Love-Rayleigh Scattering and Regional Anisotropy in USArray

与地球构造板块运动相关的地幔剪切往往会使大量地幔岩石中的矿物对齐。地幔岩石中最常见的矿物之一是橄榄石，它在某些方向上比其他方向上更坚硬。刚度的方向性变化称为各向异性。地幔岩石中橄榄石矿物的部分排列导致这些岩石是各向异性的。各向异性表现为地震波在传播方向上速度略有变化，横波获得椭圆偏振，以及入射地震波散射到其他地震波的趋势。Love波是水平极化的面波，当遇到各向异性的区域集中时，它会散射成椭圆极化的瑞利波。这些被称为准Love波或QL波的散射波，对于探测板块边界附近的剪切很有用，在那里，强大的板块在板块周围可塑性变形的地幔伸展，因为它们在大洋海沟下沉，在裂谷带扩散，或者在大陆碰撞的地方坍塌。QL波可以在单独的地震记录上观察到，并补充其他类型的地震数据。研究人员在地球望远镜的一个组成部分--陆基美国阵列的地震数据中发现，QL散射位于西北边界的卡斯卡迪亚俯冲带及其周围。在卡斯卡迪亚板块边界，胡安德富卡板块沿着太平洋西北海岸与北美板块汇合，并下沉到喀斯喀特火山线下的地幔中。胡安德富卡板块下方的近海各向异性似乎与从其他数据估计的沿海各向异性一致。这表明，胡安德富卡板块下方的地幔是由于板块的横向运动而变形的，而不是来自“板块回滚”，即板块在(自身重量的)下降到塑性变形地幔中时的逐渐后退。研究人员正在将他们的分析扩展到整个卡斯卡迪亚俯冲带，沿着追踪单个表面波进程的大圆路径为台站叠加准乐福散射波。最大散射的频率依赖性对各向异性的深度范围很敏感。卡斯卡迪亚俯冲带附近的QL散射是显著的，但内陆地区也存在横向各向异性梯度。研究人员正在将叠加程序从卡斯卡迪亚延伸到内陆，以探索沿圣安德烈亚斯系统、盆地和山脉以及黄石热点的QL交叉路径。为了对数据进行详细的解释，研究人员计算了简化地球模型几何形状的合成地震图作为快速解释工具，并采用SPECFE-GLOBAL地震波模拟器探索各向异性的三维结构，以探索假设的各向异性几何形状的影响。研究人员将绘制美国西部的地图，突出勒夫-瑞利散射强烈的位置，什么对称方向(因此剪切方向)似乎与散射一致，以及各向异性梯度是软流层(即动态的)、岩石圈(可能是化石)还是代表上地幔的更深层次的流动。

Shear in the mantle associated with the motion of Earth's tectonic plates tends to align minerals within large volumes of mantle rock. One of the most common minerals in mantle rock is olivine, which is stiffer in some direction than in others. The directional variation of stiffness is called anisotropy. The partial alignment of olivine minerals in mantle rocks causes these rocks to be anisotropic. Anisotropy expresses itself in the tendency for seismic wavespeed to vary slightly on the direction of propagation, shear-waves to acquire elliptical polarization, and for incoming seismic waves to scatter into other seismic waves. Love wave are horizontally-polarized surface waves that scatter to elliptically-polarized Rayleigh waves when they encounter regional concentrations of anisotropy. These scattered waves, called Quasi-Love, or QL waves, are useful for detecting shear near plate boundaries, where the strong plates stretch the plastically-deformable mantle surrounding the plates as they sink at oceanic trenches, spread at rift zones, or crumple where continents collide. QL waves can be observed on individual seismograms, and complement other types of seismic data. The investigators found QL scattering in and around the Cascadia subduction zone at the NW border in seismic data from the land-based USArray, a component of Earthscope. At the Cascadia plate boundary the Juan de Fuca plate converges with the North American plate along the coast of the Pacific Northwest, and sinks into the mantle beneath the line of Cascade volcanoes. Offshore anisotropy beneath the Juan de Fuca plate appears to align in agreement with coastal anisotropy estimated from other data. This suggests that the mantle beneath the Juan de Fuca plate deforms from the lateral motion of the plate, rather than from "slab rollback," that is, a progressive retreat of the plate as it falls (of its own weight) into the plastically-deforming mantle.The researchers are extending their analysis to the entire Cascadia subduction zone, stacking Quasi-Love scattered waves for stations along great-circle paths that track the progress of individual surface waves. The frequency dependence of the maximum scattering is sensitive to the depth range of anisotropy. QL-scattering proximal to the Cascadia subduction zone is significant, but lateral anisotropic gradients are present inland as well. The researchers are extending the stacking procedure inland from Cascadia to explore QL-crossing paths along the San Andreas system, the Basin and Range, and the Yellowstone hot spot. For detailed interpretation of the data the researchers compute synthetic seismograms in simplified earth-model geometries as a quick-interpretation tool, and explore 3-D structure in anisotropy by adapting the SPECFEM-GLOBE seismic-wave simulator to explore the effects of hypothetical anisotropic geometries. The researchers will produce maps of the Western US, that highlight the locations where Love-to-Rayleigh scattering is strong, what symmetry orientation (and therefore shear orientation) appears to be consistent with the scattering, and whether the anisotropic gradients are asthenospheric (i.e. dynamic), lithospheric (perhaps fossil) or represent deeper flow in the upper mantle.