印度板块挤压驱动龙门山断裂带活动的三维数值模型

Three-dimensional numberical modeling of activity of the Longmenshan fault zone driven by the India Plate

采用有限元方法模拟了印度板块的持续向北推进、挤压对龙门山断裂带及其邻区的影响,根据模拟结果 ,对比GPS、地震和地质学观测数据,定量分析了汶川大地震发生前断裂带的走滑速率以及最大主应力的分布特征,据此探讨了汶川大地震的发震机理.模型由弹性上地壳和粘弹性下地壳组成,其中上地壳包括了青藏高原、四川盆地、华南地块以及印支地块,块体内断层的处理方法主要采用了库仑摩擦模型.研究结果如下:(1)通过调整龙门山断裂带摩擦系数,发现伴随摩擦系数值的增加断裂带走滑速率逐渐降低,当其达到0.95,已接近GPS测量结果 .由此可认为龙门山断裂带应处于强摩擦环境.(2)与松潘甘孜地块中其它区域相比,整个龙门山地区处于高摩擦强度、低应力的环境下,这有利于能量缓慢而稳定的积累直至达到其破裂极限,从而引发强烈地震.(3)沿龙门山断裂带上下两盘的最大主应力分布特征具有很强的规律性,自西南到东北可划分出泸定-大邑、大邑茂县、茂县北川、北川青川四段.其中上、下两盘应力差最大值出现在大邑茂县段14～20 km处,应力差最小值出现在泸定大邑段,而北川茂县段上、下两盘的应力值则十分接近.这种最大主应力的分布特征与汶川地震余震震中的空间分布特征十分相似.

The effect of the continuous northward indentation of the India Plate along the Longmenshan fault zone and its surrounding areas is modeled using finite element analysis. Comparing the simulation result with GPS surveys,seismological data and geological analysis, we quantitatively analyze the slip rate and the distribution characteristics of major principal stress of the Longmenshan fault zone. And then, the triggering mechanics of the Wenchuan earthquake is discussed accordingly. The model consists of an elastic upper lithosphere and a vicoelastic lower lithosphere. The upper lithosphere includes the Tibetan Plateau, the Sichuan Basin, the South China Block, and the Indochina Block. The faults in the block are introduced as Coulomb-type frictional zones in the modeling. The results are as follows： （1） By adjusting the faults friction, we find that with friction coefficient increasing, the slip rate of the fault zone tends to decrease. When the friction coefficient touches 0.95, the slip rate is close to the result of GPS survey. Therefore, we assume that the Longmenshan fault zone should be under the condition with intensive friction. （2） Compared with other regions of the Songpan-Garz~ block, the Longmenshan fault zone is in a strong friction and low stress environment, which favors significant energy accumulation to trigger great earthquakes. （3） According to the distribution characteristics of major principal stress, we can divide the Longmenshan fault zone into four short segments from SW to NE as the Luding-Dayi segment, the Dayi-Maoxian segment, the Maoxian-Beichuan segment and the Beichuan-Qingchuan segment, respectively, and the maximum differential principal stress （δl） between hangingwalls and footwalls of faults appears in the Dayi-Beichuan segment, and the minimum differential principal stress（δ1 ） appears in the Luding-Dayi segment. In the Maoxian-Beichuan segment, the principal stress（δ1 ） of hanging walls of fault is almost equal to footwalls of the fault. Such distribution of major principal stresses is similar to the spatial distribution of the Wenchuan earthquake and its aftershocks.