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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.
