Implications of seismic scattering below the 8° discontinuity along PNE profile Kraton

Strong scattered arrivals are observed in the 800-1400-km offset range of the four Peaceful Nuclear Explosion (PNE) seismic sections along the 3500-km-long E-W-trending Russian profile Kraton. They give evidence of an ∼ 85-km-thick inhomogeneous scattering layer in the upper mantle, which has a slightly reduced (∼ 1%) average P-wave velocity compared to the layers above and below. The top of the anomalous zone is the 8° discontinuity at ∼ 100 km depth, which is constrained by a clear travel time delay of up to 2 s, consistently observed at ∼ 900-1000 km (∼ 8°) offset in the four PNE seismic sections. We model the inhomogeneous zone below the 8° discontinuity by random velocity fluctuations. The full waveform seismic response of a suite of stochastic velocity models is calculated using a two-dimensional finite-difference scheme. The calculated seismic sections resemble the PNE data for models where the random fluctuations have a horizontal correlation length of 5-10 km, a vertical correlation length not greater than 5 km and a standard deviation of about 2% of the average background velocity. Fluctuations with significantly larger correlation lengths give rise to coherent scattered arrivals and considerably shorter correlation lengths lead to significantly weaker scattering, none of which resemble the observed data. The preferred seismic velocity model is in agreement with an upper mantle model in which the low-velocity zone below the 8° discontinuity may contain small amounts of partially molten material. Alternatively, the combined effects of rheological weakening and small-scale anisotropy may also account for the low-velocity scattering layer.