Constructing synthetics from deep earth tomographic models

Recent studies of deep mantle structure indicate strong heterogeneity. To c onduct high-resolution waveform modelling of these structures, we have deve loped a new method to construct 2-D synthetics directly from block-style to mographic models. Unlike the WKBJ approximation, which utilizes.rays oversh ooting and undershooting receivers, our method (WKM approximation) uses ray s that arrive at the receiver. First, the ray paths from the 1-D layered re ference model are used to localize each ray segment, where the anomalous ve locities are applied by overlay, as in tomography. Next, new Pi (ti) (Pi ra y parameter, ti traveltime) are computed to satisfy Snell's law along with their numerical derivative (delta p/delta t), which is used to construct a synthetic seismogram similar to the WKBJ method. As a demonstration of the usefulness of this method, we generated WKM synthetics for the D " region o f high velocities beneath Central America based on Grand's tomography model . Reasonable fits to broad-band data are obtained by condensing his distrib uted anomalies into his lowermost mantle layer; such a 2-D model predicts s ynthetics containing a laterally varying S-cd triplication similar to obser vations.