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

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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.