Imaging structure and geometry of Alpine slabs by full waveform inversion of teleseismic body waves

利用异常密集和大口径的阿尔卑斯阵地震台网(AASN)记录的远距离地震记录的地震体波，反演了大阿尔卑斯山地区(活动区A、B、C和D以及研究主题1和4)岩石圈和地幔下地震波速的三维分布。由此产生的图像预计将解析阿尔卑斯山下至过渡带的岩石圈板块的内部结构和几何形状。它将对诸如俯冲极性转换等激烈争论的问题提供新的见解，并进一步详细说明阿尔卑斯山板块可能从下地壳拆离，它们与阿尔卑斯特提斯山脉晚白垩世至早古近纪俯冲过渡带中的高速物质的联系，以及最后过渡带中积累的俯冲物质的数量(研究主题1)。三维图像将成为岩石圈变形和地幔流动的地球动力学模型以及板块构造重建(研究主题2、3和4)的基本输入。采用基于波形敏感核的迭代高斯-牛顿全波形反演，并适应于远震成像问题，建立了阿尔卑斯山带下P波和S波速度的三维模型。远震波传播的数值模拟是以混合方式进行的，其中使用3D谱单元或3D不连续Galerkin方法来模拟目标区域中的波传播。对于地球的其余部分，采用了一种适用于球对称地球模型的快速高效的求解器。初始波速模型由自动确定的P相和S相的质量分级走时的经典层析反演法得到。后者还作为附加约束进入全波形反转，以稳定反转以防止周期跳跃。通过对其敏感度集中于目标区域的数据的线性组合进行倒置，将目标体积外的非均质性的影响降至最低。这种方法应该允许对人体波记录进行长达几秒的周期反演，这意味着分辨率长度约为20公里。

Seismic body waves from distant earthquakes recorded by the extraordinarily dense and large-aperture AlpArray Seismic Network (AASN) are inverted for the 3D distribution of seismic wave speeds in the lithosphere and sublithospheric mantle beneath the greater Alpine region (activity fields A, B, C and D and research themes 1 and 4). The resulting image is expected to resolve the internal structure and geometry of lithospheric slabs beneath the Alps down to the transition zone. It will give new insights into heavily debated issues such as subduction polarity switches, and provide further detail on possible detachments of the Alpine slabs from the lower crust, on their connection to high-velocity material in the transition zone attributed to Late Cretaceous to Early Paleogene subduction of the Alpine Tethys, and finally on the amount of subducted material accumulated in the transition zone (research theme 1). The 3D image will constitute essential input for geodynamic models of lithospheric deformation and mantle flow as well as plate tectonic reconstructions (research themes 2, 3 and 4).Iterative Gauss-Newton full waveform inversion based on waveform sensitivity kernels and adapted to the teleseismic imaging problem is employed to construct the 3D model of P and S wave speeds beneath the Alpine mountain belt. Numerical simulation of teleseismic wave propagation is performed in a hybrid way where either a 3D spectral element or a 3D discontinuous Galerkin method are used to model wave propagation in the target region. For the rest of the Earth, a fast and efficient solver valid for spherically symmetric earth models is employed. Initial wave speed models are obtained by classical tomographic inversion of automatically determined and quality-ranked travel times of P and S phases. The latter also enter the full waveform inversion as additional constraints to stabilize the inversion against cycle-skipping. Effects of heterogeneities outside the target volume are minimized by inverting linear combinations of the data whose sensitivity is focused on the target region. The approach should permit inversions of body wave records up to periods of a few seconds implying a resolution length of about 20 km.