Rheological structure of lithosphere in northern Tibet inferred from postseismic deformation modeling of the 2001 Mw 7. 8 Kokoxili earthquake

The rheological structure of lithosphere in northern Tibet has been debated for decades. The 2001 M(w)7. 8 Kokoxili earthquake greatly changed the tectonic stress field in this area, providing an opportunity to address this issue by modeling the postseismic deformation at the Earth's surface. We collect GPS data observed after the quake, and use it to invert lithosphere rheological parameters and infer deformation mechanism in northern Tibet. GPS data from 45 sites, including one reference station from the Crustal Motion Observation Network of China, are processed to produce the time series. Data acquired after the 2001 Kokoxili earthquake and prior to the 2008 Yutian earthquake are used in the study, and most of the GPS sites are located in the near to mid field, and with at most 6. 4 years of observation history. We perform a joint inversion to solve for the viscous relaxation in lithosphere and afterslip on the fault simultaneously. The Bayan Har-Qiangtang region and Qaidam Basin, located south and north of the east Kunlun fault, respectively, are assumed to have different rheological structures in the lithosphere, and viscosities of lower crust and upper mantle on each side are assumed to be the same and the values are inverted through grid search. Afterslip is constrained by both observations and the Coulomb stress distribution on the fault. Our results show the secular viscosities of 1. 5 X 10(19) Pa . s and 1. 5 X 10(20) Pa . s of lower crust/upper mantle for south and north of the fault respectively. The transient viscosities of 1. 5 X 10(18) Pa . s and 5 X 10(18) Pa . s are also found respectively. These results reveal that viscosity of lower crust/upper mantle beneath the Bayan Har-Qiangtang region is much lower than that below the Qaidam Basin, implying possible partial melt in the lower crust of the Bayan Har-Qiangtang region. The deformation pattern in the Bayan Har-Qiangtang region agrees well with the distributed deformation model, while that of the Qaidam basin is more consistent with the block deformation model. Viscosity of lower crust in northern Tibet is 2 similar to 3 orders of magnitude higher than what the lower crustal flow model requires, suggesting that lower crustal flow likely does not exist in this region.