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 similar to200 km. Epicentral distance varies from 20degrees to 60degrees, 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 (tau = deltaln upsilon(s)/delta In upsilon(p)) 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 tau near 3 and deltaln upsilon(p) near 10%, assuming only a minor density increase (1 to 2%). The value Of tau is reduced to 2 for a similar to14% density increase, but deltaln upsilon(p) 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.