汶川地震中四川盆地的地震效应研究

The Wenchuan earthquake was characterized with huge rupture fault, and successive rupture process. The damage survey results show that the damage near the western Sichuan basin edge was very severe, and attenuated rapidly toward the internal basin. The strong ground motion simulations considering the Sichuan basin is of significance for the understanding of the seismic effect of huge and complex basin during the major earthquake. The current studies of the ground motion during the Wenchuan earthquake based on global velocity structure model have demonstrated the preliminary amplification effect of the Sichuan basin. However, due to the coarse velocity model and the discrepancy with the site measurement data, these results did not reveal the effect of basin basement and Quaternary sediment. In this study, a relatively fine three dimensional Sichuan model will be established considering the laterally inhomogeneous crust structure, the Quaternary sediment, the basin crystalline basement, and the real topography. An improved spectral element method combined with the multi-transmitting formula, and the parallel computing technique is used to simulate the wave propagation behavior during the Wenchuan earthquake. The strong ground motion distribution features in the Sichuan basin are revealed, and amplification effect of the Sichuan basin and its relationship with the sediment thickness and basin structure are analyzed. The cause of the western basin edge amplification effect is investigated considering the effect of successive rupture process, and the most sensitive frequency bands of the ground motion for the Sichuan basin and Chengdu plain will be determined. Finally, the effects of material parameters on the simulated ground motion distribution field in the Sichuan basin are analyzed.

汶川地震破裂规模大，具有多次破裂过程。震害调查结果表明四川盆地西侧边缘区域震害非常严重，向盆地内部迅速衰减。考虑四川盆地的汶川地震地震动场模拟对认识大震作用下大型复杂盆地对地震动的影响具有重要作用。目前基于全球速度构造模型的汶川地震地震动场研究已初步揭示四川盆地对地震动的放大作用。但其中采用的介质模型空间分辨率低，且与基于实测资料的盆地速度构造有差异，未能反映盆地基底及第四系沉积层的影响。本研究拟考虑横向不均匀地壳构造、第四系沉积层、盆地结晶基底及真实地形，建立较精细的三维四川盆地模型。采用改进的结合多次透射边界的三维谱元法及并行计算技术模拟汶川地震中地震波的传播行为，揭示汶川地震中四川盆地强地震动的分布特征，分析盆地的放大效应及与覆盖层厚度、盆地构造的关系，探究多次破裂作用下盆地边缘放大效应的成因，明确四川盆地和成都平原主要放大的地震动频带，分析介质参数对四川盆地模拟地震动场的影响。

2008年汶川大地震中，邻近四川盆地西侧边缘区域的震害普遍较周边地区严重。为探究大震作用下大型四川盆地对地表地震动的影响，本项目采用数值模拟方法对此展开研究。主要取得以下成果：（1）基于大量资料建立了较精细的三维四川盆地模型，其中考虑了横向不均匀地壳、第四系土层、盆地基底及真实地形；与观测记录的比较验证了其合理性。（2）采用三维谱元法模拟了汶川地震中地震波的传播过程，分析了盆地内强地震动及放大系数的分布特征，揭示了盆地效应与相关因素间的关系，给出了盆地边缘严重震害的成因。探究了横向不均匀地壳、沉积层介质参数及分层构造等对盆地地震效应的影响。（3）提出了一种面波识别方法，分析了内外介质阻抗比和盆地深度对地表地震动和次生面波的影响。探究了断层和盆地的特征参数及山体地形等的影响。研究表明：（1）四川盆地对地震动放大明显，最大放大系数位于第四系覆盖层最深区域。盆地边缘效应、方向性效应及多次破裂过程的耦合是盆地边缘区域震害严重的主因。四川盆地基底基本决定强地震动区域的位置，第四系成都平原显著改变强地震动分布特征。（2）有限断层破裂下盆地地震动受不均匀地壳的影响相对不明显；第四系剪切波速的减小引起局部凹陷区域地震动幅值及范围增大，强地震动场分布特征变化，次生面波增强，而品质因子的影响相对较小。第四系土层分层对覆盖层厚度较深区域的放大系数影响较大。（3）随阻抗比增加，最大放大系数增大，盆地内多个显著放大和强面波幅值区域转为仅在盆地边缘出现。随盆地深度增加，地表最大放大系数减小，但面波幅值相应增大。随盆地倾角减小，最大放大系数增大。山体-盆地耦合显著改变盆地地震动的放大特征。（4）随断层破裂速度增大，盆地地震动强度增加，但放大系数有减小的趋势。断层位于盆地短边边缘时的地震动和放大系数相对最大。本项目的研究成果可为四川盆地区域之上城市的防震减灾服务，也可为其他沉积盆地的地震效应研究提供参考。