Modelling D'' structure beneath Central America with broadband seismic data

Deep South American events recorded at distances between 70 degrees and 96 degrees are used to constrain the lower mantle structure beneath Central America. Employing data from the TERRAscope, BDSN, IRIS and CNSN (Canadian National Seismic Network) broadband arrays, we obtain a 1-D shear model with a 200 km thick D '' layer with a 3% velocity jump at the top and a negative gradient within the layer from broadband waveform modelling of SH-polarized S, ScS, and Scd. The shear structure above D '' is similar to the Preliminary Reference Earth Model (PREM) (Dziewonski and Anderson, 1981. Preliminary Reference Earth Model. Physics of Earth & Planetary Interiors 25, 297-356) in terms of dT/d Delta and S-SKS differential times. SV synthetics for this model, assuming a PREM core, produce a satisfactory fit to the data, except at the larger ranges to the north, where the waveforms display rapid variation. This feature is indicative of lateral variation of the type suggested by Grand (1994), where the discontinuity becomes less well defined. Synthetics computed for a simplified 2-D cross-section through this structure produce the Scd phase and simple S at larger distances. While Scd is easily observed in the shear wave data there is little evidence of its counterpart in the P data. By testing several models with different P-velocity jumps at the same depth as the D '' jump in shear structure, we conclude that the velocity jump must be less than 1%. In fact, PREM provides an excellent fit both in timing and waveform shapes and therefore we conclude that lower-mantle structure beneath Central America shows clear evidence for a discontinuity in S velocity but not in P. (C) 1997 Elsevier Science B.V.