Post-seismic motion following the 1997 Manyi (Tibet) earthquake: InSAR observations and modelling

On November 8 1997 a M-w 7.6 earthquake occurred in the Manyi region of northern Tibet, near the western end of the Kunlun Fault. Over 7 m of left-lateral slip occurred on a 200-km-long fault. Here we use InSAR observations of post-seismic surface deformation following the Manyi earthquake to investigate possible causal mechanisms. Time-series of deformation are constructed from 26 interferograms, covering the entire length of the fault for nearly 4 yr after the earthquake. Three different modelling approaches are used to try and understand the observed variations in surface displacement. First order poroelastic models predict displacement fields which do not match those observed. Modelling of viscoelastic stress relaxation in a half-space with standard linear solid rheology beneath an elastic lid provides a reasonable fit to the observations, and demonstrates that two relaxation times are needed to characterize the post-seismic transient. The best-fitting viscosity is 4 pound 10(18) Pa s, and the fully-relaxed shear modulus is 32 per cent of the purely elastic value. A model with a Maxwell viscoelastic half-space underlying the lid cannot explain the observations: at later times, larger viscosities are required than at earlier times. This increase in effective viscosity with time may be consistent with stress relaxation occurring in a power-law rheology. The time-series are also inverted for distributed afterslip on an extension of the coseismic rupture. Along-strike correlation of coseismic and post-seismic slip maxima suggests that afterslip is a plausible mechanism. The maximum slip in the afterslip model is 0.72 m after 3 yr, and the equivalent moment release is approximately 20 per cent of the coseismic moment. This study shows that it is possible to rule out certain mechanisms as the dominant post-seismic process, but with the current InSAR data set it is difficult to distinguish between other plausible options.