P-SH conversions in a flat-layered medium with anisotropy of arbitary orientation

Abstract

P-SH conversion is commonly observed in teleseismic P waves, and is often attrbuted to dipping interfaces beneath the reciever. Our modelling suggests an alternative explanation in terms of flat-latered anisotropy. We use reflectivity techniques to compute three-component synthetic seismograms in a 1-D anistropic layered medium. For each layer of the medium, we prescribe values of seismic velocities and hexagonally symmetric anistropy about a common symmetry axis of arbitrary orientation. A compressional wave in an anisotropic velocity structure suffers conversion to both SV-and SH-polarized shear waves, unless the axis of symmetry is everywhere vertical or the wave travels parallel to all symmetry axes. the P-SV conversion forms the basis of the widely used 'reciever function' technique. The P-SH conversion occurs at interfaces where one or both layers are anisotrpic. A tilted axis of symmetry and a dipping interface in isotropic media produce similar amplitudes of both direct (P) and converted (Ps) phases, leaving the backazimuth variation of the P-Ps delay as the main discriminant. Seismic anisotropy with a tilted symmetry axis leads to complex synthetic seismograms in velocity models composed of just a few flat homogeneous layers. It is possible therefore to model observations of P coda with prominent transverse components with relatively simple 1-D velocity structures. Successful retrieval of salient model characteristics appears possible using multiple realizations of a genetic-algorithm (GA) inversion of P coda from several backazimuths. Using GA inversion, we determine that six P coda recorded at station ARU in central Russia are consistent with models that possess strong(> 10 per cent) anisotrpy in the top 5 km and between 30 and 40 km depth. The symmetry axes are tilted, and appear aligned with the seismic anisotropy orientation in the mantle under ARU suggested by SKS splitting.