Abstract
SUMMARY A striking feature of Russian long-range seismic refraction data from Peaceful Nuclear Explosions is the observation of a high-frequency teleseismic P, phase, which travels with a group velocity of 8.0 km s-l out to distances of several thousands of kilometres. Modelling using the reflectivity method shows that this phase can be understood as the response of an upper mantle that contains random RMS velocity fluctuations of about 44 per cent superimposed on a positive velocity gradient. This class of model explains the existence of the teleseismic P,, its high-frequency content and its coda length. A teleseismic P, can only be generated if velocity fluctuations are strong enough to cause multiple scattering and occur on a subwavelength scale. Cross-correlation properties of P- and S-wave velocity fluctuations exert a substantial influence on the wavefield. A completely unexpected phase can be observed if the fluctuations are imposed on a negative gradient
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