Radial anisotropy of the North American upper mantle based on adjoint tomography with USArray
Radial anisotropy of the North American upper mantle based on adjoint tomography with USArray
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DOI：
10.1093/gji/ggx305
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发表时间：
2017-10
影响因子：
2.8
通讯作者：
Hejun Zhu;D. Komatitsch;J. Tromp
中科院分区：
文献类型：
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作者：
Hejun Zhu;D. Komatitsch;J. Tromp
我们使用来自USAray的地震数据，基于所谓的“伴随层析成像”（一种迭代全波形反演技术）对美国下方的上地幔进行成像。反演使用4516个地震台站记录的180个区域地震的数据，产生586 185个频率相关的测量值。三分量短周期体波和长周期表面波相结合，同时约束深，浅结构。横观各向同性模型US 22是22次预处理共轭梯度迭代的结果。近似Hessian映射和点扩散函数测试表明，研究区域的照明良好，不同模型参数之间的权衡有限。我们观察到一个明显的波之间的速度对比稳定的美国东部和构造活跃的美国西部。这条边界与落基山脉前沿有很好的对应关系。稳定的超声波区的特征是250-300 km的快速异常，反映了北美岩石圈的厚度。在北美岩石圈下观察到几个快速异常，表明岩石圈拆沉的可能性。慢波速通道在岩石圈之下成像，这可能指示弱软流圈。在美国中部的地幔过渡带下方，观察到一个南北向的拉长的快速异常，这可能是古老的俯冲的Farallon板。美国西部的构造活动主要是由显著的缓慢异常所主导，其幅度大于-6%，下降到约250公里。在黄石公园之下没有观察到连续的下地幔到上地幔的隆起。此外，我们的研究结果证实了以前观察到的海洋和大陆之间的各向异性参数β =（βh/βv）2的差异。在东太平洋100至200公里深处拍摄到一个λ> 1的慢波速度通道，反映了软流圈内的水平切变。在100公里左右的较浅深度处，对水下大陆区、δ> 1的区域进行成像。它们的特点是剪切波速度快，这表明海洋和大陆下的各向异性的不同起源。与东太平洋相比，西大西洋的波速和各向异性特征与大陆区相似。此外，我们观察到在300至400 km深度之间的构造活跃的美国西部下方的深度< 1的区域，这可能反映了Farallon和Juan de Fuca板块俯冲引起的垂直流动。比较US 22与以前的几个层析模型，我们观察到相对较好的相关性长波长的功能。然而，对于小尺度特征，仍然存在很大的差异。
We use seismic data from USArray to image the upper mantle underneath the United States based on a so-called ‘adjoint tomography’, an iterative full waveform inversion technique. The inversion uses data from 180 regional earthquakes recorded by 4516 seismographic stations, resulting in 586 185 frequency-dependent measurements. Three-component short-period body waves and long-period surface waves are combined to simultaneously constrain deep and shallow structures. The transversely isotropic model US22is the result of 22 pre-conditioned conjugate-gradient iterations. Approximate Hessian maps and point-spread function tests demonstrate good illumination of the study region and limited trade-offs among different model parameters. We observe a distinct wave-speed contrast between the stable eastern US and the tectonically active western US. This boundary is well correlated with the Rocky Mountain Front. Stable cratonic regions are characterized by fast anomalies down to 250–300 km, reflecting the thickness of the North American lithosphere. Several fast anomalies are observed beneath the North American lithosphere, suggesting the possibility of lithospheric delamination. Slow wave-speed channels are imaged beneath the lithosphere, which might indicate weak asthenosphere. Beneath the mantle transition zone of the central US, an elongated north–south fast anomaly is observed, which might be the ancient subducted Farallon slab. The tectonically active western US is dominated by prominent slow anomalies with magnitudes greater than −6 per cent down to approximately 250 km. No continuous lower to upper mantle upwellings are observed beneath Yellowstone. In addition, our results confirm previously observed differences between oceans and continents in the anisotropic parameter ξ = (βh/βv)2. A slow wave-speed channel with ξ > 1 is imaged beneath the eastern Pacific at depths from 100 to 200 km, reflecting horizontal shear within the asthenosphere. Underneath continental areas, regions with ξ > 1 are imaged at shallower depths around 100 km. They are characterized by fast shear wave speeds, suggesting different origins of anisotropy underneath oceans and continents. The wave speed and anisotropic signatures of the western Atlantic are similar to continental areas in comparison with the eastern Pacific. Furthermore, we observe regions with ξ < 1 beneath the tectonically active western US at depths between 300 and 400 km, which might reflect vertical flows induced by subduction of the Farallon and Juan de Fuca Plates. Comparing US22with several previous tomographic models, we observe relatively good correlations for long-wavelength features. However, there are still large discrepancies for small-scale features.