Sharpness of upper-mantle discontinuities determined from high-frequency reflections

An understanding of the nature of seismic discontinuities in the Earth's upper mantle is important for understanding mantle processes: in particular, the amplitude and sharpness of these discontinuities are critical for assessing models of upper-mantle phase changes and chemical layering. So far, seismic studies aimed at determining the thickness and lateral variability of upper-mantle discontinuities have yielded equivocal results, particularly for the discontinuity at 410km depth^1,2. Here we present short-period (0.8–2.0 s) recordings of upper-mantle precursors to the seismic phase P′P′ (PKPPKP) from two South American earthquakes recorded by the ∼700-station short-period array in California. Our results show that the 410- and 660-km discontinuities beneath the Indian Ocean are locally simple and sharp, corresponding to transi-tion zones of 4 km or less. These observations pose problems for mineral physics models^3–5, which predict a transitional thickness greater than 6 km for the peridotite to β-spinel phase transition. In contrast to the results of long-period studies^6,7, we observe no short-period arrivals from near 520 km depth.