Earthquake cycle deformation in the Tibetan plateau with a weak mid-crustal layer

Geodetic observations of interseismic deformation across the Tibetan plateau contain information about both tectonic and earthquake cycle processes. Time-variations in surface velocities between large earthquakes are sensitive to the rheologic structure of the sub-seismogenic crust, and, in particular, the viscosity of the middle and lower crust. Here we derive a semi-analytic solution for time-dependent surface velocities resulting from viscoelastic stress relaxation in a mid-crustal layer in response to forcing by periodic earthquakes. Earthquake cycle models with a mid-crustal layer exhibit substantially more pre-earthquake strain localization than do classic 2-layer models at short Maxwell times. We apply both this 3-layer channel-like model and the classic 2-layer model to geodetic observations before and after the 2001 M_W=7.8 Kokoxili and 1997 M_W=7.6 Manyi strike-slip earthquakes in Tibet in order to estimate the viscosity of the crust below a 20 km thick seismogenic layer. For these events, interseismic stress relaxation in a weak (viscosity ≤10^18.5 Pa s) and thin (height ≤20 km) mid-crustal layer explains observations of both near-fault strain localization late in the earthquake cycle and rapid (>50 mm/yr) postseismic velocities in the years following the coseismic ruptures. We suggest that the classic 2-layer model requires a rheology with multiple relaxation time scales to simultaneously explain both pre- and post-seismic observations while the channel-like model can explain the same behavior with a single Maxwell viscosity.