered multiple different viscoelastic relaxation models and find
that these conclusions about the coseismic model are required regardless
of the viscoelastic relaxation models used. The contribution of
viscoelastic relaxation to the observed signal is not negligible, and
the early postseismic observations are best reproduced with a model that
features a 50 km thick elastic lithosphere for the overriding plate, and
an elastic cold nose to the mantle wedge.