Diffracted Sp is found to be generated under the Australian shield by deep-seated Tonga, Honshu, and Bonin events. This seismic phase is set up at the Moho as a P head wave by a critically incident upgoing SV wave at an epicentral distance of around 50¿. The simple source time history inferred from P or SH particle motion and the absence of source structure effects on SV waveforms from a deep source suggest a diffracted Sp wave for the explanation of distorted SV waves recorded at the Mundaring station. The polarity of this phase arriving immediately after direct SV indicates a P type of wave, excluding the possibility of other type of S waves like ScS. Using a WKBJ spectral method for SV wave propagation, synthetic seismograms for the direct SV and diffracted Sp are computed. With some modification of the P velocity model by Mathur and the S velocity model presented by Goncz and Cleary for the Australian shield, synthetics produce the distinctive interference between direct SV and diffracted Sp seen in the data. The arrival time difference between direct S and diffracted Sp is sensitive to the P velocity of the upper mantle. Results of the analysis reveal higher P velocities in the upper mantle along the east-west path than along the north-south path from Mundaring station. The analysis of teleseismic SV waveforms and the diffracted Sp phase is a new and useful technique for determining regional variations in upper mantle P velocity structure.