Inference of Lower Crustal Viscosity in Tibet from Geodetically Constrained Earthquake Cycle Models

Geodetic observations of deformation across the Tibetan plateau contain information about both tectonic and earthquake cycle processes. Time-dependent variations in surface velocities may occur as a result of stress relaxation in a weak lower crustal channel underneath the plateau. Previous interpretations of interseismic global positioning system (GPS) velocities in Tibet have been predicated on the assumption that the viscosity of the lower crust is the same as that of the upper mantle, and have inferred viscosities of ~10^20 Pa⋅s. We derive a semi-analytic solution for time-dependent surface velocities resulting from linear viscoelastic stress relaxation in a lower crustal channel in response to periodic earthquake forcing. Earthquake cycle models with a weak lower crustal channel reveal substantially more near-fault strain localization than do classical two-layer models. Applying a channel model to geodetic data obtained before and after the 2001 Mw=7.8 Kokoxili earthquake on the Kunlun fault and the 1997 Mw=7.6 Manyi earthquake on a western splay of the Kunlun fault, we find that stress relaxation in a lower crustal channel with a viscosity as low as 10^18 Pa⋅s can simultaneously explain both near-fault velocity gradients late in the earthquake cycle and rapid (>90 mm/yr) postseismic velocities.