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.