Postseismic deformation following the April 25, 2015 Gorkha earthquake (Nepal): Afterslip versus viscous relaxation

Example of observed and simulated postseismic time series and test of mechanisms controlling the postseismic deformation, afterslip along the northern part of the upper flat, the ramp and the flower flat of the Main Himalayan Thrust (blue) or a combination of afterslip along the northern part of the upper flat and the ramp of the MHT and a viscous relaxation controlled by a low viscosity body centered on the lower flat of the MHT (red). The simulated time series are respectively drawn in blue and red. [Display omitted] •Postseismic deformation of the 2015 Gorkha earthquake with GNSS time series.•Modeling shows afterslip along the northern part of upper flat and ramp of the MHT.•Modeling shows afterslip or a viscous relaxation along the lower flat of the MHT. The postseismic deformation consecutive to the April 25, 2015 Gorkha earthquake (Mw 7.9) is estimated in this paper based on a cGNSS network installed prior to the earthquake and supplemented by 6 cGNSS stations installed after the main shock. Postseismic displacement are obtained from daily time series corrected for interseismic deformation and seasonal variations. The maximum postseismic displacement is found north of the rupture area, where locally it reached 100 mm between the date of the earthquake and late 2016. The postseismic deformation affects the northern part of the rupture area but not the southern part, along the southern part of the Main Himalayan Thrust (MHT). Three hypotheses for the mechanisms controlling postseismic deformation are tested through numerical simulations of the postseismic time series: (i) viscous relaxation, (ii) afterslip, or (iii) a combination of these two mechanisms. We can exclude postseismic deformation controlled by viscous relaxation of a thick deformation zone along the northern and lower flat of the MHT. However, it is impossible to discriminate between postseismic deformation controlled by either afterslip along the MHT (northern part of the rupture zone, crustal ramp, and lower flat of the MHT) or a combination of afterslip along the MHT (northern part of the rupture zone, crustal ramp) and viscous relaxation controlled by a thin (∼3–4 km thick) low-viscosity body centered on the lower flat of the MHT. The occurrence of afterslip along the northern part of the upper flat of the MHT and its longitudinal variations have been established thanks to the densification of GNSS network by our team presented in this paper.