地震基岩面的选取对深厚场地地表地震动参数的影响

地震基岩面的选取对场地设计地震动参数取值的合理性有重要影响。以苏州城区的钻孔剖面为研究对象,选取剪切波速介于400~800 m/s的9个土层顶面作为地震基岩面,采用等效线性方法考虑土的非线性特性,采用一维波传法分析地震基岩面的选取对地表地震动特性的影响。结果表明:1地表峰值加速度PGA随地震基岩面剪切波速的增大而增大,PGA的增大幅度随输入地震动强度的增大而减小;2地表加速度反应谱放大系数?谱谱值也随地震基岩面土层剪切波速的增大而增大,且?谱谱值随输入地震动强度的增大而减小;对于中强地震的近场地震动作用,基岩面深度对周期小于1.0 s的?谱谱值的影响较大;而对于特大地震的远场地震动作用,基岩面深度对周期小于4.0 s的?谱谱值均有较大影响;3远场地震动作用时的?谱谱值明显大于近场地震动作用的?谱谱值;4取剪切波速不小于700m/s的土层顶面为地震基岩面较为适宜。

Effect of seismic bedrock interface depth on surface motion parameters of deep site

The seismic bedrock interface depth has great influence on design ground motion parameters. Based on the borehole profiles in the downtown area of Suzhou City, nine interfaces of soil layers, whose shear wave velocities range from 400 to 800 m/s, are selected as the seismic bedrock interfaces. The equivalent linear method is used to describe the nonlinear characteristics of soil, and the effects of seismic bedrock interface depth on site surface motion are analyzed on the basis of the one-dimensional wave propagation model. The results show that: (1) The peak ground acceleration (PGA) increases as the shear wave velocity of seismic bedrock interface grows, but the PGA increasing range decreases as the input motion PGA increases; (2) The values of ground motion acceleration response amplification factorspectrum become larger with the increase of the shear wave velocity of seismic bedrock interfaces, and thevalues become smaller with the increase of input motion PGA. Under near-field ground motion of moderate strong earthquake, the depth of seismic bedrock interface has significant influence on thevalues for the period less than 1.0 s. But under far-field ground motion of great earthquake, the depth of seismic bedrock interface has significant influence on thevalues for the period less than 4.0 s; (3) Thevalues under far-field ground motion are obviously higher than those under near-field ground motion; (4) It is appropriate to choose the interface of soil with shear velocity not less than 700 m/s as the seismic bedrock interface.