A 100-km-long record section of NTS explosions recorded in the eastern Snake River Plains (7 ¿<Δ<8 ¿) shows the cusp of critical refractions from the steepened P velocity gradient at the bottom of the upper mantle LVZ. Synthetic seismograms calculated with a modified reflectivity program have been used to derive a regional velocity model of the upper mantle beneath the eastern Great Basin. The model suggests that observed very weak Pn arrivals are due to a slight negative velocity gradient below the Moho and that no high velocity mantle lid exists in this region.