The anisotropic reflectivity technique: anomalous reflected arrivals from an anisotropic upper mantle

Abstract

Anomalous body wave arrivals due to anisotropy in a subcrustal layer are investigated using synthetic seismograms. Previously, seismograms of such anomalous phases could only be calculated for plane wavefronts, which simulated teleseismic waves. An extension of the reflectivity technique to anisotropic structures now permits the use of a curved wavefront from a local or regional source. Explosion and double couple sources at the surface are used to generate P- and SV-waves, which are incident on simple stratified models of an anisotropic upper mantle. The anisotropic subcrustal layer possesses the same degree of velocity anisotropy as has been found under Western Germany. Anomalous P to SH converted waves, and shear wave splitting, are evident on the synthetic seismograms. Characteristics of the anomalous arrivals which depend on the velocity—depth structures of the models are discussed. It is shown how the velocity contrasts at the base of an anisotropic layer may allow shear wave splitting to remain undetected even for a high degree of anisotropy. These results from simple models seem to indicate that anomalous arrivals on individual three-component seismograms are seldom likely to be of large enough amplitude to be useful for the analysis of upper mantle anisotropy.