Postseismic Deformation and Afterslip Evolution of the 2015 Gorkha Earthquake Constrained by InSAR and GPS Observations. Issue 7 (28th June 2021)

• Record Type: Journal Article
• Authors: Hong, Shunying; Liu, Mian
• Is Part Of: Journal of geophysical research. Volume 126:Issue 7(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-06-28
• Subjects: Gorkha earthquake -- afterslip -- postseismic deformation -- PS_InSAR -- GPS; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2020JB020230
• Languages: English
• ISSNs: 2169-9313

Abstract

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. Plain Language Summary: The Himalayan mountain arc is bounded by the MFT, a fault zone between the Eurasian plate and the underthrusting Indian plate. Numerous large earthquakes have been recorded along the MFT, including the April 25, 2015 Gorkha Mw7.8 earthquake that ruptured a section of the MFT in Nepal. Space geodesy has measured continued ground deformation since the Gorkha earthquake. Possible causes of the postseismic deformation include afterslip, viscoelastic relaxation, and poroelastic rebound, but their relative roles remain controversial. In this study, we used InSAR and GPS measurements to constrain the spatiotemporal evolution of afterslip; we also compared its contribution to postseismic deformation with that from viscoelastic relaxation and poroelastic rebound. Our results suggest that afterslip has been the main cause of the observed postseismic deformation in the Gorkha region. Afterslip occurred mainly downdip of the coseismic rupture zone, in the ductile crust. It has released additional seismic moment equivalent to a magnitude 7.3 earthquake. The shallow part of the MFT remains locked, thus could potentially produce future earthquakes. Key Points: We obtained ∼3 yr postseismic displacement sequences following the 2015 Gorkha earthquake by PS_InSAR analysis and fitting method Afterslip has been dominating near‐field postseismic deformation; it has released seismic moment equivalent to a MW 7.32 earthquake Afterslip occurred mainly in the ductile shear zone downdip of the coseismic rupture zone