Time-domain modelling of PKIKP precursors for constraints on the heterogeneity in the lowermost mantle

Abstract

SUMMARY Compact solutions for the scattering of plane elastic waves by spheres now exist that are valid for arbitrary levels of velocity and density perturbations and arbitrary ratios of wavelength/sphere radius. These solutions are easily incorporated into methods of seismogram synthesis that remain valid for the frequency dependence near caustics. Short-period precursors to PKIKP are synthesized by dynamic ray tracing and superposition of Gaussian beams in a representation of D″ heterogeneity by distributed spheres. Corrections to Rayleigh-Born scattering for arbitrary levels of perturbation, wavelengths on the order of or smaller than the scatterer dimension, and scattering into inner core branches of PKP tend to cancel one another, validating previous work on PKIKP precursors that assumed the validity of the Rayleigh-Born approximation. Results are similar to those studies: scalelengths on the order of 20–35 km and perturbations in P velocity and density of 10 per cent. Modelling in the time domain is helpful in interpreting the emergent arrival of the precursor wave train, constraining the depth of D″ heterogeneity. The length and excitation of precursor coda in the 120° to 130° range helps to constrain the maximum thickness of D″ heterogeneity. An apparent increase in precursor amplitude as distance increases toward the B caustic suggests a scattering mechanism that is more strongly concentrated in the forward direction than is predicted by models containing spherical scatterers or by isotropic Gaussian distributions having a single scalelength.