On the absence of an ultralow-velocity zone in the north Pacific

[1] Using an unusually large earthquake near the big island of Hawaii, we study the core mantle boundary (CMB) beneath the north-northeastern Pacific between Hawaii and North America. A dense sampling of the CMB is achieved using the core-reflected phase PcP recorded at a large number of high-quality stations in North America, including networks in California, Oregon, Washington, and Alaska, as well as at EarthScope's USArray stations. We apply an adaptive stacking technique for optimal record alignment on specific phases (namely P and PcP) and subsequently stack seismograms to obtain summation traces possessing relatively high signal-to-noise ratios of PcP and P. Anomalous ultralow-velocity zone (ULVZ) layering at the CMB has been noted to exist in various parts of the Pacific beneath the lowermost mantle large low shear velocity province imaged by tomography. ULVZ structure produces anomalous PcP waveform variations in the form of precursors to PcP. These PcP data, however, lack precursory energy, indicating either that (1) ULVZ layering is lacking or (2) that a ULVZ is present and thinner than our detection threshold, i.e., less than a few kilometers thick. We use synthetic waveform modeling to establish the sensitivity and utility of investigating the time window ahead of PcP for precursors generated from fine-scale CMB layering. These results, combined with evidence for ULVZ structure in other parts of the Pacific, suggest that ULVZs are intermittent and possibly only detectable in regions where mantle currents collect ULVZ material, whether or not partially molten, presumably beneath (or near) upwellings or plumes.