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

对特大密集大孔径AlpArray地震台网（AASN）记录的远震体波进行反演，得到大阿尔卑斯地区岩石圈和岩石圈下地幔（活动场A、B、C和D，研究主题1和4）中地震波速度的三维分布。由此产生的图像有望解析阿尔卑斯山脉下到过渡带的岩石圈板块的内部结构和几何形状。它将为俯冲极性转换等备受争议的问题提供新的见解，并提供关于阿尔卑斯板块可能从下地壳分离的进一步细节，以及它们与阿尔卑斯特提斯晚白垩世至早古近纪俯冲作用导致的过渡带高速物质的联系，以及最终在过渡带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）的重要输入。采用基于波形灵敏度核的、适应远震成像问题的迭代高斯-牛顿全波形反演方法，建立了高寒山带下纵、横波速度三维模型。采用三维谱元或三维不连续伽辽金方法模拟目标区域内的远震波传播，采用混合方法进行远震波传播数值模拟。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求解器。初始波速模型是通过P相和S相的经典层析反演得到的。后者也作为附加约束进入全波形反演，以稳定反周期跳变。通过将灵敏度集中在目标区域的数据进行反向线性组合，将目标体积外的非均质性的影响降至最低。这种方法应该可以在几秒钟的时间内反演体波记录，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.