Array studies ofPphases and the structure of theD″ region of the mantle

Slowness (dT/dΔ) and azimuth measurements for P and pP phases have been derived for 57 earthquakes at distances between 82° and 115° recorded at the Yellowknife array to determine whether such data are capable of elucidating the structure of the lowest 300 km of the mantle. To show that systematic errors in the azimuths of arrival and slight differences in dT/dΔ values from those associated with the ‘Seismological Tables for P Phases’ (1968) are unlikely to be produced in the source region, the data have been supplemented by slowness and azimuth measurements of PcP, ScP, PKP, P2KP, P3KP, and P4KP phases from 28 events at distances where the expected dT/dΔ values are greater than 4.0 sec/deg. Evidence for regional variations in structure at depths greater than 2500 km is presented, together with one velocity model that will explain an offset in the dT/dΔ curve near 90° for rays that have penetrated to the base of the mantle below the Caribbean Sea and surrounding regions. The occurrence of large-scale anomalies in dT/dΔ and azimuth for some azimuth ranges has been demonstrated; such anomalies in dT/dΔ are produced by lateral variations in structure or irregularities within the crust and upper mantle beneath the array, but comparable anomalies in azimuth may accumulate over the whole of the ray path or may be caused by structure below the array.