Study On The Deformation Characteristics Of The M_w 9.0 Japan Earthquake Based On The GPS And GRACE Observations

The Mw9.0 Japan Earthquake,which occurred on March 11th,2011,is the largest earthquake in Japan in historical record.This megathrust earthquake happened in the subduction zone where the Pacific plate subducts beneath to the Eurasian plate and the North America plate.The earthquake generated not only huge co-seismic and post-seismic deformation,but also giant tsunami that destroyed heavy economic losses and casualties.At the same time,this event also provided a rare earthquake example for studying on deformation of large earthquake using geodetic measurements.Modern geodetic techniques,particularly GPS and satellite gravity,can provide accurate and detailed models of the displacement field and the gravity field which have become the solid foundations for studying the temporal and spatial evolution regularity of displacement and gravity.The study of post-seismic deformation characteristics based on the GPS and satellite gravity observations has become an important approach to recognize and understand the physical processes of large earthquakes in subduction zone.These are the starting points of this thesis.In this thesis,the post-seismic deformation characteristics and the physical mechanism of the Mw 9.0 Japan Earthquake are studied by using the GPS and GRACE observations before December 2015.The main work is as follows:(1)Co-seismic and post-seismic displacement and gravity changes are extracted based on the GPS and GRACE observations,and the temporal and spatial evolution regularity of the displacement and gravity change caused by Japan earthquake is analyzed.(2)By studying the predecessors' research work and comparing the simulation calculation,the layered earth model of the Japanese region is determined,and the optimal fault slip model is selected.(3)Combining the theories of afterslip and viscoelastic dislocation,the deformation mechanism of the Japan earthquake is studied,and the method of comprehensive application of the GPS and GRACE observations in the simulation research is explored,and the mantle viscosity structure of earthquake regions in Japan is estimated by using GPS and GRACE observations.The results of this study show that:(1)The post-seismic displacements observed by GPS are characterized by the exponential function,and the rate of change accords with the attenuation characteristic of Omori formula.The post-seismic horizontal displacements accumulated to 60?165 cm for the east component,and 20?65 cm for the south component in nearly 5 years following the main shock until December 2015.For GPS sites(G104?G105 and J192)farther away from the epicenter,the post-seismic displacements have exceeded their co-seismic changes.Moreover,the post-seismic deformations are still continuing.The co-seismic gravity changes observed by GRACE are significant and show positive and negative distribution on both sides of the fault.The co-seismic gravity changes of ocean area are obviously weaker than subduction zone.The post-seismic gravity changes observed by GRACE are also significantly,and the trend of gravity change on both sides of seismic rupture is nearly the same,overall upward.But there still exist differences,the post-seismic gravity changes of ocean area have increased significantly,and the rate of change is faster.(2)Combined with the theories of afterslip and viscoelastic dislocation,the post-seismic deformation is simulated and found that under the time scale of this thesis,the afterslip and viscous relaxation effect can make a reasonable explanation for the deformation and gravity changes caused by the Japan earthquake.It is illustrated that the post-seismic deformation of large earthquake is mainly caused by these two physical mechanisms.(3)Similar to the Sumatra earthquake region,there exist the lateral inhomogeneity of the crust mantle structure and the rheological parameters are different on both sides of the fault rupture.The viscosity of the continental side is of the order of 1.0×1019 Pa s,while the viscosity of the ocean side is of the order of 6.O×1018 Pa s.(4)In the simulation of post-seismic deformation,the method of estimating the regional viscous structure is developed.Firstly,the viscous relaxation model is established by GPS observation data,then the deep viscosity coefficient is modified by GRACE observation data,and the shallow viscosity coefficient is fine turned by synthetically using the GPS and GRACE observations.Finally,the regional viscous structure can be estimated.The results show that it is a feasible method to inverse the viscous structure of the megathrust earthquake region with GPS and GRACR observations.