龙门山地区及其断裂带系统形变模拟研究

Longmenshan, which defines the eastern margin of the Tibetan plateau, exhibits great topographic relief and includes an active thrust fault system. A diverse suite of models (e.g., lower crustal flow; brittle crustal thickening by thrust faults; and midcrust detachment layer) were proposed to account for the processes that produce and maintain the topography of this mountain belt. A rich assembly of geophysical data and interpretations from studies of the Ms 8.0 2008 Wenchuan Earthquake reveals the lithospheric structure of the Longmenshan fault system and thus investigations of the Wenchuan Earthquake provide opportunities to test the models that describe the tectonics of Longmenshan. We will simulate deformation of the Longmenshan and its fault system by constructing finite element models that simulate fault-slip and the resulting viscoelastic relaxation. The mechanical structure of these models will be constrained by the results of previous studies of seismic tomography and rheologic structure. Controlling parameters, such as the viscosities of the middle crust, lower crust, and mantle and friction characteristics of the faults will be calibrated to geodetic data (e.g., GPS and InSAR) using inverse methods. The model will be driven by a load to simulate tectonic kinematics and the faults will be simulated as frictional contact surfaces. Friction law will be applied on faults, which controls the mechanical process on faults. We will use the calibrated model to predict the evolution of stress and strain in the model domain and relative slip along the fault segments. Results of this study will improve understanding of the tectonic deformation mechanisms at depth and help researchers to discriminate among the alternative models that seek to account for horizontal shortening and maintenance of topographic relief in the eastern margin of the Tibetan plateau.

龙门山地区地壳缩短增厚机制问题是有关青藏高原大陆地壳形变模式争论的关键问题之一。先后发展有下地壳流、全地壳逆冲推覆、中地壳滑脱层等模型。但这些模型没能同时考虑龙门山地区岩石圈流变结构和龙门山断裂带断层活动的相互影响。 近年来，通过对汶川地震的研究，在龙门山地区积累了空前丰富的有关断裂带和岩石圈的地质、地球物理、形变等资料和成果。 针对龙门山地区形变模式问题，本申请课题拟在选取应用可靠流变研究结果基础上, 结合已有断裂带几何结果，建立龙门山断裂带系统黏弹性有限元模型；分别以震间、震中和震后GPS和InSAR观测资料为模型约束，反演断层面摩擦系数，并同时反演断裂带系统各断层摩擦系数和岩石圈流变参数。以得到的最佳模型，模拟龙门山地区的中、下地壳变形和断裂带系统中各断层面的相互运动情况。以帮助理解和评估青藏高原东缘、龙门山地区的地壳形变模式，以及理解龙门山断裂带上的的地震动力学演化过程。

我们以动力学模型模拟了龙门山断层的地震准周期行为。基于模拟地震周期行为中的应力/应变演化过程，我们发现，断层几何、构造边界加载和断层上的摩擦本构关系，都会对大陆内铲形逆冲断层上的构造活动和地震形变产生重要的影响。并且，不同于近垂直的走滑断层上的地震周期行为特征，铲形逆冲断层上的构造应力的积累和释放过程更复杂、有其独特性，比如：在断层破裂的深部和浅部，同震滑动大小和构造应力释放大小并非同步，而是差异悬殊；在断层底部会形成永久变形，及震后应力释放。对大陆内铲形逆冲断层上这些变形特征的了解，有助于我们在其地震周期行为中评估地震危险性。.紫坪铺水库在汶川地震发生前2.7年蓄水，位置与汶川地震震中相距仅约12公里，震后10年来，有关两者之间是否存在触发关系一直争论不休。我们用完全耦合孔隙压力弹性模型模拟了紫坪铺水库蓄水对于邻近的龙门山断层上库仑应力的时空演化影响，结果显示水库加载，对龙门山断层8公里以上的影响，相当于约60至450年的构造应力加载。则地壳深处的普通构造小震会触发上部处于临界状态的断层发生大面积连锁失稳，形成大震。本研究的结果显示：紫坪铺水库蓄水间接触发了汶川地震。