Near-Surface Structure of the Continental United States Using Distant Earthquakes

该项目使用一种利用EarthScope USArray地震数据的新方法来研究美国最浅的结构。纵波和横波速度的深度相关估计不仅有助于地下成像研究，而且有助于地震危险性评估和地震工程。新方法是非侵入式的，因此不需要钻井或使用炸药进行现场实验，并且可以应用于任何具有单个地震站或密集阵列的地点。此外，如果不寻求深度依赖，可以使用易于安装和廉价的仪器，从而以低成本揭示土壤/地壳结构的详细变化。从本研究中获得的近地表结构可以用作地面运动模拟的输入模型，并将有助于地震预警等需要在关键位置预测震动的项目。该项目还将促进对美国大陆地质研究的多学科研究。由于该技术可以解析顶部几公里或更小的地震构造，因此本研究揭示的横向变化可以与地面观测到的地质省份进行比较和解释。目标的浅层也开始将全球地震学分析（使用遥远的地震数据）与近地表临界带研究结合起来，并有助于我们理解发生在地球表面的复杂过程。这项新技术利用体波偏振来解析地震台站下方的波速。与旅行时间等其他量相比，地震波极化信息相对未被开发，将建立并验证远震体波极化信息与近地表波速度之间关系的基本理论框架。与直觉相反，远震纵波的极化对横波速度敏感，而S波的极化对纵波和横波速度都敏感。此外，检查极化数据的频率依赖性，可以构建浅地下的深度依赖波速模型。该方法将应用于USArray数据，以产生美国大陆近地表波速模型。通过不同的频率滤波器，结合可移动阵列、参考网络和柔性阵列的数据，在不同的尺度上研究波速在垂直和水平维度上的空间变化。该结果将更好地约束地壳性质，并可用于改进用于其他类型研究（如全球或区域地震层析成像）的地壳校正。

This project examines shallowest structure of the United States using a novel approach that utilizes EarthScope USArray seismic data. Depth-dependent estimates of both compressional- and shear-wave speeds will contribute not only to the studies of the subsurface imaging, but also to seismic hazard assessment and earthquake engineering. The new method is non-invasive, such that well drilling or field experiments using explosives are not necessary, and can be applied to any site with a single seismic station or dense arrays. Furthermore, if depth dependence is not sought, easy-to-install and inexpensive instruments can be used, allowing detailed variationsin the soil/crust structure to be revealed at low cost. The near-surface structure obtained from this study can be used as an input model for ground motion simulations, and will contribute to such programs as earthquake early warning that requires shaking predictions at critical locations. This project will also promote multidisciplinary research on the geological study of the continental United States. Since the technique can resolve seismic structure of top few kilometers or less, the lateral variation revealed from this study can be compared to and interpreted with geological provinces observed at the surface. The shallowness of the target also starts to bring together the global seismological analysis (using distant earthquake data) with near-surface critical zone studies, and contribute to our understanding of complex processes that occur at the Earth's surface.The new technique utilizes body-wave polarization to resolve wave speed immediately beneath a seismic station. Seismic wave polarization information has been relatively unexploited compared to other quantities such as travel times, and the essential theoretical framework for relating the polarization information of teleseismic body waves to near-surface wave speeds will be established and tested. Counter-intuitively, the polarization of teleseismic P waves is sensitive to shear-wave speed, while that of S waves is sensitive to both compressional and shear-wave speeds. Moreover, examining the frequency dependence of the polarization data allows a depth-dependent wave speed model of the shallow subsurface to be constructed. The method will be applied to the USArray data to produce the near-surface wave speed model of the continental United States. The spatial variation of the wave speeds, in both vertical and horizontal dimensions, will be studied at different scales through different frequency filters and combination of data from Transportable Array, Reference Network, and Flexible Array. The results will provide better constraints on the crust properties, and can be used to improve crustal corrections used for other types of studies such as global or regional seismic tomography.