Observations ofPcPandPphases at lasa at distances from 26° to 40°

Short-period P and PcP phases were studied from 45 earthquakes in the distance range 26° < Δ < 40° from the Large-Aperture Seismic Array (Lasa) in Montana. Steered Lasa beams were made for each phase and reduced to NOAA magnitude 6.0. No reversals of PcP polarity for Δ < 32° and no minimum in amplitudes at 32° were observed. This seems to rule out the core-mantle boundary model suggested by Buchbinder with a density ratio of core to mantle equal to 1.0. Although beamed data are used, PcP/P ratios show enough scatter to prohibit a precise estimate of a core-mantle model and show no clear trend within the range 26°–40°. A theoretical PcP/P curve that passes through the average data level can be obtained by using the Bullen-Haddon model B1 of the core-mantle boundary, the Johnson model CIT 208 for the P velocity in the earth, and a Q model more attenuative than the short-period Q model of Archambeau, Lambert, and Flinn.