We analyzed P wave travel-time data of earthquakes along a north-south vertical cross section in the central Tibetan plateau to search for high velocity zones related to downwellings in the upper mantle. Differential travel-time residuals of an event in southern Tibet vs. north central and Himalayan events show relatively fast arrivals at stations in the northwest--southeast direction. Results of numerical simulation with simple velocity anomaly models suggest that a narrow, nearly vertical, high velocity zone extending to at least 400 km beneath the southern plateau can account for the observed fast arrivals. This high velocity zone may result from cold downwellings predicted by the delamination model for continental convergence. ¿ American Geophysical Union 1996