T-phases are acoustic signals in which the primary portion of the propagation path is through tile oceans. Energy sources resulting m the generation of T-phases include earthquakes, submarine volcanism, and underwater explosions. Once seismic energy is cot, pled into the water column. T-phases propagate as compressional waves primarily through the Sound Fixing and Ranging (SOFAR) Channel, and may be reconverted, at continental shelves, into short-period elastic waves that travel through the sediments and basement of the continents. The wave-guide effect of the sound channel combined with the low attenuation of sound m seawater permit T-phases to propagate over great distances through the oceans. It is this fundamental property of T-phases that make them ideal for the study of love-magnitude seismicity within the oceans. The frequency content of T-phases (1 – >100 Hz) is such that attenuation within the crust and upper mantle precludes long-range inland propagation: therefore, the best instruments for the detection and analysis of T-phases are underwater hydrophones which record seismo-acoustic signals that cannot be detected by conventional short-period seismographs (Shurbet, 1962).