Coseismic and early postseismic deformation due to the 25 April 2015, M_w 7.8 Gorkha, Nepal, earthquake from InSAR and GPS measurements

Abstract

Analysis of Interferometric Synthetic Aperture Radar (InSAR) and Global Positioning System (GPS) data reveals coseismic and early postseismic (4–88 days) surface displacements associated with the 25 April 2015, Mw 7.8 Gorkha, Nepal, earthquake. The pattern of early postseismic surface uplift and subsidence is found to be opposite to that of the coseismic motion. InSAR and GPS data were jointly inverted for coseismic and postseismic slip on the Main Himalayan Thrust (MHT). We consider a complex ramp‐flat‐ramp‐flat subsurface structure of the MHT with four connected fault planes dipping toward north from the Main Frontal Thrust (MFT). The inverted coseismic slip distribution follows an elliptical pattern, extending east‐southeastward from the hypocenter with maximum amplitude of 5.7 m above the upper edge of the midcrustal ramp. We infer early postseismic afterslip (4–16 days) of 0.2–0.47 m toward downdip of the coseismic slip asperity and another patch with 0.1–0.2 m slip toward east. The shallow portion of the MHT toward south is found to have remained unruptured during the earthquake, nor did it slip aseismically after the earthquake, suggesting possibility of large events in the future.

Constraints on Source Parameters of the 25 April 2015, M_w = 7.8 Gorkha, Nepal Earthquake from Synthetic Aperture Radar Interferometry

“…The modelled slip value on the fault is comparable with the average of the distributed slip values (0-6 m) derived using inversion of InSAR and GPS data by Lindsey et al 27 , and Wang and Fialko 30 , though their model has shallower dip (7-10). However, we notice that the steeper dip angle ~15 obtained in the present study is analogous to that of Sreejith et al 31 . A more detailed discussion on the implications of steeper dip angle on earthquake nucleation in frontal Himalaya is provided later in the text.…”

Dominant Controls of Downdip Afterslip and Viscous Relaxation on the Postseismic Displacements Following the Mw7.9 Gorkha, Nepal, Earthquake

“…Several afterslip models have been published to understand the earlier postseismic mechanisms based on GPS observations in Nepal and/or very short intervals of InSAR data (Gualandi et al, ; Mencin et al, ; Sreejith et al, ). These studies only consider afterslip alone as the postseismic mechanism and ignore contributions from other mechanisms including viscous relaxation and poroelastic rebound.…”

“…These studies only consider afterslip alone as the postseismic mechanism and ignore contributions from other mechanisms including viscous relaxation and poroelastic rebound. Sreejith et al () inferred a downdip afterslip distribution based on kinematic inversion of only 12 days of InSAR data and 13 days of GPS observations at four Nepal stations starting 4 days after the mainshock, finding a gap between the coseismic and afterslip slip zones. Such a slip pattern with a large spatial separation of the coseismic and afterslip is hard to understand using physically reasonable stress‐driven afterslip models, which shows that slip should occur immediately beneath the main asperity (Figure b).…”

“…Several studies have explored the early postseismic deformation following the Nepal earthquake using GPS and/or InSAR data. All these works relied on afterslip modeling to explain the transient postseismic deformation (Gualandi et al, ; Mencin et al, ; Sreejith et al, ), ignoring the effects from viscous relaxation in the lower crust and upper mantle and potential contributions from poroelastic relaxation in the shallow crust.…”

Coseismic Traveling Ionospheric Disturbances during the Mw 7.8 Gorkha, Nepal, Earthquake on 25 April 2015 From Ground and Spaceborne Observations

“…Joint inversion of teleseismic, geodetic, and waveform data sets suggests that the rupture front velocity of 3.3 km/s controlled the timing of the rupture propagation toward southeast direction (Kobayashi et al, ; Koketsu et al, ). The maximum coseismic slip due to thrust faulting occurred at the hypocenter was about 5.7 m at a depth of ~12 km (Sreejith et al, ). The surface deformations observed from coseismic InSAR (interferometric synthetic aperture radar) interferograms and ground‐based GPS observations show an uplift of ~1–1.3 m near Khatmandu (central Nepal) and a subsidence of ~0.5–0.8 m on the northern region of Nepal as shown in Figure (after Lindsey et al, ).…”