Postseismic Processes in the Region of the July 29, 2021 Chignik Earthquake, Alaska: Part I. Modeling Results

Abstract—We analyze the postseismic processes in the region of the M_w 8.2 Chignik earthquake, Alaska, which occurred on July 29, 2021. Using the model of the seismic rupture surface constructed in our previous work (Konvisar et al., 2023), we have modeled the viscoelastic relaxation process. The results indicate that by reducing the viscosity of the asthenosphere to 10¹⁸ Pa s we an achieve the displacement velocities that are close to those recorded at the coastal GPS sites. However, the displacements on the islands near the source zone of the earthquake differ significantly not only in magnitude but also in direction. At the same time, the constructed postseismic creep (afterslip) model agrees well with displacement data at the GPS sites and with the line-of-sight (LOS) displacement map derived from radar images taken from the descending orbit of the Sentinel-1A satellite. We also analyze temporal variations of gravity field in the earthquake region. The obtained coseismic anomaly is consistent with the anomaly calculated from the rupture surface model. Isolating the postseismic anomaly it is not yet possible due to the insufficiently long time series of gravity models for the time period after the earthquake. The analysis of the postseismic processes is continued in the second part of this work (Smirnov et al., 2024), where we compare development over time of the postseismic displacements of various GPS sites with the aftershock activity, which allows us to draw conclusions about the creep nature of the postseismic processes in the source region of the Chignik earthquake.