Reconciling viscoelastic models of postseismic and interseismic deformation: Effects of viscous shear zones and finite length ruptures

We have developed a suite of earthquake cycle models for strike-slip faults to investigate how finite ruptures and lithosphere-scale viscous shear zones affect interseismic deformation. In particular, we assess whether localized and stationary interseismic deformation and large-scale, rapidly decaying postseismic transients may be explained with models incorporating either or both of these features. Models incorporating viscous shear zones give more stationary interseismic deformation than layered half-space, Maxwell viscoelastic models producing similar early postseismic deformation in the near field. This tendency is accentuated when the effective viscosity per unit width of the shear zone increases either with depth or with interseismic time. Models with finite (200 km long) ruptures produce time-dependent deformation similar to that from models with infinitely long ruptures, though with smaller-magnitude and somewhat more localized surface velocity fluctuations. Models incorporating a stiff lithosphere, a low-viscosity mantle asthenosphere, and a crust- to lithosphere-scale viscous shear zone can replicate postseismic and interseismic deformation typical of large, strike-slip earthquakes. Such models require depth-dependent, power law, or transient rheologies for the viscous shear zone material in the crust, as long as the effective viscosity per unit shear zone width is approximately 1015 Pa s/m at crustal depths during the postseismic interval. Below the Moho, the shear zone effective viscosity per unit width must be higher throughout the seismic cycle, at least over a short depth interval. For example, if shear zone viscosity per unit width is 5×1016 Pa s/m in the mantle lithosphere, a model with a 50 km thick lithosphere and asthenosphere effective viscosities of 1 to 5×1018 Pa s can reproduce reference velocity profiles for both postseismic and later interseismic deformation.