Teleseismic P_n arrivals: Influence of mantle velocity gradient and crustal scattering

Teleseismic P_n phases with a long, high-amplitude coda, are observed to offsets >3000 km along the Peaceful Nuclear Explosion seismic profiles Quartz and Ruby. For the high-frequency (>5 Hz) part of the Quartz data, which is shown here, coda lengths of more than 10 s can be visually observed in the seismic sections. We interpret the teleseismic P_n arrivals as multiple sub-Moho refractions (whispering-gallery phases), which travel over large distances due to a positive upper-mantle velocity gradient. The coda of these arrivals is explained by lower crustal scattering of the whispering-gallery phases from the mantle. The modelling is based on calculation of reflectivity and 2-D visco-elastic finite-difference synthetic seismograms. Our models are in agreement with other deep seismic data from continental areas, which typically show a reflective lower crust and an almost transparent uppermost mantle. The lower crustal heterogeneity is described by von Karman distribution with a standard deviation of 0.5 km s^-1 for the P-wave velocity and correlation lengths of 2.4 km (horizontally) and 0.6 km (vertically). The positive vertical upper-mantle velocity gradient of ∼0.004 km s^-1 km^-1, which is characteristic for profile Quartz, forms a waveguide which gives rise to the teleseismic P_n. It is not necessary to include upper-mantle heterogeneity in order to explain the characteristics of this arrival.