Absence of short-period ULVZ precursors to PcP and ScP from two regions ofthe CMB

We present results from a study of the core-mantle boundary (CMB) underneat h Mexico, Central America, and the northeast Pacific using core-reflected p hases PcP and ScP, which provide a direct means of sampling the deep mantle . Precursor arrivals to these phases produced by reflection from the top of an ultra-low velocity layer (ULVZ) at the base of the mantle would contain information about the thickness of such a layer and the velocity reduction s within it. Stacks of these phases from 17 earthquakes recorded by the Nor thern and Southern California Seismographic Networks do not exhibit coheren t or incoherent precursor arrivals from a ULVZ. In precursor time windows, ratios of stack amplitudes to those of the CMB-reflected phases fall below predicted values from published ULVZ models. We conclude that ULVZs, if pre sent in these regions, have V-P and V-S reductions less than 10%, are less than five km thick, or have transitions greater than five km wide at their upper boundaries.