Postseismic Deformation and Afterslip Evolution of the 2015 Gorkha Earthquake Constrained by InSAR and GPS Observations

Abstract In the past few years, space‐geodesy has measured continued postseismic deformation in the region of the 2015 Gorkha Mw 7.8 earthquake. Previous studies have suggested that the postseismic deformation is dominated by afterslip, but the spatiotemporal evolution of the afterslip, hence its cause and impact on regional seismicity are disputed, and contribution from other postseismic processes has not been well constrained. Here, we constructed an afterslip model using InSAR and GPS observations over three years after the Gorkha earthquake. First, the Sentinel‐1A satellite data were processed using Permanent Scatterer Interferometric Synthetic Aperture Radar method to extract deformational signals at high‐coherence points, and a fitting method was used to extract the postseismic displacement sequences. We then used InSAR and GPS data in a joint inversion for the evolution of afterslip, and compared its contribution to the observed postseismic displacement to that of viscoelastic relaxation and poroelastic rebound. Our results confirm that afterslip has dominated the observed postseismic deformation in the Gorkha region. Within 3 years of the Gorkha earthquake, afterslip released ∼1.20 × 10 20 N m, about 14% of that released by the mainshock and the largest aftershock, or equivalent to an Mw 7.32 earthquake. The afterslip has occurred mostly downdip of the coseismic slip zone, in the ductile shear zone, apparently controlled by rheological properties. The updip shallow portion of the Main Frontal Thrust (MFT) zone has not been ruptured by coseismic slip or unlocked by afterslip, thus could potentially produce future earthquakes.