No abstract

Knowledge of lithospheric rheology can provide fundamental insights into crustal deformation near the Longmen Shan fault (LMSF). Based on viscoelastic deformation models constrained by interseismic geodetic observations, we obtain an optimal crustal shortening rate of 4.8 ± 0.4 mm/a across the LMSF and an upper mantle viscosity of 5.0 × 1020−21 Pa · s beneath eastern Tibet. More importantly, we find a high‐viscosity zone (>1021 Pa · s) in the lower crust beneath the LMSF, where the steady‐state viscosity is significantly higher than the transient viscosity derived from postseismic deformation. Further investigations with a power‐law rheology suggest that, due to the stress loading of the Wenchuan earthquake and the relaxation afterwards, the effective lower crustal viscosity decreases to ∼1018 Pa · s immediately after the earthquake and finally recovers to interseismic level (∼1021 Pa · s). Our results highlight the stress‐dependent behavior and the viscoelastic effect of rheological structure beneath the LMSF during the earthquake cycle.

Based on a viscoelastic earthquake-cycle deformation model, we revisit the fault locking of the central Himalayan thrust using geodetic data acquired in the past three decades. By incorporating the viscoelastic relaxation effect induced by stress buildup and release, our viscoelastic model is capable of explaining the far-field observation with similar fault locking width obtained in previous studies. Elastic models underestimate the far-field deformation and consequently underestimate the fault slip rate by attributing the far-field deformation to stable intraplate deformation. A steady-state viscosity of ∼1019  Pa·s is required for the lower crust beneath south Tibet to best fit the crustal velocity. The optimal slip rate and locking width of the central Main Himalayan Thrust are estimated to 18.8 ± 1.6 mm/a and 85 ± 2.1 km, respectively. The inferred fault locking width, along with the down-dip rupture extension of the 2015 Gorkha earthquake, agrees well with the identified mid-crustal ramp, which leads to an interpretation that the fault geometry of the central Himalayan thrust plays an important role on fault kinematics. Our results highlight that viscoelastic relaxation during the earthquake cycle should be incorporated for robust estimation of fault locking parameters and reasonable data fitting.