Postseismic deformation is considered to be a motion which is a response to the redistribution
of stresses induced by an earthquake. In other words, the motion is evidence of the relaxation
stress somewhere in the upper part of the lithosphere (*Ryder et al*, 2007). There are various
hypotheses suggesting the relaxation procedure, such as: afterslip on a discrete plane,
creep in a viscous or viscoelastic shear zone, viscoelastic relaxation in the lower
crust/upper mantle, and poroelastic rebound, etc. In this study, we will apply viscoelastic
modeling using the VISCO1D code (*Pollitz*, 1992), and compare different rheological
configurations with their responses to the postseismic surface deformation. The aim for doing
numerical modeling is to find out a relevant rheological forward model
that could explain the geodetic observations. In this way, we can understand the key
parameters, especially the viscosity, which control the deformation process following an earthquake.

*Shen et al.*, 2009 apply GPS and InSAR (Interferometric Synthetic Aperture Radar)
inversion to find the best fault geometry and slip distribution.
Here we simplify the Wenchuan coseismic fault geometry provided by
*Shen et al.*, 2009 into 5 fault segments with different fault properties. According
to coseismic studies (e.g., *Shen et al.*, 2009), all segments
(Pengguan fault, Beichuan fault, and Wenchuan-Maowen fault) of the Longmenshen fault strike about
229and dip to the NW. However, the dipping angle ranges from 43in the SW to nearly
vertical in the NE. Moreover, the sense of motion varies from thrusting in the SW to right
lateral strike-slip in the NE.
Most of the GPS stations were installed just
after the Wenchuan main shock. We estimate the one year postseismic slips in terms of linear least square
fit to the GPS time series. In this study, we consider viscoelastic rebound to be the main driving force
of the postseismic deformation. The viscoelastic model calculation is based on the software VISCO1D
(*Pollitz*, 1992). This program can build up the earth model with either elastic or viscoelastic
properties specified by users for Maxwell or Burgers rheological models. We consider the rheological
model suggested by *Ryder et al.*, 2007 and *Godard et al.*, 2009 as the starting conditions.
[1] *Ryder et al.*, 2007 study the post 1997 7.6 Manyi (Tibet) earthquake surface deformation
in terms of SAR interferometry. They assume a 15 km elastic lid over a viscoelastic half-space, and the
shear modulus is held constant at 5x Pa. Their best fitting viscosity of the half-space
(Maxwell rheology) is 7x Pa s. [2] *Godard et al.*, 2009 explore the viscoelastic
properties of the borders of Tibetan Plateau (the Sichuan and Tarim basins, for example) in terms of
thermomechanical modeling of continental lithosphere coupled with fluvial denudation. They propose a
viscoelastic model that has a 15 km elastic lid over the lower crust (viscosity: Pa s) and
the upper mantle with viscosity decreasing in depth. We simplify their proposed viscosity
model to include 4 layers: elastic lid, lower crust (viscosity: Pa s), upper-most mantle (viscosity: Pa s), and upper mantle (viscosity: Pa s).

Afterslip and poroelastic rebound are additional important mechanisms that contribute to
postseismic deformation. Poroelastic rebound generally shows relatively localized postseismic
slips near the fault rupture zone and is related to Poisson's ratio and shear modulus, etc.
(*Freed et al.*, 2006). Afterslip represents accelerated aseismic slip on and adjacent
to the main shock rupture. In this study, we also test for contributions from these two
processes to the postseismic deformation. The afterslip distribution inverted from the
postseismic GPS data can explain the near-field postseismic displacement in terms of afterslip
on the deeper part of the Beichuan fault. However, this model is not able to fit the far-field
transients, which might be dominated by the viscoelastic deformation. A first-order model of
poroelastic rebound in terms of changing the Poisson's ratio in coseismic forward dislocation
models from 0.29 (undrained) to 0.25 (drained) leads to surface deformation with only up
to 6 mm horizontal and 1 mm vertical deformation at near-field GPS stations. The modeled
poroelastic rebound can only contribute about 1/5 of the observed postseismic deformation
and thus cannot be the primary source of the postseismic deformation. Our future work will involve
careful exploration of how a combination of viscoelastic flow, poroelastic rebound and afterslip
produces the observed transients, so that we will be able to characterize the relative importance
of each mechanism at different scales of postseismic deformation and quantify the rheological
parameters of the underlying processes.

We thank Z.K. Shen for providing the GPS time series. This work is supported by NSF grant (EAR 0738298).

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Shen, Z.K., J. Sun, P. Zhang, Y. Wan, M. Wang, R. Bürgmann, Y. Zeng, W. Gan, H. Hiao, and Q. Wang, Slip maxima at fault junctions and rupturing of bariers during the 2008 Wenchuan earthquake, *Nat. Geosci., 2,* 718-724, doi:10.1038/NGEO636, 2009.

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