双俯冲地球动力学模型的各向异性和XKS分裂:测试解释的局限性

IF 2.8 3区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS
Jan Phillip Kruse, Georg Rümpker, Frederik Link, Thibault Duretz, Harro Schmeling
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引用次数: 0

摘要

摘要 XKS相分裂特征的分析对于约束地震各向异性模式至关重要,尤其是在外倾双俯冲等复杂的俯冲环境中。地中海中部就是一个自然的例子,亚平宁板块和迪纳里德板块以相反的方向俯冲,亚得里亚海板块将它们隔开。为了评估 XKS 分裂分析在揭示各向异性地震特性(如快速极化方向和剪切波各向异性(以百分比表示))方面的能力,我们使用了三维数值地球动力学模型并结合纹理演变模拟。在这些模型中,两个完全相同的向外倾斜的大洋板块被一个大陆板块隔开。利用从质地演化模拟中直接导出的全弹性张量,我们计算了各向异性地震特性和合成远震波形。根据这些波形确定合成观测值,包括表观分裂参数(快速极化方向和延迟时间)和分裂强度。根据这些观测数据,我们(1)推导出单层各向异性层的模型(单层模型),(2)确定具有显著深度各向异性地震特性的区域,以及(3)根据两个各向异性层(双层模型)在选定位置进行反演。我们考虑了两个地球动力学模型:一个是强大陆板块模型(M1),另一个是弱大陆板块模型(M2)。模型 M1 显示俯冲板块大幅后退,大陆板块没有水平拉伸;而模型 M2 显示俯冲板块后退幅度较小、水平拉伸幅度较大和脱离。这些不同的俯冲方式导致了上地幔不同的流动和变形模式,并反映在各向异性的地震特性中。在模型 M1 中,大陆板块下方的快速极化方向主要与海沟平行,而在模型 M2 中,则主要与海沟正交。在这两个模型的大部分区域,单层模型足以解析各向异性地震特性,因为这些特性随深度变化几乎不变。然而,对于这两个模型,我们发现了一些孤立的区域--主要是俯冲板块顶端附近和大陆板块下方--快速极化方向随深度的变化表现出显著的差异。将这些区域的表观分裂参数反转,可以在每个位置得到多个双层模型,这些模型都能很好地拟合观测数据。然而,它们的各向异性地震特性可能会有很大差异,并非所有这些双层模型都能充分逼近真实的深度变化。通过选择剪切波各向异性总和与其中一个单层模型接近的双层模型,可以部分减少这种模糊性,因为这些模型能更好地捕捉真实的变化。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Anisotropy and XKS-splitting from geodynamic models of double subduction: testing the limits of interpretation
Summary The analysis of the splitting signature of XKS phases is crucial for constraining seismic anisotropy patterns, especially in complex subduction settings such as outward-dipping double subduction. A natural example of this is found in the Central Mediterranean, where the Apennine and the Dinaride slabs subduct in opposite directions, with the Adriatic plate separating them. To assess the capability of XKS-splitting analysis in revealing anisotropic seismic properties, such as fast polarization directions and shear wave anisotropy (in per cent), we use three-dimensional numerical geodynamic models combined with texture evolution simulations. In these models, two identical outward-dipping oceanic plates are separated by a continental plate. Using the full elastic tensors—directly derived from the texture evolution simulations—we compute anisotropic seismic properties and synthetic teleseismic waveforms. From these waveforms synthetic observables are determined, including apparent splitting parameters (fast polarization directions and delay times) and splitting intensities. Based on these observables, we (1) derive models for a single anisotropic layer (one-layer model), (2) identify regions with significant depth-dependent anisotropic seismic properties, and (3) perform inversions at selected locations in terms of two anisotropic layers (two-layer model). We consider two geodynamic models: one with a strong (M1) and one with a weak (M2) continental plate. Model M1 exhibits significant retreat of the subducting plates with no horizontal stretching of the continental plate, whereas Model M2 shows less retreat, substantial horizontal stretching, and detachment of the subducting plates. These different subduction styles result in distinct flow and deformation patterns in the upper mantle, which are reflected in the anisotropic seismic properties. In Model M1, the fast polarization directions below the continental plate are predominantly trench-parallel, whereas in Model M2, they are mostly trench-normal. In most regions of both models, the one-layer models are sufficient to resolve the anisotropic seismic properties, as these properties are nearly constant with depth. However, for both models, we identify some isolated regions—primarily near the tips of the subducting plates and beneath the continental plate—where fast polarization directions exhibit significant variations with depth. Inverting the apparent splitting parameters in these regions yields multiple two-layer models at each location that excellently fit the observables. However, their anisotropic seismic properties can vary significantly, and not all these two-layer models adequately approximate the true depth variations. This ambiguity can be partially reduced by selecting two-layer models in which the summed shear wave anisotropy closely matches that of one of the one-layer models, as these models better capture the true variations.
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来源期刊
Geophysical Journal International
Geophysical Journal International 地学-地球化学与地球物理
CiteScore
5.40
自引率
10.70%
发文量
436
审稿时长
3.3 months
期刊介绍: Geophysical Journal International publishes top quality research papers, express letters, invited review papers and book reviews on all aspects of theoretical, computational, applied and observational geophysics.
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