Arctic abyssal T phases: Coupling seismic energy to the ocean sound channel via under-ice scattering

Abstract

Earthquakes along the mid-arctic ridge radiate earthborne compressional and shear waves that couple into the waterborne arctic acoustic channel and propagate as T phases. The T-phase energy is transmitted above the epicenter through the seafloor at near-vertical angles. Scattering from the ice surface couples some of this energy into waterborne angles. At 5 Hz, the coupled energy is about 40 dB down from the incident levels, and at 15 Hz it is about 30 dB down. Scattering accounts for the characteristic spectral shape of the abyssal T phase that is more energetic at 15 Hz than 5 Hz. Several T-phase arrivals were recorded on hydrophones during the FRAM II experiment. From the measured acoustic levels, a transmission loss prediction between source and receiver, and the scattering conversion losses, the microearthquake acoustic source level in the water column is estimated to be about 250 dB re 1 μPa2-m/Hz at 5 Hz and 235 dB re 1 μPa2-m/Hz at 15 Hz. These levels lead to a crustal attenuation estimate of 0.52 dB/km at 5 Hz, which is consistent with FRAM II refraction measurements.