The horizontal to vertical spectral ratio (HVSR) of seismic noise is often used to investigate site effects, and it is usually assumed to be a stable feature of the site considered. Here we show that such an assumption is not always justified, and may lead to incorrect conclusions. The HVSR analysis was performed on ambient seismic noise recordings lasting from weeks to months at many sites in Calabria, Italy. Results show a variety of site effects, from the resonance of a shallow sedimentary layer to the polarized amplification of horizontal ground motion associated with topographic effects. We describe the results of seven sites whose HVSR is characterized by dual content: one that is persistent, and another appearing only occasionally. Two sites very near the coast of the Tyrrhenian sea and five sites in the Calabrian Arc mountains show the most remarkable results. The shape of the HVSR changes significantly at these sites when the amplitude of background noise increases in a broad frequency band during periods of bad weather. The occasional contribution to the HVSR consists of one or more peaks, depending on the site, that appear only when the amplitude of ambient noise is higher than usual. The seven sites where we observe the HVSR variability are all located in complex geological environments, on mountains, ridges or foothills. A variation of the HVSR correlated with the day–night cycle is also observed at some of these sites.

中文翻译：

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靠近或位于地形高度的地点的噪声HVSR不稳定

地震噪声的水平与垂直频谱之比（HVSR）通常用于调查场地影响，通常认为这是所考虑场地的稳定特征。在这里，我们表明这种假设并不总是合理的，并且可能导致错误的结论。HVSR分析是在意大利卡拉布里亚的许多地点对持续数周到数月的环境地震噪声记录进行的。结果显示出各种场地效应，从浅沉积层的共振到与地形效应相关的水平地面运动的极化放大。我们描述了HVSR具有双重内容特征的七个站点的结果：一个站点持续存在，另一个站点仅偶尔出现。最接近第勒尼安海海岸的两个地点和卡拉布里亚弧形山脉的五个地点显示了最显着的结果。在恶劣天气期间，当背景噪声的幅度在较宽的频带中增加时，HVSR的形状在这些位置处发生明显变化。HVSR的偶尔影响是由一个或多个峰值组成，具体取决于位置，仅当环境噪声的幅度高于平常时才会出现。我们观察到HVSR变异性的七个地点都位于复杂的地质环境中，包括山脉，山脊或山麓。在这些站点中的某些站点，还观察到与昼夜周期相关的HVSR变化。在恶劣天气期间，当背景噪声的幅度在较宽的频带内增加时，HVSR的形状将在这些位置发生明显变化。HVSR的偶尔影响是由一个或多个峰值组成，具体取决于位置，仅当环境噪声的幅度高于平常时才会出现。我们观察到HVSR变异性的七个地点都位于复杂的地质环境中，包括山脉，山脊或山麓。在这些站点中的某些站点，还观察到与昼夜周期相关的HVSR变化。在恶劣天气期间，当背景噪声的幅度在较宽的频带内增加时，HVSR的形状将在这些位置发生明显变化。HVSR的偶尔影响是由一个或多个峰值组成，具体取决于位置，仅当环境噪声的幅度高于平常时才会出现。我们观察到HVSR变异性的七个地点都位于复杂的地质环境中，包括山脉，山脊或山麓。在这些站点中的某些站点，还观察到了与昼夜周期相关的HVSR变化。我们观察到HVSR变异性的七个地点都位于复杂的地质环境中，包括山脉，山脊或山麓。在这些站点中的某些站点，还观察到与昼夜周期相关的HVSR变化。我们观察到HVSR变异性的七个地点都位于复杂的地质环境中，包括山脉，山脊或山麓。在这些站点中的某些站点，还观察到与昼夜周期相关的HVSR变化。