THE CARIBBEAN ANOMALY - SHORT-WAVELENGTH LATERAL HETEROGENEITY IN THE LOWER MANTLE

Data from the Archean-Proterozoic Transect Experiment 1989 (APT89) are used to constrain smaller-scale velocity heterogeneity in the lower mantle beneath the Caribbean. We observe large variations in travel time residuals for paths from South America to portable broadband and short-period stations distributed along a 1500 km-long transect in central North America. After accounting for near-source and near-reciever contributions, we find variations as large as 3s for P and 8s for S for rays that bottom around 1200 km depth. This rapid spatial variation in travel times suggests that we are seeing the western edge of the Caribbean anomaly, a feature previously observed by several investigators. The gradient appears to be about 300 km wide. Assuming a path length of 1500 km through the anomaly (suggested from previous studies), we find that deltanu(P)/nu(P) = 2.1% and deltanu(S)/nu(S) = 3.6% which for S waves is about three times the estimate from long period body waves. The relative velocity ratio for this anomaly is deltalnnu(S)/deltalnu(P) almost-equal-to 1.7, a value slightly larger than typical laboratory values but lower than previous recent seismological studies of lower mantle heterogeneity. With increasing turning depth the observed anomaly appears to decrease in size, although it continues to be visible for shear waves with bottoming depths as great as 2200 km. The presence of multiple direct S phases for several records of one of the events studied suggests multipathing induced by the large observed lateral velocity gradients required by the travel time data.