2015年尼泊尔地震震后形变动力学机理及岩石流变性质研究

地壳和地幔岩石对大地震同震应力扰动的变形响应主要取决于岩石流变性质，研究深浅部岩石流变性质对厘清震后变形物理机理具有重要意义。2015年尼泊尔Mw7.8和Mw7.3地震为探测喜马拉雅主逆冲断裂带的摩擦参数和深部岩石的黏滞系数提供了难得的机会。但由于藏南GPS站分布稀疏，震后形变模型简单等原因，已有研究对岩石流变参数的约束存在较大不确定性。本研究将搜集研究区域尤其是藏南更多的GPS资料，获得远近场互补震后变形过程。再应用速率强化摩擦定律控制的余滑模拟技术和三维黏弹性有限元模拟技术，建立余滑和黏弹性松弛耦合的震后形变动力学模型。以GPS震后形变为约束，估计断层面摩擦参数和深部岩石的黏滞系数，厘清尼泊尔震后形变的动力学机理。最后计算同震、余滑和黏弹性松弛引起的断层面应力变化，探讨其对余震序列的触发作用。该研究结果有助于深化对喜马拉雅断裂带地震周期形变物理机理的认识，为地震风险评估提供依据。

The deformation of mantle and crustal rocks in response to stress perturbations such as large earthquakes is controlled mainly by the rock rheology, which also plays an important role in the dynamics of the plate tectonics, glacial isostatic adjustment, and earthquake cycle deformation and so on. Generally, the rate-and-state friction law and power-law, which are derived from laboratory rock experiments, are the most widely accepted constitutive laws to simulate seismic and aseismic slips on the fault interface and bulk viscous flow in the lower crust and upper mantle. However, detailed knowledges of rock rheology in the continent remains elusive as it is difficult to measure. The occurrence of the 2015 Gorkha, Nepal Mw7.8 mainshock and the largest Mw7.3 aftershock in the eastern Himalayan collision zone provides an unprecedented opportunity to investigate the frictional parameter of the Main Himalayan Thrust fault (MHT) and bulk viscosity of lower crust and upper mantle. A number of publications by different research groups have analyzed the postseismic deformation mechanisms following the Gorkha earthquake depending heavily on the GPS-derived postseismic deformation in Nepal or on InSAR data. However, there are apparent discrepancies of afterslip distribution, frictional parameter and heterogeneous rheological structure between models due to the lack of enough GPS data in south Tibet, short time-span of GPS data and simple simulation models...To overcome the these limitations, we will collect more GPS data from a dense local network covering the entire study region, especially our own GPS observations in south Tibet as well as additional measurements in Nepal. All available GPS data will be processed using the GAMIT/GLOBK 10.7 software and isolate the postseismic transients from raw position time series in the ITRF2014. Once the three-dimensional postseismic deformation is derived, we will develop a numerical method for simulating the dynamic postseismic deformation with mechanically coupled frictional afterslip on the MHT and viscous relaxation in the lower crust and upper mantle, which are driven by the coseismic shear stress loading. The frictional afterslip evolution following the Gorkha earthquake, which is mainly governed by the loaded coseismic shear stress and frictional parameter (a-b)σ, will be resolved under the velocity-strengthening friction framework using our python-coded numerical code. Whereas, the viscous flow controlled by rheological stratification of the lithosphere will be simulated using three-dimensional viscoelastic finite element method realized in the PyLith software. In order to reduce the number of parameters to be explored, we will firstly refine the induced coseismic slip model and determine the stratification of the lithosphere by a large number of trial-and-error. The rock rheological parameters will be determined using a simple grid search or Monte Carlo method through finding the minimum data misfit of model predictions to GPS observations, especially the vertical motions. In our dynamic models, we will consider the interaction between the afterslip on the fault plane and bulk viscoelastic flow. Finally, we will investigate the time-dependent shear stress or Coulomb stress changes on the MHT induced by the coseismic rupture, the largest aftershock, frictional afterslip and viscoelastic flow using the optimal dynamic postseismic deformation model, and investigate the relationship between the stress evolution and aftershock sequence. Our expected research results will shed light on the mechanism of earthquake cycle deformation in the Himalayan collision zone, the mechanism for the uplift of Tibetan Plateau, crustal deformation pattern over the Tibetan Plateau, and future seismic hazard.