Lateral variations in D'' below the Caribbean

Broad-band data from the Canadian National Seismograph Network (CNSN) shows evidence for a laterally-varying S-wave discontinuity in the lowermost mantle beneath the Caribbean. The magnitude of the velocity contrast across this D '' discontinuity varies inversely with its distance from the core-mantle boundary (CMB). Clear evidence for a 2.75% increase in shear velocity 250 km above the CMB is observed in a region 10 degrees N and 60 - 85 degrees W; a thicker D '' layer (290 km) with a less pronounced topside discontinuity (2.45%) is interpreted in the region 80 degrees - 90 degrees W and 20 degrees N; and there is no evidence for a D '' discontinuity in the region 65 degrees - 80 degrees W and 25 degrees N. Considerable 3D structure within the D '' region is suggested by traveltime scatter in ScS arrivals and S arrivals which turn within D ''. Transverse-component SKS signals due to upper-mantle anisotropy can be confused with D '' signals; however, we show that false interpretations of D '' structure can be mitigated through the careful analysis of travel-time moveout and low-pass filtering.