Postseismic motion after the 2001 MW 7.8 Kokoxili earthquake in Tibet observed by InSAR time series

On November 14th 2001, a Mw 7.8 earthquake occurred in the Kokoxili region of northern Tibet. The earthquake ruptured more than 400 km along the western part of the Kunlun fault with a maximum of 8 m left-lateral slip. In this paper, we use a multitemporal Interferometric SAR (InSAR) time series technique to map the postseismic motion following the large Kokoxili event. SAR data from Envisat descending orbits along five adjacent tracks covering almost the entire ruptured fault length are used to calculate the displacement time series for a period between 2 and 6 years after the earthquake. A peak-to-trough signal of 8 cm in the radar line of sight is observed during the period between 2003 and 2008. Two different mechanisms are employed to explain the observed surface displacements, namely afterslip and viscoelastic relaxation. The observations inverted for afterslip on and below the coseismic rupture plane shows that the maximum slip in the afterslip model is 0.6 m. The position of the maximum postseismic slip is located in the middle of two relatively high coseismic slip patches, which suggests that afterslip is a plausible mechanism. Models of viscoelastic stress relaxation in a Maxwell half-space give a best fitting viscosity for the mid-to-lower crust of 2-5 - 10(19) Pa s, and the principal postseismic relaxation process is due to viscous flow in the lower crust to upper mantle. However, the InSAR observations are incapable of distinguishing between localized (afterslip) and distributed (viscoelastic relaxation) deformation. And the lowest misfits are produced by mixed models of viscoelastic relaxation in the mantle below 70 km and afterslip in the crust. Modeling of viscoelastic relaxation in a Maxwell half-space, and also a mixed mechanism model, enables us to place an effective viscosity of 2 x 10(19) Pa s on the lower crust to mantle of northern Tibet.