Abstract
Summary. Several studies have suggested that there are lateral heterogeneities in the velocity structure at or near the base of the mantle. Such heterogeneities can be studied through the analysis of amplitude ratios of core phases PKPAB and PKPDF for given earthquake-station pairs. In the epicentral distance range 155-175 these arrivals are well separated in time, and PKPAB has near-grazing incidence at the mantle-core boundary 80 that it is highly sensitive to lateral changes in velocity structure in that region, while PKPDF with its near-normal incidence is less sensitive. The observed amplitude ratios vary by well over an order of magnitude, but the size of the ratio is found to be correlated with the region of the core-mantle boundary sampled by the PKPAB rays: beneath the eastern Pacific and western Atlantic, for which abundant data are available, well-defined, contiguous regions (millions of square kilometres in extent) are found which correlated with predominantly larger or smaller amplitudes. We interpret this result as indicating differences in the degree to which the velocity structure of the lowermost mantle is heterogeneous.
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