利用震后黏弹性位错理论研究苏门答腊地震(M_w9.3)的震后重力变化

Retrieve post-seismic gravity changes induced by Sumatra earthquake(M_w9.3)based on the viscoelastic dislocation theory

本文利用2003—2011年的GRACE RL05数据提取了苏门答腊地震(Mw9.3)引起的震后重力变化,发现断层两侧震后重力变化速率存在明显差异,断层下盘总体变化率为0.55μGal/yr,断层上盘为0.16μGal/yr.基于子断层叠加的编程思想,本文将Tanaka的黏弹球体位错理论配套计算程序(简称黏弹位错程序)加以改造,克服了其近场计算精度不足(甚至错误)的缺陷,可用来研究大地震引起的近场震后位移与重力变化.本文利用改造后的黏弹位错程序计算了2004年苏门答腊地震(Mw9.3)产生的同震重力变化,计算结果在空间分布和量级上均与利用弹性位错程序计算获得的结果一致,验证了我们对黏弹位错程序进行改造的正确性.最后,结合GRACE卫星观测数据,本文利用Tanaka的黏弹位错理论研究了苏门答腊地区的地幔黏性因子.结果表明,该地区地幔黏滞性具有显著的横向差异,当发震断层上下两盘的地幔黏滞性系数分别取8×1018 Pa·s和1×1018 Pa·s时,模拟的震后重力变化在总体空间分布和变化趋势上与GRACE卫星观测结果更接近.

The GRACE（Gravity Recovery and Climate Experiment）mission has continually provided the data of temporal variability of the global gravity after its launch.Several coseismic gravity changes have been successfully retrieved by the GRACE mission.The visco-elastic dislocation theory is used to study the co-and post-seismic deformations.In this paper,we studythe post-seismic gravity changes due to the 2004 Sumatra earthquake（Mw9.3）based on the viscoelastic spherical dislocation theory and the GRACE data,and then analyze the viscosity of the mantle and horizontal difference at this area.We adopt the RL05 data provided by UT/CSR（Center for Space Research,University of Texas）,and the data spans from January 2003 to December 2014.We replaced the Earth′s oblateness values（C20）with those from Satellite Laser Ranging because of their poor accuracy.After the de-correlation filter using polynomials of degree 3for coefficients with orders 6or higher to alleviate longitudinal stripes and the Gaussian smoothing with averaging radius of 350 km to reduce short wavelength noises,we obtain the post-seismic gravity changes due to the 2004 Sumatra earthquake（Mw9.3）based on the difference method and time series of two points respectively located on the hanging side and the heading side.We adopt the visco-elastic spherical dislocation theory to validate the GRACE-derived result.We first use the integration of the finite fault models to solve the problem of large errors in the near-field computation using the viscoelastic spherical dislocation theory,and comparing the modified results with the results of elastic spherical dislocation theory.After that,we retrieve post-seismic gravity changes induced by the earthquake.The result shows that the peak of the gravity changes is from-8.3to 4.6μGal in the period of 2004—2005based on the GRACE,which is consistent with the result derived by the elastic spherical dislocation theory in the spatial distribution and the magnitude.Based on the spatial distribution of the gravity changes and the analysis of time series,we find that the rate of the post-seismic gravity change at the heading side is 0.55μGal/yr and the rate is 0.16μGal/yr on the other side,which demonstrates that the viscosity of the two sides of the fault is different.The differences of coseismic vertical displacement and gravity changes based on the modified visco-elastic theory are respectively 0.06% and 0.1%,compared with those derived from the elastic spherical dislocation theory. We also analyze the Green functions of the vertical displacement and gravity changes at the source depth of 32 km in two different periods of 0～0.7years and 0～7years,which demonstrate that we have successfully solved the problem of large errors in the near-field computation using the visco-elastic spherical dislocation theory.Based on the corrected method,we simulate the post-seismic gravity changes due to the 2004 Sumatra earthquake（Mw9.3）,finding that only when the viscosity of the heading side is 1×1018Pa·s and the viscosity of the hanging side is 8×1018 Pa·s,the simulating results are consistent with the GRACE-derived results in the spatial distribution and the magnitude.Based on the theory results,we find that the rate of the post-seismic gravity changes at the heading side is 0.52μGal/yr and the rate of the post-seismic gravity changes at the hanging side is-0.12μGal/yr.Comparing the observing results with the simulating results,it demonstrates that the viscosity of the mantle at the Sumatra region is laterally inhomogeneous.The viscosity at the left side（the heading side）of the fault is smaller than the one at the right side（the hanging side）of the fault,and the viscosity is respectively 1×1018Pa·s and 8×1018Pa·s.