Interrogation of global seismic models with 3D wave simulations

层析成像是一种强大的地震技术，用于约束地球内部的三维(3-D)结构[例如，Romanowicz，2003；瑟伯和RitSema，2005]。三维地震结构模型在地球科学家的研究中占有重要地位，走时(体波或面波)是层析成像中最常用的数据类型。走时可以在地震学目录中找到，也可以用相对直接的方式来测量。此外，走时可以以线性方式与速度非均质性相关，从而简化了层析反问题。数以百万计的旅行时间构成了现代数据集。然而，层析成像模型是否解释了波传播的其他属性，以及简化的波理论如何影响模型的准确性？这些都是我们在研究中要解决的问题。我们正在分析三维频谱元素法(SEM)波模拟[Komatitsch和Tromp，1999；Komatitsch等人，2002]，以调查如何使用传统上不用于层析反演的观测值来改进3D地震模型。(1)体波幅度我们已经证明[RitSema等人，2002年；Komatitsch等人，2002年]，对于S20RTS模型，SS/S幅度比的模式(但不是幅度)可以用地幔中的剪切速度不均匀来解释。我们正在研究这些体波振幅比是否可以用来限制地幔中剪切速度的(横向)梯度，我们怀疑断层成像模型低估了这一梯度。(2)与频率相关的体波传播时间我们正在观察到体波传播时间在20-100兆赫之间的系统变化。虽然这种变化在理论上是预期的[Dahlen等人，2000]，但我们正在使用扫描电子显微镜模拟来研究这种信号是否可以用来限制地幔中非均质性的幅度和频谱。(3)“小尺度”非均质性的分辨率我们正在分析扫描电子显微镜波形，以进行新的分辨率测试，以探索“小尺度”(即200公里)结构是否产生可观察到的旅行时或波形变化。我们对限制冰岛的地幔结构特别感兴趣，冰岛是一个覆盖在异常地幔上隆起的地区。广泛的影响：这项研究正在支持一名地球物理学学生在地震数据分析和高级科学计算方面的培训。这项研究加强了P.I.S与全球波传播理论和地球动力学专家的合作。

Tomography is a powerful seismic technique for constraining the three-dimensional (3-D) structure of the Earth's interior [e.g., Romanowicz, 2003; Thurber and Ritsema, 2005]. Models of 3-D seismic structure take a central place in the research of Earth scientists from a wide range of disciplines.The traveltime (of a body- or surface wave) is the most popular data type in tomography. Traveltimes are available in seismological catalogs, or they can be measured in a relatively straightforward manner. Moreover, traveltimes can be related to velocity heterogeneity in a linear fashion, simplifying the tomographic inverse problem. Millions of traveltimes constitute modern data sets. However, do tomographic models explain other attributes of wave propagation and how do simplified wave theories compromise model accuracy? These are the questions we are addressing in our research.We are analyzing 3-D Spectral-Element Method (SEM) wave simulations [Komatitsch and Tromp, 1999; Komatitsch et al., 2002] to investigate how 3-D seismic models can be improved using observables that are traditionally not used in tomographic inversions.(1) Body wave amplitudesWe have demonstrated [Ritsema et al., 2002; Komatitsch et al., 2002] that the pattern (but not the amplitude) of SS/S amplitude ratios can be explained by shear velocity heterogeneity in the mantle for model S20RTS. We are investigating whether these body wave amplitude ratios can be used to constrain the (lateral) gradients of shear velocity in the mantle, which we suspect to be underestimated by tomographic models.(2) Frequency-dependent body wave traveltimesWe are observing a systematic variation of body wave traveltimes between 20-100 mHz. While such a variation is expected on theoretical grounds [Dahlen et al., 2000], we are using SEM simulations to investigate whether this signal can be used to constrain the amplitude and spectrum of heterogeneity in the mantle.(3) Resolution of "small-scale" heterogeneityWe are analyzing SEM waveforms to conduct new resolution tests that explore whether "small-scale" (i.e., 200 km) structures produce observable traveltime or waveform variations. We are particularly interested in constraining the mantle structure of Iceland, a region overlying an anomalous mantle upwelling.Broader Impact: The research is supporting the training of a Geophysics student in seismological data analysis and advanced scientific computing. The research enhances the P.I.'s collaboration with specialists in global wave propagation theories and geodynamics.