Observations and Modeling of Coseismic and Postseismic Deformation Due To the 2015 M_w 7.8 Gorkha (Nepal) Earthquake

Abstract

We use space geodetic data to investigate coseismic and postseismic deformation due to the 2015 Mw 7.8 Gorkha earthquake that occurred along the central Himalayan arc. Because the earthquake area is characterized by strong variations in surface relief and material properties, we developed finite element models that explicitly account for topography and 3‐D elastic structure. We computed the line‐of‐sight displacement histories from three tracks of the Sentinel‐1A/B Interferometric Synthetic Aperture Radar (InSAR) satellites, using persistent scatter method. InSAR observations reveal an uplift of up to ∼70 mm over ∼20 months after the main shock, concentrated primarily at the downdip edge of the ruptured asperity. GPS observations also show uplift, as well as southward movement in the epicentral area, qualitatively similar to the coseismic deformation pattern. Kinematic inversions of GPS and InSAR data and forward models of stress‐driven creep suggest that the observed postseismic transient is dominated by afterslip on a downdip extension of the seismic rupture. A poroelastic rebound may have contributed to the observed uplift and southward motion, but the predicted surface displacements are small. We also tested a wide range of viscoelastic relaxation models, including 1‐D and 3‐D variations in the viscosity structure. Models of a low‐viscosity channel previously invoked to explain the long‐term uplift and variations in topography at the plateau margins predict opposite signs of horizontal and vertical displacements compared to those observed. Our results do not preclude a possibility of deep‐seated viscoelastic response beneath southern Tibet with a characteristic relaxation time greater than the observation period (2 years).

Changes in Groundwater Level Possibly Encourage Shallow Earthquakes in Central Australia: The 2016 Petermann Ranges Earthquake

“…Considering that the poroelastic relaxation contributes most to the vertical displacement ( Figure S10) and our data are dominated by uplift, we select a poroelastic rebound model with a difference between undrained and drained Poisson's ratio of 0.06 that provides the best fit (Figures 3f and 3g). The estimated difference in Poisson's ratios is consistent with that of previously published studies (Table S5; see also Fialko, 2004;Fielding et al, 2009;Jónsson et al, 2003;Peltzer et al, 1998;Wang & Fialko, 2018). The maximum estimated value of 1.2 and 1.3 cm for Sentinel-1 and ALOS-2 data seen near the bends along the fault agrees well with the observations (Figures 3a and 3b).…”

“…Localized residuals are seen close to small-scale fault bends within an~1-km-wide zone due to heterogeneous shallow slip and possible inelastic off-fault ground deformation ( Figure S2). The time decaying behavior approximately conforms to the expected exponential time-dependent evolution of postseismic deformation, indicating that the results are robust although a relatively strong smoothing factor was adopted in the Sentinel-1 time series (e.g., Wang & Fialko, 2018). The deformation pattern is similar to that of the coseismic rupture with dominant hanging wall uplift and is characterized by high gradients close to the fault bends.…”

Structural Control on Downdip Locking Extent of the Himalayan Megathrust

“…Thus, while a residual viscoelastic transient from large megathrust earthquakes in Nepal could bias the inferred long‐term slip rate (by making it appear too large) or the depth of the coupling transition (by making it appear too shallow), it should not bias the inferred horizontal location of the coupling transition. We note that the late postseismic velocity anomaly modeled here is different from the immediate postseismic deformation following the 2015 Gorkha earthquake, which is dominated by afterslip in the downdip area just to the north of the rupture zone and therefore has a much less symmetric pattern (e.g., Gualandi et al, ; Wang & Fialko, ; Zhao et al, ).…”

Geometric Controls on Pulse‐Like Rupture in a Dynamic Model of the 2015 Gorkha Earthquake

“…The shallow velocity‐strengthening zone (i.e., on the updip ramp) in our model serves as a barrier to rupture on the coseismic timescale (~60 s). The actual geophysical mechanism restricting the upward extent of rupture is not known and may instead be related to insufficient stress accumulation on a fully coupled shallow region (Gualandi et al, ; Michel et al, ; Stevens & Avouac, ; Wang & Fialko, ). Our simulated ruptures are not affected by the choice of confinement mechanism (e.g., velocity strengthening versus low stress accumulation).…”