J.E. Vidale H.M. Benz 1993 <p><span>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</span>^1,2<span>. 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&nbsp;</span><span class="stix">∼</span><span>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</span>^3–5<span>, which predict a transitional thickness greater than 6 km for the peridotite to&nbsp;</span><i>β</i><span>-spinel phase transition. In contrast to the results of long-period studies</span>^6,7<span>, we observe no short-period arrivals from near 520 km depth.</span></p> application/pdf 10.1038/365147a0 en Springer Nature Sharpness of upper-mantle discontinuities determined from high-frequency reflections article