2008年汶川大地震中北川地区极重震害的物理机制研究

doi:10.6038/cjg2018M0111

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摘要 2008年汶川特大地震中北川地区受灾特别严重，而该地区远离汶川主震震中超过100多千米.尽管汶川地震发生已近10年，但这种不正常现象一直困扰着地学工作者，至今没有给出合理的解答.为此，本文利用有限单元方法模拟汶川地震的主要发震断裂带（即：映秀—北川断层）的自发破裂动力学过程.模型中，几乎平直的映秀—北川断裂带在高川地区发生拐折，形成高川右弯.模拟结果显示，破裂沿着映秀—北川断裂带向东北方向前行，当破裂前端到达高川右弯时，破裂形态发生剧烈变化，破裂传播速度由亚剪切破裂突然转化为超剪切破裂.与断层阶区促进超剪切破裂明显不同的是，这种超剪切破裂形态的转化不需要时间停顿.由于超剪切地震破裂的产生，破裂辐射的地震波发生相长干涉，形成马赫波，地震动被大大放大.计算给出的强地面运动峰值加速度在北川地区不仅数值高，而且高值区分布范围广，造成了北川地区的震害特别严重.模拟结果还表明，若高川右弯不存在（即映秀—北川断层面为平直），则北川地区的断层不会形成超剪切破裂；如果高川右弯附近断层的不连续程度较大，则破裂会被终止；这些情况下，北川地区的震害都没有那么严重.所以，高川右弯的几何形状对于超剪切破裂的形成起着决定性作用，也是造成远离震中的北川地区震害特别严重的主要原因.因此，分析发震断层几何对于研究震源动力学过程及震害评估等有着非常重要的科学意义和实际应用价值.

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关键词 ：汶川地震, 映秀—北川断裂带, 高川右弯, 超剪切破裂, 北川地区, 地震灾害

Abstract：The Beichuan area suffered the most serious seismic damage during the 2008 Wenchuan earthquake although over 100 km away from the instrumental epicenter of the mainshock. The mechanism for this peculiar phenomenon remains unclear even though nearly 10 years has passed since the event. To address this issue, we construct a spontaneous rupture model in which the Gaochuan right bend (GRB) in the middle of the Yingxiu-Beichuan fault, a major seismogenic structure for the Wenchuan event, is included. The simulation results show that the complex geometry of the GRB plays a first-order role in controlling the rupture propagation. While rupture is initiated at the epicenter of the Wenchuan mainshock, it propagates spontaneously northeastward at a speed slower than the shear wave velocity of local media. When the rupture front spreads near the end of the Yingxiu-Gaochuan fault, a new rupture is re-nucleated at the curve section of the Gaochuan bend, and propagates in the NE direction with the speed greater than the S wave velocity. In particular, this rupture speed transition from subshear to supershear does not need time delay, much different from the case of a fault step over which was studied well by previous work. Due to the curved geometry of GRB, the stress regime on the fault section favors the occurrence of super-shear rupture. Once the super-shear rupture occurs along the Beichuan fault, seismic waves are focused and strongly amplified with the form of Mach waves. The numerical results also illustrate that high values of spatial distribution of the strong ground motion acceleration are mainly located in the Beichuan area, directly leading to grave seismic catastrophe. Therefore, this work may give some insight into why the most serious seismic damage occurred in the Beichuan area, and may have important implications for understanding earthquake dynamics and assessing seismic hazards.

Key words： Wenchuan earthquake Yingxiu-Beichuan fault Gaochuan right bend Supershear rupture Beichuan area Seismic hazard

收稿日期: 2018-02-21

PACS:

P315

基金资助:国家自然科学基金项目（41574041），国家重点研发计划（2017YFC1500104）及中央级公益性科研院所基本科研业务专项（ZDJ2017-08）联合资助.

作者简介: 朱守彪,男,1964年生,理学博士,研究员,博士生导师.现主要从事地球动力学、地震活动性及地震预报方法研究.E-mail:zhusb@pku.edu.cn;zhushoubiao@gmail.com

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http://manu39.magtech.com.cn/Geophy/CN/10.6038/cjg2018M0111 或 http://manu39.magtech.com.cn/Geophy/CN/Y2018/V61/I5/1863

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