Azimuthal Anisotropy of the North American Upper Mantle Based on Full Waveform Inversion
Azimuthal Anisotropy of the North American Upper Mantle Based on Full Waveform Inversion
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DOI：
10.1029/2019jb018432
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发表时间：
2020-02
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影响因子：
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通讯作者：
Hejun Zhu;Jidong Yang;Xueyan Li
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文献类型：
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作者：
Hejun Zhu;Jidong Yang;Xueyan Li
一个新的方位各向异性模型的北美和加勒比板块，即，US 32，构造全波形反演和记录的基础上，从USAray和其他临时/永久网络部署在研究区域。在反演中总共使用了180个地震和4，516个地震台站，以同时约束地壳和地幔内的径向和方位各向异性模型参数：L，N，Gc和Gs。使用了32次预处理共轭梯度迭代来最小化三分量短周期（15-40 s）体波和长周期（25-100 s）表面波的观测和预测地震图之间的频率相关相位差。模型US 32显示了美国西部和东部地下各向异性组构的复杂变化，特别是在浅于100公里的深度。例如，模型US 32中的快轴方向表明卡斯卡迪亚俯冲带下方存在海沟垂直地幔流，并且还遵循蛇河平原、沃希托造山带前缘以及格伦维尔和阿巴拉契亚造山带的走向。方位各向异性的幅度在深度大于200公里时减小到1%左右，方位与落基山脉以东的全球板块运动方向近似平行，但加拿大中部和东部的差异较大。在700 km的深度处，快轴沿着五大湖地区和墨西哥湾下方的Farallon板块的轨迹变化，这可能表明在最上部的下地幔内垂直于下沉板块走向的二维极向模地幔流的发展。考虑到不同数据集和成像技术的使用，从环境噪声层析成像和SKS分裂测量的模型US 32与美国西部模型之间的比较表明，快速轴方向具有相对良好的一致性。然而，在模型US 32中方位各向异性的绝对大小可能被低估，特别是在更大的深度，考虑到预测和观测SKS分裂时间的幅度不一致。在目前阶段，不同的方位各向异性模型之间的协议在全球和大陆尺度仍然是穷人，即使是美国与密集的站覆盖。
A new azimuthal anisotropy model for the North American and Caribbean Plates, namely, US32 , is constructed based on full waveform inversion and records from the USArray and other temporary/permanent networks deployed in the study region. A total of 180 earthquakes and 4,516 seismographic stations are employed in the inversion to simultaneously constrain radially and azimuthally anisotropic model parameters: L , N , Gc , and Gs , within the crust and mantle. Thirty‐two preconditioned conjugate gradient iterations have been utilized to minimize frequency‐dependent phase discrepancies between observed and predicted seismograms for three‐component short‐period (15–40 s) body waves and long‐period (25–100 s) surface waves. Model US32 exhibits complicated variations in anisotropic fabrics underneath the western and eastern United States, especially at depths shallower than 100 km. For instance, the fast axis orientations in model US32 suggest the presence of trench‐perpendicular mantle flows underneath the Cascadia Subduction Zone and also follow the strikes of the Snake River Plain, the Ouachita Orogenic Front, and the Grenville and Appalachian Orogenic Belts. The amplitudes of azimuthal anisotropy reduce to around 1% at depths greater than 200 km, and the orientations are subparallel to the global plate motion directions to the east of the Rocky Mountain, except for large discrepancies in central and eastern Canada. At a depth of 700 km, the fast axes change along the trajectory of the Farallon slab underneath the Great Lakes region and Gulf of Mexico, which might indicate the development of 2‐D poloidal‐mode mantle flows perpendicular to the strike of the sinking slab within the uppermost lower mantle. Comparisons between model US32 with a western U.S. model from ambient noise tomography and SKS splitting measurements demonstrate a relatively good agreement for the fast axis orientations, considering the usage of different data sets and imaging techniques. However, the absolute magnitude of azimuthal anisotropy in model US32 might be underestimated, especially at greater depths, given the poor agreement on the amplitudes of predicted and observed SKS splitting times. At the current stage, the agreement among different azimuthal anisotropy models at global and continental scales is still poor even for the United States with a dense station coverage.