P-Wave Scattering in the Earth's Crust and Upper Mantle

Summary The codas which follow P-phase onsets on seismic records can be interpreted in terms of waves scattered from random inhomogeneities in the Earth's crust and upper mantle. In particular, the so-called precursors to PP have travel times and slownesses consistent with the interpretation of these signals as waves scattered from direct P-waves incident at the Earth's surface at distances greater than about 20° from either the source or receiver. Significant differences between recordings at the Warramunga array of PP precursors from two Novaya Zemlya explosions are extremely difficult to account for on any interpretation previously suggested, but are fully consistent with the random variations to be expected on the proposed scattering mechanism. The present interpretation accounts for the major observed features of P codas, including their durations and the variations in slowness along the trains. Precursors to P°P can also be interpreted as scattering phenomena. Thus the interpretation removes the need to postulate sharp reflecting discontinuities in the uppermost few hundred kilometres of the upper mantle.