We investigate seismic site response by inverting seismic ground‐motion spectra for site and source spectral properties, in a region of central Oklahoma, where previous ground‐motion studies have indicated discrepancies between observations and ground‐motion models (GMMs). The inversion is constrained by a source spectral model, which we computed from regional seismic records, using aftershocks as empirical Green’s functions to deconvolve site and path effects. Site spectra across the region exhibit multiple, strong, low‐frequency (⁠f<2 Hz⁠) resonances. Modeling of vertically propagating SH waves reproduces the mean amplitudes and frequencies of the site spectra and requires a deep (⁠∼1–2 km⁠) impedance contrast. Comparison of regional seismic velocity models and geologic profiles indicates that the seismic impedance contrast is, or is in proximity to, the Great Unconformity, which marks the interface between Precambrian basement rocks and overlying Paleozoic sedimentary rocks. Depth to Precambrian basement increases to the southwest across the study region (⁠∼1500–4500 m⁠), and the fundamental frequencies of the site spectra are anticorrelated with basement depth. The first higher‐mode resonance also exhibits dependence on basement depth; although modeling suggests that the second higher mode should depend on basement depth, site spectra do not support this. The low‐frequency resonances in central Oklahoma are not represented in the GMMs used in current seismic hazard analyses for tectonic earthquakes, though approaches to account for such features are under consideration in other regions of the central and eastern United States. Given the broad spatial extent of the Great Unconformity underlying eastern North America, it is likely that similar effects on seismic site response also occur in other areas. This study highlights the impact of regional geologic structure on earthquake ground motions and reiterates the need for modeling regional effects to improve ground‐motion predictions and seismic hazard assessments.

中文翻译：

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俄克拉荷马州中部地震现场响应的频谱反演：来自极大不整合面的低频共振

我们通过反转俄克拉荷马州中部地区的地震动和震源频谱特性的地震地动谱来调查地震现场的响应，以前的地震动研究已经表明了观测值与地震动模型（GMM）之间的差异。该反演受源谱模型的约束，该源谱模型是我们根据区域地震记录计算出来的，使用余震作为经验格林函数来消除场地和路径影响的卷积。整个区域的站点频谱显示出多个强烈的低频（f <2Hz⁠）共振。垂直传播的SH波的模型再现了现场频谱的平均幅度和频率，并且需要很深的（〜1-2km⁠）阻抗对比。区域地震速度模型和地质剖面的比较表明，地震阻抗对比是，或与之接近，这标志着前寒武纪基底岩石与上覆古生界沉积岩石之间的界面。前寒武纪基底的深度在整个研究区域内向西南方向增加（⁠〜1500-4500m⁠），且站点频谱的基频与基底深度呈反相关。第一次高模共振也表现出对基底深度的依赖性。尽管建模表明第二个更高的模式应该取决于地下室的深度，但现场光谱不支持这一点。尽管在美国中部和东部的其他地区正在考虑解决此类特征的方法，但俄克拉荷马州中部的低频共振并未在当前地震构造地震分析中使用的GMM中得到体现。考虑到北美东部潜在的巨大不整合面的广泛空间范围，其他地区也可能对地震现场响应产生类似影响。这项研究强调了区域地质结构对地震地震动的影响，并重申需要对区域影响进行建模，以改善地震动预测和地震灾害评估。