Direct measures of lateral velocity variation in the deep Earth
Current tomographic models of the Earth display perturbations to a radial stratified reference model. However, structures in the deep mantle that are chemically dense with low Rayleigh numbers can develop enormous relief, perhaps with boundaries closer to vertical than to radial. Such features are hard to detect with present tomographic modeling techniques because the timing anomalies are based on long-period filtered waveforms with complexity removed. Here we develop a new tool for processing array data on the basis of a decomposition referred to as a multipath detector, which can be used to distinguish between horizontal structure (in-plane multipathing) and vertical (out-of-plane multipathing) directly from processing array waveforms. A lateral gradient coefficient based on this detector provides a direct constraint on the sharpness of the boundaries and material properties. We demonstrate the usefulness of this approach by processing samples of both P and S data from the Kaapvaal array in southern Africa, which are compared with synthetic predictions from a metastable dynamic model containing sharp edges. Both data and simulations produce timing gradients larger than 2 s/deg in azimuthal changes for S waves, where only minor effects are obtained for P waves. These results further validate the case for distinct chemistry inside the African Low Shear Velocity Province.
Additional Information© 2009 American Geophysical Union. Received 14 June 2008; accepted 17 March 2009; published 19 May 2009. We would like to thank two reviewers and the Associate Editor. Their suggestions and comments were greatly appreciated and made significant improvements to the manuscript. We also thank Tarje Nissen-Meyer for helpful discussions. Data were provided by IRIS data centers. All maps in this paper were produced using GMT developed by Paul Wessel and Walter H. F. Smith. This work was supported by National Science Foundation grants NSF-EAR0636012. Contribution 10014 of the Division of Geological and Planetary Sciences, California Institute of Technology.
Published - Sun2009p4476J_Geophys_Res-Sol_Ea.pdf