A broadband seismic study of the lowermost mantle beneath Mexico: Constraints on ultralow velocity zone elasticity and density

Broadband seismic stacking is performed for PcP waves sampling the lowermost mantle and core‐mantle boundary beneath central Mexico and the western Gulf of Mexico. The data contain evidence of a 10 to 20‐km‐thick ultralow velocity zone on the western edge of the study area, tapering (or vanishing) to the east to less than 5‐km thickness over a horizontal distance of ∼200 km. Epicentral distance varies from 20° to 60°, a range over which the amplitude of reflections from the top of the ultralow velocity zone responds strongly to changes in the ratio of shear to compressional velocity reduction (τ=δlnυs/δlnυp) and to density within the zone. Where the zone is present and thick enough to separate PcP and the precursory reflection, our modeling is consistent with a value of τ near 3 and δln υp near 10%, assuming only a minor density increase (1 to 2%). The value of τ is reduced to 2 for a ∼14% density increase, but δln υp remains large (7%). Very low amplitude multiply converted phases are synthetically well reproduced with extreme attenuation within the zone, suggesting the presence of a melt component.