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

Abstract. We present results from a study of the core-mantle boundary (CMB) underneath Mexico, Central Amer-ica, and the northeast Pacic using core-reflected phases PcP and ScP, which provide a direct means of sampling the deep mantle. Precursor arrivals to these phases pro-duced by reflection from the top of an ultra-low velocity layer (ULVZ) at the base of the mantle would contain infor-mation about the thickness of such a layer and the velocity reductions within it. Stacks of these phases from 17 earth-quakes recorded by the Northern and Southern California Seismographic Networks do not exhibit coherent or inco-herent 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 present in these regions, have VP and VS reductions less than 10%, are less than ve km thick, or have transitions greater than ve km wide at their upper boundaries