Imaging of shallow multiple basins and middle to lower crustal structure beneath the Mississippi embayment is still limited. No tomography studies have delineated multiple basins as effectively as those from reflection and drilling experiments. Several studies have constructed different velocity models for the middle to lower crust. We take advantage of the Northern Embayment Lithosphere Experiment dataset and long-term deployment of 277 3-component broadband stations installed from 1990 to 2018 to image crustal shear wave velocity and radial anisotropy in the Mississippi embayment. We first construct 5–40s group velocity maps for Rayleigh wave and 5–30s ones for Love wave. In the 5 s group velocity map, we observe that low velocity features correlate spatially with the sediment thickness from the Crust 1.0 model. High velocity anomalies are associated with the Ozark uplift and the Nashville Dome. In the 32 s group velocity map, high-low velocity boundaries align with 40 km crustal thickness contours from the Crust 1.0 model. We then invert Rayleigh and Love group velocity dispersion for shear wave velocity and radial anisotropy at each grid point and construct a 3D model accordingly. At the depth of 0–5 km of the model, we find that low velocity features characterize the Mississippi Valley Graben, Rough Creek Graben, Black Warrior Basin, and coastal flood plain. High velocity features mark thinly sedimented regions, like the Ozark uplift and the Nashville Dome. Regions with high gravity anomalies are characterized with negative anisotropies. At the depth of 5–12 km, we observe low velocity features in the Missouri Batholith, centering around 10 km. This region is characterized with negative radial anisotropy (Vsh < Vsv), inferring possible intrusions. High-low velocity boundaries correlate with the Appalachian-Ouachita thrust front. At the depth of 12–24 km, the Mississippi Valley Graben is filled with high velocity material, which may be related to the formation of the graben due to isostatic equilibrium. At the depth of 24–43 km, high velocity anomalies characterize the lower crust of the Mississippi embayment, consistent with high density features from the inversion of free air anomaly. The high velocity anomaly might be responsible for the formation of the embayment due to isostatic adjustment. The average velocity in the lower crust is approximately 4.3 km/s. Mafic Pikwitonei granulite is a possible material, indicating relative strong lower crust. This work provides useful information to explain earthquake occurrence in the New Madrid Seismic Zone and regional tectonic settings.

中文翻译：

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来自环境噪声断层扫描的密西西比河湾下方的地壳剪切波速度和径向各向异性

密西西比湾下方浅层多盆地和中下地壳结构的成像仍然有限。没有任何断层扫描研究能够像反射和钻探实验那样有效地描绘出多个盆地。一些研究已经为中下地壳构建了不同的速度模型。我们利用北海湾岩石圈实验数据集和 1990 年至 2018 年安装的 277 个三分量宽带站的长期部署，对密西西比海湾的地壳剪切波速度和径向各向异性进行成像。我们首先为瑞利波构建 5-40s 的群速度图，为拉夫波构建 5-30s 的群速度图。在 5 s 群速度图中，我们观察到低速度特征与 Crust 1.0 模型中的沉积物厚度在空间上相关。高速异常与奥扎克隆起和纳什维尔圆顶有关。在 32 s 群速度图中，高低速度边界与 Crust 1.0 模型中的 40 km 地壳厚度等值线一致。然后，我们对每个网格点处的剪切波速度和径向各向异性进行 Rayleigh 和 Love 群速度色散的反演，并相应地构建 3D 模型。在模型的 0-5 公里深度，我们发现密西西比河谷地堑、Rough Creek 地堑、黑武士盆地和沿海洪泛平原具有低速特征。高速特征标志着薄沉积区域，如奥扎克隆起和纳什维尔圆顶。具有高重力异常的区域具有负各向异性的特征。在5-12公里的深度，我们观察到密苏里基岩的低速特征，以10公里左右为中心。该区域具有负径向各向异性 (Vsh < Vsv)，推断可能存在侵入。高低速度边界与阿巴拉契亚-沃希托逆冲锋相关。在12~24 km深度处，密西西比河谷地堑充满了高速物质，这可能与地堑由于等静压平衡而形成有关。在24-43公里深度，高速异常是密西西比湾下地壳的特征，与自由空气异常反演的高密度特征一致。高速异常可能是由于均衡调整而形成海湾的原因。下地壳的平均速度约为4.3公里/秒。镁铁质 Pikwitonei 麻粒岩是一种可能的物质，表明下地壳相对较强。这项工作为解释新马德里地震带的地震发生和区域构造环境提供了有用的信息。