Dominant Controls of Downdip Afterslip and Viscous Relaxation on the Postseismic Displacements Following the M_w7.9 Gorkha, Nepal, Earthquake

Abstract

We analyze three‐dimensional GPS coordinate time series from continuously operating stations in Nepal and South Tibet and calculate the initial 1 year postseismic displacements. We first investigate models of poroelastic rebound, afterslip, and viscoelastic relaxation individually and then attempt to resolve the trade‐offs between their contributions by evaluating the misfit between observed and simulated displacements. We compare kinematic inversions for distributed afterslip with stress‐driven afterslip models. The modeling results show that no single mechanism satisfactorily explains near‐ and far‐field postseismic deformation following the Gorkha earthquake. When considering contributions from all three mechanisms, we favor a combination of viscoelastic relaxation and afterslip alone, as poroelastic rebound always worsens the misfit. The combined model does not improve the data misfit significantly, but the inverted afterslip distribution is more physically plausible. The inverted afterslip favors slip within the brittle‐ductile transition zone downdip of the coseismic rupture and fills the small gap between the mainshock and largest aftershock slip zone, releasing only 7% of the coseismic moment. Our preferred model also illuminates the laterally heterogeneous rheological structure between India and the South Tibet. The transient and steady state viscosities of the upper mantle beneath Tibet are constrained to be greater than 1018 Pa s and 1019 Pa s, whereas the Indian upper mantle has a high viscosity ≥1020 Pa s. The viscosity in the lower crust of southern Tibet shows a clear trade‐off with its southward extent and thickness, suggesting an upper bound value of ~8 × 1019 Pa s for its steady state viscosity.

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

“…Assuming that the evolution of afterslip is governed by the rate and state friction law, Gualandi et al (2016) estimated that after full relaxation, the moment release due to afterslip would amount to approximately one third of the seismic moment of the main shock. This is in good agreement with results of Zhao et al (2017) but a factor of ∼2-3 smaller than the moment release in the limit of full relaxation predicted by Gualandi et al (2016). This is in good agreement with results of Zhao et al (2017) but a factor of ∼2-3 smaller than the moment release in the limit of full relaxation predicted by Gualandi et al (2016).…”

“…The inferred afterslip distribution is similar to the results of previous studies that used GPS observations spanning shorter epochs (e.g., Gualandi et al, 2016;Mencin et al, 2016;Zhao et al, 2017). The inferred afterslip distribution is similar to the results of previous studies that used GPS observations spanning shorter epochs (e.g., Gualandi et al, 2016;Mencin et al, 2016;Zhao et al, 2017).…”

“…The lower bound on the effective viscosity of the ductile substrate in the northwestern Himalaya estimated by Wang and Fialko (2014) was 10 19 Pa s, although their model did not consider possible lateral variations in the rheological structure (e.g., Bendick et al, 2015). Zhao et al (2017), based on analysis of GPS data from the Nepal network as well as a few additional sites in southern Tibet over a time period of 1 year after the 2015 Gorkha earthquake, estimated the long-term viscosity beneath the southern Tibet of ∼10 19 Pa s, consistent with the lower bound estimates of this study. The effective viscosities in the interior of the Tibetan Plateau constrained by geodetic observations of postseismic transients are thus much higher than the values of 10 16 -10 17 Pa s implied by 1-D fluid mechanical models relating the topography variations across the plateau margins to viscosity of the channel material (e.g., Clark & Royden, 2000;Royden et al, 1997).…”

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, ).…”

Can the Updip Limit of Frictional Locking on Megathrusts Be Detected Geodetically? Quantifying the Effect of Stress Shadows on Near‐Trench Coupling

“…Based on thermal models of the megathrust, the fault should reach a temperature of 80°C at 4-5 km depth or 22-28 km from the frontal fault (which we consider analogous to the trench in a subduction zone; Ader et al, 2012;Herman et al, 2010). If we consider the updip limit of the Gorkha rupture as the maximum extent of the frictionally locked portion of the fault, then W F /W T = 0.4, although we note that almost no afterslip has been observed updip of the Gorkha earthquake in 2015, suggesting that this part of the fault is velocity weakening (e.g., Mencin et al, 2016;Zhao et al, 2017). If we consider the updip limit of the Gorkha rupture as the maximum extent of the frictionally locked portion of the fault, then W F /W T = 0.4, although we note that almost no afterslip has been observed updip of the Gorkha earthquake in 2015, suggesting that this part of the fault is velocity weakening (e.g., Mencin et al, 2016;Zhao et al, 2017).…”