Clyde E. Nishimura
Naval Res. Lab., Code 7420, Washington, DC 20375
Christopher W. Clark
Cornell Univ., Ithaca, NY 14850
Earthquakes in oceanic regions commonly generate acoustic signals known as T-phases which are similar but not identical, to man-made explosions. As earthquake-producing areas are also regions where marine mammals congregate (e.g., Aleutians and Caribbean margin), knowledge of the characteristics of T-phases may provide some additional information on the possible effect of noise on these animals. T-phases are generally characterized by acoustic energy below 100 Hz with most of its energy in the 10- to 30-Hz range. The duration of the T-phase is, to the first order, linearly related to the source earthquakes magnitude; durations of several minutes are common. The T-phase source signal level, which can exceed 200 dB re: 1 (mu)Pa for a magnitude 4--5 earthquake, is a complex function of the source magnitude, focal depth, and the complexity of the bathymetry at the water--rock radiation area. This radiation area is not a point source and can extend over a radius of several tens of km. As part of the Whales '93 program, analysis has begun as to whether there is any correlation between the occurrence of earthquakes and observable changes in the acoustic signature form, and the positioning of large cetaceans.