A large, high quality P-wave data set comprising short-period and broadband signals sampling four separate regions in the lowermost mantle beneath the Cocos plate, Mexico, the central Pacific, and the north Pacific is analyzed using regional one-dimensional double-array stacking and modelling with reflectivity synthetics. A data-screening criterion retains only events with stable PcP energy in the final data stacks used for modelling and interpretation. This significantly improves the signal stacks relative to including unscreened observations. allows confident alignment on the PcP arrival and allows tight bounds to be placed on P-wave velocity structure above the core-mantle boundary (CMB). The PcP reflections under the Cocos plate are well modelled without any ultra-low velocity zone from 5 to 20 degrees N. At latitudes from 15 to 20 degrees N, we find evidence for two P-wave velocity discontinuities in the D '' region. The first is similar to 182 km above the CMB with a delta In V-P of +1.5%, near the same depth as a weaker discontinuity (<+0.5%) observed from 5 to 15 degrees N in prior work. The other reflector is similar to 454 km above the CMB, with a delta In V-P of +0.4%: this appears to be a shallower continuation of the joint P- and S-wave discontinuity previously detected south of 15 degrees N, which is presumed to be the perovskite to post-perovskite phase transition. The data stacks for paths bottoming below Mexico have PcP images that are well matched with the simple IASP91 structure, contradicting previous inferences of ULVZ presence in this region. These particular data are not very sensitive to any D '' discontinuities, and simply bound them to be <similar to 2%, if present. Data sampling the lowermost mantle beneath the central Pacific confirm the presence of a similar to 15-km thick ultra-low velocity zone (ULVZ) just above the CMB, with delta In V-P and delta In V-S of around -3 to -4% and -4 to -8%, respectively. The ULVZ models predict previous S-wave data stacks well. The data for this region indicate laterally varying V-P discontinuities in D '', with one subregion having a delta In V-P of 0.5% 140 km above the CMB. Beneath the north Pacific, the PcP arrivals are compatible with only weak ULVZ (delta In V-P similar to 0 to -3%), and there is a weak D '' reflector with delta In V-P = 0.5%, near 314 km above the CMB. These results indicate localized occurrence of detectable ULVZ structures rather than ubiquitous ULVZ structure and emphasize the distinctiveness between the large low shear velocity province under the central Pacific and circum-Pacific regions. (c) 2008 Elsevier B.V. All rights reserved.
