Postseismic vertical deformation followingthe 1990 MS7.0 Gonghe earthquake

We model leveling deformation data observed on the Earth's surface following the 1990 MS7.0 Gonghe earthquake, Qinghai province, to infer the postseismic deformation source and mechanism. By analyzing the leveling data observed one epoch before and six epochs after the quake across the coseismic fault, we find the following characteristics of the postseismic deformation: ① the near field on the hanging wall side of the coseismic rupture continued to uplift at large scale after the earthquake, with the highest rate measured in the first year; ② the scope of postseismic uplifting region was larger than that of coseismic uplifting, but did not change during the postseismic deformation period; ③ the time series of elevation change between two adjacent benchmarks can be modeled by a logarithmic relaxation function or an exponential relaxation function, and the relaxation time constants are 0.165 years and 1.344 years, respectively. In the paper we also introduce a new method using leveling data and dislocation model in continuum to investigate the source and mechanism of postseismic deformation.The new method utilizes the raw observations of elevation differences between two adjacent benchmarks, not their integrals with respect to a reference point, to constrain a dislocation model in a continuum, effectively reducing biases introduced from cumulative errors due to data integration and making full use of the leveling data. Our preliminary result suggests that two mechanisms operate simultaneously to produce the postseismic vertical deformation of the surface: (a) afterslip on the coseismic rupture fault plane of the main shock and its peripheral extensions, particularly upward to the sediment layer above the main rupture, and (b) viscoelastic relaxation of the lower crust and upper mantle, with a viscosity in the order of 1020 74Pamiddot;s.