2.3 Campaign GPS Data
We also utilize campaign GPS data collected three weeks after the
Chignik earthquake, at survey marks that had long-term pre-earthquake
interseismic campaign measurements (Li and Freymueller, 2018). Eight
sites in the near field of the Chignik rupture were surveyed as part of
the Chignik earthquake repid respond activity from 08/17/2021 –
08/25/2021. These data were analyzed using the same methods as the
continuous site data to estimate daily positions. We estimated
displacemements that combine the coseismic displacement and 3 weeks of
posteismc displacement by fitting a model that included the
pre-earthquake trend and an offset at the time of the earthquake.
3 Methods
3.1 Coseismic Slip Inversions
We estimated the coseismic slip model using exactly the same data and
method used by Elliott et al. (2022), except for the variations in the
fault plane as noted below. The details of the inversion technique,
which is based on the method of Ji et al. (2002). are described by Xiao
et al. (2021). The epicenter of this event was taken from the U.S.
Geological Survey (USGS) estimate, and we pinned its depth (26.2 km) to
the slab depth as estimated by Slab2.0. The model fault plane is a local
average of the Slab2.0 geometry which has a strike angle of 235° and a
dip angle of 15°. All of the fault planes have an along-strike length of
320km, and the downdip widths of the fault vary from 100km to 160km. The
subfaults are all 10km by 10km squares, and we imposed zero slip
conditions around all edges of the model plane; this affects the
structure of the Laplacian smoothing operator at the fault edges. For
the bottom edge, we also specifically imposed zero slip on the deepest
row of subfaults. As a result, the effective wdith of the slip model is
10 km smaller than the width of the plane, and we identify models by
this smaller effective width where slip is allowed to happen.
The fault model plane used by Elliott et al. (2022) had a total width of
160km, which extended well beyond the likely maximum depth of the
coseismic rupture so that edge effects from the zero slip condition
would not affect the estimated model. In this study, we repeat the slip
inversion assuming alternate definitions of the fault plane, varying
either the geometry (depth, dip angle) or the spatial extent of the
model fault plane.