Directionality of acoustic T-phase signals from shallow submarine earthquakes

Acoustic transients radiated from shallow undersea earthquakes were studied in an experiment carried out using a towed horizontal line array operating in the South Fiji Basin. The transient signals consisted of P, S, and T phases, with the T-phase signal from each earthquake lasting for about 5 min. During this period, the directionality of the T-phase signal was determined by processing the array data with a conventional beamformer. The weakest part of the signal arrived first on a direct bearing between the earthquake source and the array. However, subsequent stronger components of the T phase arrived from different directions farther south of the source, in a region where a number of seamounts rose within the sound channel. A simple model based on ray-path travel times for p-wave travel in the earth and in the water suggests that the later components of the T-phase signal are radiated into the water by downslope propagation from the seamounts and ridges. The initial weaker components may be scattered into the sound channel by leakage from the P and S phases relatively closer to the array.