Bokelmann, G.H.R., Silver, P.G., 1993, The Caribbean Anomaly: Short-Wavelength
Lateral Heterogeneity in the Lower Mantle, Geophysical Research Letters,
20, 12, 1131-1134
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 1500km-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 1200km 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 300km
wide. Assuming a path
length of 1500km through the anomaly (suggested from previous studies), we find that dvp/vp ~2.1% and
dvs/vs ~ 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 dlnvs/dlnvp ~ 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 2200km. 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.
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