| 2008年汶川大地震单侧破裂过程的动力学机制研究 |
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| 引用本文： | 朱守彪,袁杰.2008年汶川大地震单侧破裂过程的动力学机制研究[J].地球物理学报,2016,59(11):4063-4074. |
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| 作者姓名： | 朱守彪 袁杰 |
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| 作者单位： | 1. 中国地震局地壳应力研究所, 北京 100085;
2. 中国科学院计算地球动力学重点实验室, 北京 100049;
3. 中国地震局地球物理研究所, 北京 100081 |
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| 基金项目： | 国家自然科学基金项目（41574041）、北京市自然科学基金项目（8152034）以及广西科技攻关项目（桂科攻1377002）共同资助. |
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| 摘 要： | 2008年汶川大地震的破裂过程极其发杂，向东北方向的破裂距离长达300 km，而向西南方向的破裂长度很小，呈现出单侧破裂的主要特征.尽管汶川地震破裂呈单侧传播的现象引起许多地震学家的关注，但其物理机制至今还不是十分清楚.本文利用有限单元计算方法，模拟了汶川地震的破裂过程.模型中根据龙门山断裂带两侧（东南侧为四川盆地，西北侧为川西高原）实际的地震波速度来确定模型的介质物性参数，利用目前观测的应力环境来选定初始应力条件.模拟结果表明：破裂在汶川地震的震中处成核后，先向断层两侧自发传播，但向东北方向的传播距离明显大于向西南方向；断层面上的正应力在东北方向（破裂的正方向）随着传播距离的增大而不断减小，位错速率随着破裂的传播距离而越来越大，其脉冲变得越来越尖锐，即产生了Weertman脉冲.研究结果显示：由于这种脉冲的出现，破裂在正方向上（东北方向）能够自己放大、自己愈合、自行维持，摩擦热极小，所以破裂能够沿着东北方向一直传播，直到应力场方位发生变化，不利于破裂时才最后终止.但在西南方向，破裂过程中断层面上的正应力增大，阻碍破裂继续扩展.最后就出现了汶川地震中破裂朝东北方向单侧优势传播的基本格局.模拟结果还表明：若断层面两侧介质均匀，则破裂向两侧是对称传播，且破裂距离很短，因此这种情况无法产生像汶川大地震那样的特大地震.因此，文中的模拟结果表明龙门山断裂带两侧的物性差异是造成汶川大地震单侧传播的决定性因素.断层两侧物性差异（bimaterial contrast）影响断层破裂过程的研究对于深入认识地震动力学过程、地震灾害预测及评估等有重要的科学意义.
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| 关 键 词： | 破裂动力学过程 断层两侧物性差异 Weetman脉冲 单侧破裂 汶川地震 有限单元 |
| 收稿时间： | 2016-04-05 |
Mechanisms for the fault rupture of the 2008 Wenchuan earthquake (MS=8.0) with predominately unilateral propagation |
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| ZHU Shou-Biao,YUAN Jie.Mechanisms for the fault rupture of the 2008 Wenchuan earthquake (MS=8.0) with predominately unilateral propagation[J].Chinese Journal of Geophysics,2016,59(11):4063-4074. |
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| Authors: | ZHU Shou-Biao YUAN Jie |
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| Institution: | 1. Institute of Crustal Dynamics, China Earthquake Administration, Beijing 100085, China;
2. Key Lab of Computational Geodynamics, Chinese Academy of Sciences, Beijing 100049, China;
3. Institute of Geophysics, China Earthquake Administration, Beijing 100081, China |
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| Abstract: | The rupture processes of the 2008 Wenchuan earthquake are very complex. The rupture propagated northeastward a large distance as long as 300 km, while the rupture length is short in the southwest direction. Although a great deal of attention has been paid to why the rupture propagated preponderantly northeastward during this event, the physical mechanism remains unclear. By means of a finite element model in which bimaterial contrast across the Longmen Shan fault is taken into account, this study suggests that the nucleation process is initiated at the center of the fault, ruptures then spread out outward spontaneously in both directions. The simulation results show that the different materials between both sides of the fault lead to tensile changes of normal stresses on the fault, which enhances the propagation of the rupture. We find that bimaterial mechanism is important for the earthquake rupture, and the mode II rupture evolves with propagation distance along a bimaterial interface to a unilateral wrinkle-like Weertman pulse in the direction of slip on the more compliant side of the fault, namely in the positive direction (i.e, the northeast direction). This Weertman pulse can be self-amplified, self-sustained and self-healing, which gives rise to little frictional energy and long propagation distance. This may be the reason why the 2008 Wenchuan earthquake is a unilateral fault rupture and has a high seismic magnitude. In addition, the modelling results suggest that the rupture distance would be much smaller if the material in the model is homogeneous, in which no huge earthquakes can occur like the 2008 Wenchan event. Hence, the study of the bimaterial effect will play an important role in understanding fault dynamics, seismic hazard estimation and prediction. |
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| Keywords: | Dynamic rupture process Bimaterial contrast Weertman pulse Unilateral fault rupture Wenchuan earthquake Finite element |
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