H.M. Benz J.E. Vidale 1993 <p><span>The Earth's core–mantle boundary (CMB) juxtaposes liquid iron and crystalline silicates, and is a region of large vertical thermal gradients. The D" region, which extends up to 200–300 km above the CMB, often has elevated shear-wave velocity and suggestions of lateral variations in structure</span>¹<span>. Recent improvements in our ability to assemble and analyse records from regional seismic networks have allowed us to examine long profiles of travel times, amplitudes and waveforms from more than a thousand short-period seismometers</span>²<span>. We observe, across Canada and the United States, P waves that have grazed the CMB from the powerful nuclear test in Lop Nor, China, on 21 May 1992. First-arrival travel times and large secondary arrivals are consistent with a 1.5% compressional velocity increase with depth&nbsp;</span><span class="stix">∼</span><span>&nbsp;130 km above the CMB—about half the thickness of D" in this locality</span>³<span>. Our observations, together with evidence for the absence of such a thin, fast layer in neighbouring regions, suggest the presence of lateral heterogeneity in composition or phase at the base of the mantle.</span></p> application/pdf 10.1038/361529a0 en Nature Seismological mapping of fine structure near the base of the Earth's mantle article