Regional and teleseismic P wave travel times from surface foci entail relatively fast crustal and upper mantle velocities beneath continental shields, and, in contrast, slow velocities beneath cordillera.
On the average, teleseismic readings from continental and oceanic sources give arrival times 2 sec earlier than the Jeffreys-Bullen zero depth curve. In addition to this overall offset an empirical surface focus curve shows prominent departures from the J-B predictions in the neighborhood of 30° and 60°, where the observations are approximately 2 1/2 to 3 sec earlier than the J-B curve.
As defined in this paper the term “transition zone” refers to the range from 12° to 18° (circa) where the first apparent arrival depends on signal-to-noise conditions. Given sufficient signal strength Pr, an emergent phase which fits a slope of 8.6 to 8.8 km/sec on travel time plots, arrives first. Amplitudes associated with the Pt (P-teleseismic) phase dominate the longitudinal portion of recordings from the transition zone and, if noise levels are high, the observer often interprets Pt as the first arrival.