Abstract:
Five T-phase events were detected in April 1980 on a horizontal hydrophone array suspended from the arctic icecap at a distance of 300 km from the mid-Arctic ridge. The hydrophone array localized the bearing of the event source and was able to detect P-wave arrivals with 5000- to 8000-m/s phase velocities in addition to the dominant T-phase signature with 1400- to 1700-m/s phase velocities. Scattering from the ice surface is considered as a mechanism to convert the near-vertical seismic energy into propagating acoustic energy. At 5 Hz, the scattering losses are about 40 dB, and at 15 Hz they are about 30 dB. The received acoustic levels of the T-phase spectral peaks ranged from 103--119 dB re: 1 (mu)Pa, 1 Hz, and the propagation losses over the 300-km distance between source and receiver are on the order of 90--100 dB. The earthquake source level in the water column is estimated to be about 250 dB re: 1 (mu)Pa, 1 Hz at 5 Hz, and 235 dB re: 1 (mu)Pa, 1 Hz at 15 Hz. These levels are consistent with refraction measurements of crustal attenuation of 0.52 dB/km at 5 Hz [R. E. Keenan and L. R. L. Merriam, J. Acoust. Soc. Am. 89, 1128--1133 (1991)].