Love-Rayleigh Scattering and Regional Anisotropy in USArray

与地球构造板块运动相关的地幔剪切力往往会排列大量地幔岩石中的矿物。 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更硬。 刚度的方向变化称为各向异性。 地幔岩石中橄榄石矿物的部分排列导致这些岩石具有各向异性。 各向异性表现为地震波速在传播方向上略有变化、剪切波获得椭圆偏振以及入射地震波散射成其他地震波的趋势。拉夫波是水平偏振表面波，当遇到各向异性的区域集中时会散射为椭圆偏振瑞利波。 这些散射波被称为准洛夫波或 QL 波，可用于检测板块边界附近的剪切力，当板块在海沟下沉、在裂谷区扩散或在大陆碰撞处破裂时，强板块会拉伸板块周围的塑性变形地幔。 QL 波可以在单独的地震图上观察到，并补充其他类型的地震数据。研究人员在来自陆基 USArray（Earthscope 的组成部分）的地震数据中发现了西北边界卡斯卡迪亚俯冲带及其周围的 QL 散射。 在卡斯卡迪亚板块边界，胡安德富卡板块与沿太平洋西北海岸的北美板块会合，并沉入喀斯喀特火山线下方的地幔中。 胡安德富卡板块下方的近海各向异性似乎与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，而不是由于“板块回滚”，即板块在（由于自身重量）落入塑性变形的地幔时逐渐后退。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，沿大圆路径堆叠准洛夫散射波，跟踪单个表面波的进展。最大散射的频率依赖性对各向异性的深度范围敏感。卡斯卡迪亚俯冲带附近的 QL 散射很显着，但内陆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至内陆，以探索沿着圣安德烈亚斯系统、盆地和山脉以及黄石热点地区的 QL 交叉路径。 为了详细解释数据，研究人员以简化的地球模型几何形状计算合成地震图作为快速解释工具，并通过采用 SPECFEM-GLOBE 地震波模拟器探索假设的各向异性几何形状的影响来探索各向异性的 3D 结构。 研究人员将制作美国西部的地图，突出显示拉夫-瑞利散射较强的位置、什么对称方向（以及剪切方向）似乎与散射一致，以及各向异性梯度是否是软流圈（即动态）、岩石圈（可能是化石）或代表上地幔中更深的流动。

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.