A Bayesian approach to assess the importance of crustal corrections in global anisotropic surface wave tomography
A Bayesian approach to assess the importance of crustal corrections in global anisotropic surface wave tomography
复制标题
评估全球各向异性表面波层析成像中地壳修正重要性的贝叶斯方法
DOI：
10.1093/gji/ggv401
复制
发表时间：
2015
影响因子：
2.8
通讯作者：
C. Beghein
中科院分区：
文献类型：
--
作者：
Z. Xing;C. Beghein
最近的几项研究表明，在反演面波数据以模拟地幔径向各向异性的横向变化时，地壳校正是重要的。它还表明，选择先前的地壳模型来校正数据会强烈影响各向异性模型，并可能导致不同的地球动力学解释。然而，在比较由不同地壳校正获得的径向各向异性层析模型时，这些研究没有确定定量模型的不确定性。然而，只有当反问题的非唯一性引起的后验模型误差小于地壳修正本身的影响时，不同先前地壳修正产生的地幔模型才在统计上不同。在这里，我们将模型空间搜索方法应用于全球基波和高阶瑞利波和乐夫波相速度图，以确定可靠的、定量的关于地震速度和径向各向异性的模型不确定性。所采用的技术使我们能够用后验概率密度函数来描述模型空间，从而检验从不同地壳改正获得的模型是否在统计上不同。因此，我们通过将后验模型误差与不同地壳修正引起的地幔结构差异进行比较，评估了选择地壳模型的意义。我们测试了以前的地壳模型CRUST2.0、CRUST1.0和3SMAC。我们的研究表明，使用不同地壳模型的先前地壳改正，在50公里深度附近的地幔速度和100公里以下的径向各向异性方面存在显著差异。地壳改正对径向各向异性的影响在某些地区可以延伸到250公里以下。我们发现，选择3SMAC模型对地幔模型的影响更大，但CRUST1.0和CRUST2.0在所有深度得到统计上相同的各向异性模型，除了少数网格单元。重要的是，地壳模型的影响在大陆地区最显著，而不是在海洋以下，这对确定大陆根部的深度具有重要影响。因此，我们的结果表明，改善对大陆地壳结构的制约对于我们理解大陆的形成至关重要。我们的工作还表明，先前的地壳模型对100公里以上深度的径向各向异性和速度没有显著影响。这意味着，如果100公里深度以下的径向各向异性的地球动力学解释能够解释层析模型的不确定性，它们将不依赖于先前地壳模型的选择。因此，重要的是地球动力学家和地震学家应通力合作，在解释结果之前努力确定定量层析模型的不确定性。我们的结果也告诫不要使用3SMAC来校正大陆岩石圈研究中的面波数据，并表明固体地球社区将受益于为建立修订后的3SMAC所做的一些努力。基于3SMAC地壳改正建立的地幔模型与基于CRUST1.0或CRUST2.0建立的地幔模型之间的差异也应该有助于阐明在构建类似3SMAC的模型时所做的地球动力学假设的有效性。
Several recent studies have demonstrated the importance of crustal corrections when inverting surface wave data to model lateral variations in mantle radial anisotropy. It has also been shown that the choice of the prior crustal model to correct the data can strongly influence the anisotropy model and potentially lead to different geodynamic interpretations. In comparing tomographic models of radial anisotropy obtained from different crustal corrections, these studies did not, however, determine quantitative model uncertainties. Nevertheless, mantle models resulting from different prior crustal corrections are statistically different only if the posterior model errors stemming from the non-uniqueness of the inverse problem are smaller than the effect of the crustal correction itself. Here, we applied a model space search approach to global fundamental and higher mode Rayleigh and Love wave phase velocity maps to determine reliable, quantitative model uncertainties on seismic velocities and radial anisotropy. The technique employed enabled us to describe the model space with a posterior probability density function, and therefore to test whether models obtained from different crustal corrections are statistically different. We thus assessed the significance of the choice of the crustal model by comparing the posterior model errors to the differences in mantle structure resulting from different crustal corrections. We tested prior crustal models CRUST2.0, CRUST1.0, and 3SMAC. Our study shows that the use of prior crustal corrections from different crustal models yields significant discrepancies in mantle velocities around 50 km depth and in radial anisotropy down to 100 km. The impact of the crustal correction on radial anisotropy can extend down to 250 km in some locations.We found that choosing 3SMAC instead of the other crustal models has a stronger influence on the mantle model, but that CRUST1.0 and CRUST2.0 yield statistically identical anisotropy models at all depths, except at a few grid cells. Importantly, the effect of the crustal model is most significant in continental regions and not so much beneath oceans, which has important consequences for determining the depth of continental roots. Our results therefore suggest that improving constraints on crustal structure in continents is essential for our understanding of continent formation. Our work also demonstrates that the prior crustal model does not significantly affect radial anisotropy and velocities at depths greater than 100 km. This implies that if geodynamic interpretations of radial anisotropy below 100 km depth were to account for tomographic model uncertainties, they would not depend on the choice of the prior crustal model. It is therefore important for geodynamicists and seismologists to work in concert and to put effort into determining quantitative tomographic model uncertainties before interpreting the results. Our results also caution against the use of 3SMAC to correct surface wave data for studies of the continental lithosphere, and suggest that the solid Earth community would benefit from putting some efforts toward building a revised 3SMAC. The discrepancies between mantle models built based on 3SMAC crustal corrections and those based on CRUST1.0 or CRUST2.0 should also help shed light on the validity of the geodynamical assumptions made in the construction of models like 3SMAC.