考虑土层非线性效应的四川地区场地放大系数模型

Site amplification factor model for Sichuan region considering nonlinear soil effects

为研究四川地区场地放大效应对地震动的影响,选取了该地区105个场地的钻孔资料和897条强震记录,结合强震数据分析和等效线性化计算结果讨论了地震动强度、场地条件对地震动场地放大的影响,建立了考虑土层非线性反应的场地放大系数模型,并探讨了模型在地震动估计中的应用效果。对模型结果的分析表明:当地震动强度较小时,场地放大系数随Vs30增大而减小,减小的幅度与反应谱的周期相关,当周期为0.2 s时,减小得最为显著;当地震动强度较大时,在同一场地上,场地放大系数随地震动强度增大而减小,减小的幅度与反应谱的周期相关,当周期大于1 s时,场地放大系数与地震动强度无关;此外,土层非线性反应对地震动的影响随场地变硬而不明显,当Vs30大于500 m/s时,可忽略这一影响。与只考虑线性场地反应影响的地震动衰减关系相比,利用本文模型建立的地震动衰减关系可显著地减小估计四川地区地震动时的标准差,尤其对于近场短周期地震动减小幅度可达12%。

To investigate the effect of site amplification on ground motion in Sichuan region, strong motion data and calculated equivalent-linear results are analyzed using 897 strong motion records and 105 shear-wave velocity profiles from the region. Based on the analysis results, the influence of the ground motion intensity and site conditions on site amplification is studied, and site amplification factor model considering nonlinear soil effects is established. Then the application effect of the model to evaluate ground motion is discussed. The results show that the decreasing extent of site amplification factor with the increasing Vs30 is period-dependent at low shaking levels. The decrease is mostly significant at period of 0.2 s. Under high shaking levels the decreasing extent of site amplification factor with the increasing ground motion intensity is spectral period-dependent but the site amplification factor is independent of the ground motion intensity at a periods larger than 1 s. In addition, the soil nonlinearity has less influence on ground motion with the increasing Vs30. When value of Vs30 is larger than 500 m/s, the influence can be neglected. In conclusion, compared with attenuation relationship considering only linear soil effects, the standard deviation of ground motion estimates in Sichuan region using the proposed model can be reduced significantly. Especially for the near field and the short period ground motion, the reduction can reach 12%.