TIBULEAC, I. M. and HERRIN, E., Department of Geological Sciences, Southern Methodist University, Dallas, Texas, TX 75275-0395

ileana@passion.isem.smu.edu; herrin@passion.isem.smu.edu

Anomalous slowness residuals (2-3 s/deg) and azimuth residuals of P and PcP waves from teleseismic events, recorded at selected seismic arrays, are mapped and used to study sharp lateral heterogeneity in the lower mantle.

We used the database provided by GSETT-3, available on the WWW, beginning with January 1, 1995, to study short period ( ~ 1 sec) P and PcP arrivals from events with magnitude mb ³ 4.0 located in specific regions. The statistical significance of the slowness anomalies at each array was tested and the 'best' arrays (low standard deviation of the slowness residual) were chosen.

Maps of the slowness anomalies in the lower mantle, obtained using events recorded at TXAR (Lajitas, Texas) and YKA (Yellowknife, Canada), show definite patterns, with changes of sign between adjacent regions. The variations are reflected also in the azimuth residuals but not in travel time residuals.

The results are interpreted in terms of geometry of velocity gradients, possible lateral temperature variations and structure of the lowermost mantle and are compared to results from other methods. The advantage of our method is a resolution of several tens of kilometers which will provide important constraints for models of lower mantle properties.