High-Resolution Upper-Mantle Tomography Using Seismic Arrays and Full-Wave Frechet Kernels: Southern Africa

本提案的重点是开发地震阵列的高分辨率层析成像方法。通过反演相邻台站之间的相位和振幅变化，而不是单个台站之间的相位和振幅变化，提高了对子阵结构的分辨率，从而最大限度地减少了远离阵列的横向非均质性的不确定性造成的偏差。数据的Frechet核是通过耦合的正态求和来计算的，这样所有的有限频率效应都没有高频或平均近似。还将开发一种新的行波方法来计算车站附近的精确核。用渐近Legendre函数表示波从源到子阵的非均质性（散射体），用精确Legendre函数表示从散射体到台站的波。层析成像方法正在为南非地震阵列开发，用于研究Kaapvaal克拉通，并对上地幔各向异性结构进行了三维反演。测量了51次远震事件平均50个三分量台站的地表和上地幔体波数据。这一结果将为最近的一些区域模型中相互矛盾的特征提供新的线索，例如克拉通下方存在海洋型低速带和横波各向异性。这里开发的多站全波方法也非常适合满足即将到来的USArray的需求。该方法在相邻台站之间的平均间距约为70公里，它将能够在整个美国大陆的上地幔上提供约50公里的几乎均匀的横向分辨率。

Zhao, LiThe focus of this proposal is to develop a high-resolution tomography method for seismic arrays. Resolution to sub-array structure is enhanced by inverting the phase and amplitude variations between adjacent stations, rather than at individual stations, in order to minimize biases caused by uncertainties in lateral heterogeneities far away from the array. Frechet kernels of the data are calculated by coupled normal-mode summation so that all finite-frequency effects are accounted for with no high-frequency or averaging approximations. A new traveling-wave method will also be developed to calculate exact kernels near stations. Asymptotic Legendre functions are used to express waves from the source to sub-array heterogeneity (scatterer), but exact Legendre functions are adopted for waves from the scatterer to the station.The tomography method is being developed for the Southern Africa Seismic Array, deployed to study the Kaapvaal craton, and 3-D inversions are conducted for upper-mantle anisotropic structure. Data are measured for surface and upper-mantle body waves at an average of 50 three-component stations from 51 teleseismic events. The result will shed new light on the conflicting features in some of the recent regional models, such as the presence of an oceanic-type low-velocity zone beneath the craton and shear-wave anisotropy. The multi-station, full-wave method developed here is also well suited to serve the need of the upcoming USArray. With its proposed average spacing of about 70 km between adjacent stations, this method will be able to provide a nearly uniform lateral resolution of about 50 km in the upper mantle over the entire continental United States.