D” Shear Velocity Heterogeneity, Anistropy, and Discontinuity Structure Beneath the Caribbean-fig. 2

D” Shear Velocity Heterogeneity, Anistropy, and Discontinuity Structure Beneath the Caribbean-fig. 2 Measured shear wave travel time anomalies plotted at the raypath midpoint for all of our data, superimposed on a large-scale tomographic image for the lowermost mantle from Steve Grand’s 2002 model. The anomalies are from differential times of ScS-S and S-SKS, with the velocity anomaly attributed to that portion of the path within a 250-km-thick layer at the base of the mantle.

The D” region in the lowermost mantle beneath the Caribbean and Central America has been investigated using shear waves from South American earthquakes recorded by GSN, CNSN, BDSN, and TRInet stations in North America. We develop a composite map of volumetric shear velocity heterogeneity, shear wave anisotropy, and lateral extent of the D” discontinuity in the region, with some extension out into the Atlantic. Corrections for aspherical mantle structure shallower than 250 km above the core-mantle boundary are applied to the differential travel time and shear wave splitting observations. The region below the Caribbean is characterized by having large-scale regions of higher than average shear velocity, but there are localized zones of reduced velocity beneath northern South America and the Caribbean. There is extensive shear wave splitting in the region, with some variability in character. While much of the region has early SH arrivals, there are some waveforms that appear to involve fast arrivals that are predominantly, but not purely polarized as SH. The reflections from the top of D” are relatively uniformly observed, but on a local scale we observe waveforms that show no clear reflector. Whether this is due to lateral disruption of the reflector, topographic defocusing of the reflections, attenuation or some other cause is not yet resolved.

Garnero, E. J., and T. Lay, D” shear velocity heterogeneity, anisotropy and discontinuity structure beneath the Caribbean and Central America, Phys. Earth Planet. Inter., 140, 219-242, 2003.

Supported by NSF grants EAR-9814554, EAR-9996302, and EAR-0125595.


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