Study Of The Post-seismic Gravity Changes Induced By 2004 Sumatra Earthquake(Mw9.3) Based On The Viscoelastic Spherical Dislocation Theory And GRACE Data

The crustal deformation induced by earthquakes is the important research area of the internal structure and the mechanism of earthquake occurrence. With the development of modern geodetic techniques, the post-seismic deformation caused by earthquakes can be observed by the GPS and In SAR. Besides, we can also monitor the co- and post-seismic gravity changes which have ~300km spatial resolution caused by huge subduction earthquake based on GRACE. Since been launched successfully in 2002, GRACE has continuously supplied abundant global gravity field data for more than 10 years for the study of focal mechanism, earthquake prediction, tomographic inversion and geodetic results explanation. So far, most scholars believe that viscoelastic relaxation, afterslip, poroelastic rebound are the mechanism of post-seismic deformation. The post-seismic deformation mechanism should be explained combined with the dislocation theory, and presently, one of the most completely dislocation theory is the viscoelastic spherical dislocation theory proposed by Tanaka. The theory can consider the compressibility and the infinite stratification of the earth at the same time. The main purpose of this paper is to extract the coseismic and postseismic gravity changes induced by huge earthquake based on the GRACE, and explained these changes by the viscoelastic spherical dislocation theory, to provide support for the study of earth’s internal structure and earthquake breeding mechanism. The main work in this paper is as follows: 1) Research on the GRACE Level02 Rlease05 data preprocessing methodWe process the GRACE Level02 Rlease05 global gravity field data released by the CSR(Center for Space Research University of Texas) using de-correlation filter. And by this way, we remove the correlation error between the even and odd order of spherical harmonic coefficient of gravity field, and weaken the strikes in the spatial distribution. Then, we smooth the GRACE data using Gaussian smoothing with different radius, and the result shows that different smooth radius having great influence on the smooth results. So that the smooth radiuses needs to be carefully selected according the research area. We suggest that Gaussian smoothing and de-correlation filter should be used at the same time when we are processing the GRACE data. 2) Study on the method of extracting the co-seismic and post-seismic gravity changes from the GRACE data.We extract the co-seismic gravity changes spatial distribution induced by the Sumatra Earthquake(Mw9.3) with the difference method and the least square fitting method, comparing these results with the gravity changes simulated by the elastic spherical dislocation theory. Result shows that the difference method can extract seismic gravity changes better than the least square method. Then we extract the co-seismic and 7 years post-seismic gravity changes with difference method, and the result shows that the viscosity in mantle is different between both sides of the fault. Additionally, we extract the gravity filed time series of single points A(95.0°E, 1.5°N) and B(97.0°E, 7.0°N) in both sides of the fault, and analyze the characteristic of the time series. 3) The viscoelastic spherical dislocation theoryStudy on the theoretical basis and calculation method of viscoelastic spherical dislocation theory. Formulas for calculating the post-seismic deformation(vertical displacement and spatial fixed point gravity changes) of any point caused by arbitrary fault at the Arctic and the formulas for calculating the post-seismic deformation of any point caused by the arbitrary fault in the earth are given.4) Modifying the integration program of viscoelastic spherical dislocation theoryPoint out the defect of large near field deformation error when calculating the post-seismic deformation by the initial viscoelastic spherical dislocation theory. Then we modify the integration program with the finite fault subdivision method proposed by Sun and Okubo(1998), comparing the vertical displacement Green Function and gravity changes Green Function of four independent sources in depth 10 km and 32 km calculated by the modified viscoelastic theory and the elastic theory, and the result shows that they are generally in agreement: the mean difference of vertical displacement between viscoelastic and elastic theory is 0.05%, and the mean difference of gravity changes between viscoelastic and elastic theory is 0.03%. At last, we analyze the 0~0.7 year and 0~7 year gravity changes Green Function of four independent sources in depth 32 km with the mantle viscosity 1×1018 Pa?s. 5) The application of the modified viscoelastic spherical dislocation theoryWe simulate the post-seismic gravity changes induced by the Sumatra Earthquake(Mw9.3) with the modified viscoelastic spherical dislocation theory, and compare the simulated result with the observation result. The compared result shows that the simulated result is generally agreement with the observation result in peak spatial distribution and post-seismic gravity changes trend. The mantle viscosity in this region is obviously laterally different, the mantle viscosity in the footwall area is about 1×1018 Pa?s, and the mantle viscosity in the hanging wall area is about 8×1018 Pa?s.