A velocity anomaly in the lower mantle

Abstract

A study of ScS-S and PcP-P differential travel times and absolute S wave travel times from two deep focus South American earthquakes indicates that the scatter observed in ScS-S times by Hales and Roberts (1970) and Jordan (1972) results from lateral heterogeneities in the lower mantle along the path of the S phase. Both ScS-S and PcP-P times delineate a region of anomalously high velocity in the lower mantle beneath the Caribbean, which may be responsible for some observations of body wave multipathing at Lasa. The total velocity variation associated with this anomaly is estimated to be 1% or greater, and the ratio of P velocity variation to S velocity variation is about 1:1. The observed velocity contrasts may be due to a thermal anomaly, in which case lateral temperature differences of at least 200°–300°C are required. We suggest that the high-velocity anomaly beneath the Caribbean marks a site of descending material in the convecting mantle.