Rheology of the Northern Tibetan Plateau Lithosphere Inferred from the Post-Seismic Deformation Resulting from the 2001 Mw 7.8 Kokoxili Earthquake

Time-series observations from Sentinel-1 A/B InSAR spanning November 2014 to July 2021 were used to study the late post-seismic deformation velocity field arising from the Kokoxili earthquake. The deformation velocity caused by the interseismic slip along the major active faults in Tibet was first simulated. Comparing the simulated deformation velocity with the observed one, the maximum ratio of the simulated deformation velocity to the observed one was found to be 42%, indicating continuity in the viscoelastic relaxation caused by the 2001 Kokoxili earthquake. Subsequently, the rheological structure of the Kokoxili region was explored using a mixed model comprising the viscoelastic relaxation mechanism and the buried elastic dislocation model. The best estimated viscosities for the lower crust and upper mantle were eta(lc)=1(-0.44)(+0.78)x10(19) Pas and eta(um)=1(+0)(+0.78)x10(20) Pas, respectively. The results obtained in this study were compared with those of previous studies that used the early post-seismic displacement ranging from 0 to 6.5 years following the earthquake. The obtained value was largely the same as the previously estimated steady-state viscosity, which means that the viscosities of the viscoelastic layer beneath the Kokoxili regions have almost reached their stable state. Furthermore, the effective lower crustal viscosity of the Kokoxili region exhibited a logarithmic trend with time.