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

Elizabeth H. Hearn, & Wayne R. Thatcher

Published April 30, 2015, SCEC Contribution #1956

We have developed a suite of earthquake-cycle models for strike-slip faults, to in- vestigate whether localized and stationary interseismic deformation and large-scale, rapidly de- caying postseismic transients may be explained with models incorporating finite ruptures and lithosphere-scale viscous shear zones. Models incorporating viscous shear zones give more sta- tionary interseismic deformation than layered halfspace, 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 inter- seismic time. Our models with finite (200-km-long) ruptures produce similar time-dependent deformation, though with smaller magnitude and somewhat more localized surface velocity fluc- tuations. Models incorporating a stiff lithosphere, a low-viscosity mantle asthenosphere, and a crust- to lithosphere-scale viscous shear zone can replicate postseismic and interseismic deforma- tion 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 10^15 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 η_sz/w is 5 x 10^16 Pa s /m in the mantle lithosphere, a model with a 50-km-thick lithosphere and as- thenosphere effective viscosities of 1 to 5 x 10^18 Pa s can reproduce reference velocity profiles for both postseismic and later interseismic deformation.

Citation
Hearn, E. H., & Thatcher, W. R. (2015). Reconciling viscoelastic models of postseismic and interseismic deformation: Effects of viscous shear zones and finite-length ruptures. Journal of Geophysical Research, 120(4), 2794-2819.