Mechanisms of transient postseismic deformation following the 2001 Mw 7.8 Kunlun (China) earthquake

摘要

Using global positioning system (GPS) technology, significant postseismic surface displacements were observed within the first 4 months after the 2001 Mw 7.8 Kunlun earthquake which occurred in China. In this study, we investigated the mechanisms that may have possibly contributed to the postseismic deformations that have been observed. Based on the modeling results, we find that an after slip model can interpret postseismic displacements in the near field even when the fault plane is extended to the bottom of the crust (~70 km). Models based on the viscoelastic relaxation theory showed a large discrepancy in the spatial pattern of the deformation compared with what has been observed. Thus, we infer that both mechanisms cannot interpret the observed postseismic deformation independently. A combination of after slip and viscoelastic relaxation can further improve the data fit, especially at sites far from the fault. With maximum after slip of ~0.4 m occurring at a depth of 10 km in the central section, the combined model shows that the estimated after slip occurred mostly on and below the coseismic rupture plane, as well as on its eastern extension. The estimated moment released by the after slip in the first 4 months is almost 40% of that released by the coseismic slip. The best-fitting viscoelastic relaxation model shows a "weak" upper mantle with a viscosity of ~1.0 × 10~(18) Pa s. The combined model also suggests the existence of a lower crust with viscosity larger than 1.0 × 10~(18) Pa s, although it cannot be constrained accurately.