Koroni Maria, Paulssen Hanneke, Trampert Jeannot
Department of Earth Sciences ETH Zürich Zürich Switzerland.
Department of Earth Sciences Universiteit Utrecht Utrecht The Netherlands.
Geophys Res Lett. 2019 Jan 28;46(2):698-707. doi: 10.1029/2018GL081592. Epub 2019 Jan 23.
We analyze the sensitivity of PP precursor traveltimes that are often used to infer lateral variation in the depths of the 410- and 660-km discontinuities in the mantle. Previous results were inconclusive due to complex wave phenomena, such as multiple energy conversions and focusing/defocusing, that hamper their interpretation. Using spectral-element synthetics and Fréchet derivatives calculated with adjoint methods, we compute sensitivity kernels for volumetric and boundary parameters in a 1-D model for representative epicentral distances of past studies, and a dominant period of 11-25 s. Our results indicate that the boundary sensitivity of PP precursors is low and that these phases are not coherently seen in exact synthetics. Our most important finding is the strong sensitivity to both shear and compressional wave speeds, indicating that wave interference and wave conversions are dominant. The PP precursor traveltimes appear more sensitive to structural parameters, that is, compressional and shear wave speed, than to the boundaries; therefore, they are unlikely sources for valuable insight into discontinuity topography.
我们分析了常用于推断地幔中410千米和660千米不连续面深度横向变化的PP前驱波走时的敏感性。由于复杂的波动现象，如多次能量转换和聚焦/散焦，妨碍了对其的解释，先前的结果尚无定论。利用谱元合成地震图和用伴随方法计算的弗雷歇导数，我们针对过去研究中的代表性震中距以及11 - 25秒的主导周期，计算了一维模型中体积参数和边界参数的敏感性核。我们的结果表明，PP前驱波的边界敏感性较低，并且在精确合成地震图中这些震相并不能一致地被看到。我们最重要的发现是对剪切波速和压缩波速都有很强的敏感性，这表明波的干涉和波的转换起主导作用。PP前驱波走时似乎对结构参数（即压缩波速和剪切波速）比对边界更敏感；因此，它们不太可能成为深入了解不连续面地形的有价值信息来源。
