不均匀软弱夹层对沉积盆地地震动放大的影响研究

Study on the effect of inhomogeneous soft interlayer on ground motion amplification in sedimentary basins

含有倾斜软弱夹层的盆地将引起地震波复杂散射,这较大程度改变地表地震动的强度和分布。目前,盆地内倾斜夹层对地震动放大影响较少。通过自编ABAQUS插件程序实现对黏弹性边界快速准确施加,并基于黏弹性和修正Drucker-Prager理想弹塑性本构,研究了SV波垂直入射下横向不均匀软弱夹层对沉积盆地地震动放大的影响。研究结果表明:横向不均匀软弱夹层降低了地表PGA与Arias烈度,使二者空间分布显著不均匀,软弱夹层较厚一侧下降程度更高。考虑土层非线性时,软弱夹层隔震效应增强;存在横向不均匀软弱夹层的二维沉积盆地的基频接近夹层较厚一侧一维土层基频,显著低于夹层较薄一侧的一维土层基频。随着平均阻抗比越大,横向不均匀软弱夹层对二维盆地基频、f 1d/f 2d及放大系数的影响越小;随着软弱夹层倾角增大,传递函数主要放大区域均向夹层较厚一侧移动,且主要放大频带向低频移动。随着地震动幅值增大,横向不均匀软弱夹层非线性增强,地表PGA放大系数与传递函数下降,同时地表地震动不均匀分布程度降低。

Basins containing inclined soft interlayers will cause complex scattering of seismic waves and change the intensity and distribution of ground shaking at the surface to a large extent.At present,tilted interlayers in basins have less influence on the amplification of ground shaking.Intra-basin tilted interlayers have less influence on ground-shaking amplification.In this paper,the effect of transverse inhomogeneous soft interlayer on ground shaking amplification in sedimentary basins under vertical incidence of SV waves is investigated based on viscoelasticity and modified Drucker-Prager ideal elastic-plastic ontology by self-programmed ABAQUS plug-in program to achieve fast and accurate application of viscoelastic boundaries.The results are as followings.The transverse inhomogeneous soft interlayer reduces the surface PGA and Arias intensity,results in significantly uneven spatial distribution,and decreases more at the thicker side of the soft interlayer.When considering the nonlinearity of the soil layer,the seismic isolation effect of the soft sandwich layer is enhanced.The fundamental frequency of the two-dimensional sedimentary basin with laterally inhomogeneous soft interlayer is close to that of the one-dimensional soil layer on the thicker side of the interlayer and significantly lower than that of the one-dimensional soil layer on the thinner side of the interlayer.The larger the average impedance ratio of the soft interlayer,the smaller the influence on the fundamental frequency,f 1d/f 2d and amplification coefficient of the 2D basin.As the dip angle of the soft interlayer increases,the main amplification area of the transfer function moves to the thicker side of the interlayer,and the main amplification band moves to the lower frequency.As the amplitude of ground shaking increases,the non-linearity of soft interlayer increases,and the PGA amplification coefficient and transfer function weaken and reduce the degree of uneven distribution of PGA amplification coefficient at the surface.