Modelling teleseismic waves in dipping anisotropic structures

The existence of seismic discontinuities within the continental upper mantl e has long been recognized, with more recent studies often indicating an as sociation with elastic anisotropy. Their near-vertical sampling renders tel eseismic P and S waves suitable for characterization of mantle discontinuit ies, but computationally efficient methods of calculating synthetic seismog rams are required for structures that exhibit lateral variability. We consi der lithospheric models consisting of planar, homogeneous anisotropic layer s with arbitrary dip. We adopt the traveltime equation of Diebold for dippi ng, plane-layered media as the basis for a high-frequency asymptotic method that does not require ray tracing. Traveltimes of plane waves in anisotrop ic media are calculated from simple analytic formulae involving the depths of layers beneath a station and the vertical components of phase slowness w ithin the layers. We compute amplitudes using the reflection and transmissi on matrices for planar interfaces separating homogeneous anisotropic media. Modelling indicates that upper-mantle seismic responses depend in a comple x fashion on both layer dip and anisotropy, particularly in the case of con verted phases. Azimuthal anisotropy generally displays a distinctive 180 de grees backazimuthal periodicity in Ps conversion amplitude, as opposed to t he 360 degrees symmetry produced by dip. In contrast, anisotropy with a ste eply plunging axis may under certain conditions be difficult to distinguish from interface dip, as both exhibit a 360 degrees symmetry. We demonstrate the application of the method on Ps and Sp conversion data from the Yellow knife Array, which show evidence for both dipping and anisotropic layering, attributed to layers of anisotropi c fabric in the upper mantle associated with ancient subducted slabs.