Finite difference modelling of P-wave scattering in the upper mantle

Scattering within the upper mantle is prominently documented in the so-call ed high-frequency teleseismic P-n phase, generated by velocity fluctuations in the upper mantle. This phase is seen in many data sets from active and passive seismology. Whenever the source-generated frequencies are high enou gh (5-10 Hz) to allow their observation at large distances (> 2000 km), thi s phase appears in the recordings of the Russian PNE programme, in other lo ng-range refraction experiments, and in numerous recordings of earthquakes. Using a 2-D finite difference scheme we calculate complete synthetic seismo grams in order to relate the scales of the fluctuation of the elastic param eters to wavefield properties. We discuss the influence of critical paramet ers on the scattering properties of the upper mantle and on the propagation mechanism of the teleseismic P-n: variations in vertical and horizontal co rrelation lengths, rms velocity fluctuations, thickness of the heterogeneou s layer, and cross-correlation of P- and S-wave velocities. A teleseismic P -n phase arises only if the velocity fluctuations are strong enough to caus e multiple scattering and if the horizontal correlation length exceeds the vertical by an order of magnitude. Our favourite model for the Russian PNE profiles includes a 100-150-km-thick zone of scatterers below the Moho, con taining heterogeneities with a horizontal correlation length of 20 km, a ve rtical correlation length of 0.5 km and an rms velocity perturbation of 2 p er cent, while keeping the P- and S-wave velocities correlated. As the tele seismic P-n is often observed, this scale of velocity fluctuations seems to be characteristic of the uppermost mantle. Anisotropy is likely to contrib ute to the inferred velocity fluctuations.