High frequency response of a sediment-filled valley for seismic waves

本文研究了沉积谷在SH波作用下的时域响应和频域响应，重点研究了地震工程中感兴趣的高频（bbb10 ~ 1hz）场地响应。我们用边界积分方程法计算它们，其中格林函数用离散波数法求值。通过计算充沙河谷的频域响应和时域响应，详细讨论了河谷倾斜界面对波的放大模式、Love波的传播和河谷地震动持续时间的影响。本文还根据几个站点的高主频人工地震地面输入运动引起的加速度响应计算了地面运动与基岩运动的谱比，并将其与平坦层对基岩运动响应的谱比进行了比较。结果表明，由于倾斜界面产生的Love波对山谷响应的影响较大，因此填沙山谷的谱比比平坦层响应的谱比更宽。通过对地下不规则地基上的钢筋混凝土模型地基强制振动引起的地面运动的详细测量，研究了地下不规则地基中谐波的传播特性。在三个不同的基础上，每一个都在三个方向上被激发，以产生不同的波传播模式。利用粒子轨道、相干函数和相邻点间交叉功率谱的相位信息，构建相速度色散曲线，研究波在地下不规则地面上的传播特征。结果表明，在不太不规则的地下地面上测得的频散数据与理论值吻合较好。

Time-domain responses as well as frequency-domain responses of a sediment-filled valley for incident SH waves are studied with the emphasis of site responses for high frequencies(>1 Hz)which are of interest in earthquake engineering. We calculate them using a boundary integral equation method in which Green's functions are evaluated using the discrete wave number method. Calculating frequency-domain responses as well as time-domain responses of the sediment-filled valley, we discuss in details effects of inclined interfaces of the valley upon wave amplification patterns, propagation of Love waves and duration of the ground shaking in the valley.Some spectral ratios of surface ground motion to bedrock motion are also calculated based on the acceleration responses at several sites due to an artificial earthquake ground input motion which has relatively high predominant frequencies and they are compared with spectral ratios of the flat layer response to bedrock motion. It is shown that since Love waves generated by the inclined interface have a great influence on the responses of the valley, spectral ratios of sediment-filled valley become more broad-band than that of the flat layer response.Propagation characteristics of harmonic waves by subsurface irregularities is studied experimentally with the detailed measurements of a surface ground motion generated by forced vibration of a reinforced concrete model footing placed on a subsurface irregular ground. Each of three different footings was excited in three kinds of directions to generate different patterns of wave propagation. We evaluated particle orbits, coherence function and phase information from the cross-power spectrum between adjacent sites and constructed phase velocitv dispersion curves to investigate wave propagation characteristics on subsurface irregular ground. It is shown that dispersion data measured on less irregular subsurface ground are in good agreement with the theoretical values.