Postseismic processes in the region of the july 29, 2021 Chigni earthquake, Alaska. Part I: modeling results

Abstract

We analyze the postseismic processes in the region of the Mw 8.2 Chignik earthquake, which occurred on July 29, 2021. Using the seismic rupture surface model constructed in our previous paper (Konvisar et al., 2023), we have simulated the viscoelastic relaxation process. The results of the simulation have shown that reducing the viscosity of the asthenosphere to 1018 Pa ‧ s in the calculations gives displacement velocities close to those recorded at the GPS coastal points. However, the displacements on islands close to the earthquake source region differ significantly not only in magnitude but also in direction. At the same time, the constructed model of the postseismic creep is closely consistent with the GPS displacement data and with the LOS (line-of-sight) displacement map derived from radar images acquired from the descending orbit of Sentinel 1A satellite. We also analyze temporal variations of the gravity field in the earthquake region. The obtained coseismic anomaly agrees with the anomaly calculated from the rupture surface model. Due to the insufficiently long series of gravity models after the earthquake, it is not yet possible to isolate the postseismic anomaly. The analysis of the postseismic processes is continued in the second part of this work (Smirnov et al., 2024), where we present the compare the time evolution 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.