Preliminary Study on Basin Amplification Effect for Vertical Incidence of Two Kinds SH Pulse Waves

Study of basin effect has important reference significance for future urban the planning of earthquake prevention and disaster reduction. The amplification effects of three ideal simple basin models, “square” shape, “inverted convex” shape and “curved” shape, were studied by using the finite element numerical simulation method of decoupled nearfield wave combined with transmission boundary. The results show that: 1. Two kinds of basin models, “square” and “inverted convex”, show different rules of seismic response of basin position along with the depth of basin: the square basin model shows similar characteristics to the existing site response, while the calculation results of inverted convex basin model are more fit our existing basin knowledge. Therefore, it is not appropriate to study the amplification effect of basin earthquake ground motion only by using simple “square” basin model. 2. The seismic response of a more realistic "curved-edge" basin model is studied. The calculation results of the symmetric model show good regularity. A relatively large seismic response occurs in the center of the basin, which reflects the role of the “focusing effect” of the basin. The asymmetric basin model is located in the center of the basin and at the edge of the basin. Greater ground motion responses have been observed on both sides of the basin, even on the steeper edge of the basin, which is related to the “edge effect” of the basin; with the softening of the medium in the basin, the ground motion responses of the site in the basin gradually increase; with the increase of the stratification of the medium in the basin, the ground motion responses of the basin are larger. On the whole, the seismic response also tends to increase gradually, but there are also large differences in some locations of the basin, which reflects the complexity of the amplification effect of the seismic basin. Keywords—Basin amplification effect; basin models; numerical simulation; SH pulse waves, data analysis 摘要—盆地放大效应研究对于未来城市的防震减灾规划, 具有重要的参考意义。利用结合透射边界的解耦近场波动有 限元数值模拟方法,以 SH 波的形式在模型底部垂直入射δ脉 冲波和 Ricker 脉冲波,对二维“方形”,“倒凸形”和“曲 边形”三种理想简单盆地模型的放大效应进行了研究,结果 表明:1.“方形”和“倒凸形”两种盆地模型中盆地场点的 地震动响应随场点盆地深度的变化,表现出了不同的规律: “方形”盆地模型表现出了与现有场地反应相类似的特征, 而“倒凸形”盆地模型的计算结果则更符合我们现有对于盆 地放大效应的认识,因此,仅用简单的“方形”盆地模型进 行地震动盆地放大效应的研究是不太合适的。2.对更接近实 际的“曲边形”盆地模型的地震动响应进行了研究,对称模 型的计算结果展示了很好的规律性,在盆地中央位置出现比 较大的地震动响应,体现了盆地“聚焦效应”的作用;非对 称盆地模型在盆地中央和盆地边缘处均出现了比较大的地震 动响应,在较陡的一侧边缘,甚至出现了比盆地中央更大的 地震动响应,这与盆地的“边缘效应”有关;随着盆地内介 质变软,盆地内场点的地震动响应逐渐变大;随着盆地内介 质分层的增加,盆地的地震动响应整体上也有逐渐增大的趋 势,但在盆地内的一些位置也出现了比较大的差异,体现了 地震动盆地放大效应的复杂性。 关键词—盆地放大效应,盆地模型,数值模拟,SH 波