The directionality of acoustic T-phase signals from small magnitude submarine earthquakes

Acoustic transients radiated from small magnitude 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 up to 7min. The directionality of the T-phase signal was determined by processing the array hydrophone 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 arrived from different directions farther south of the source, in a region where a number of seamounts and ridges rose within the sound channel. A simple model based on ray path travel times for elastic wave travel in the earth and acoustic wave propagation in the water suggests that the later components of the T-phase signal were 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 close to the array.