Sound-channel observations of ice-generated tremor in the Indian Ocean

[1] Mid to low southern latitude hydrophone stations within the Indian Ocean have recorded two distinct types of low-frequency (< 100 Hz) tremor that can be correlated with drifting icebergs and glacial features along the Wilkes Land coast of eastern Antarctica. The most common of these signals is a variable harmonic tremor (VHT), with spectral peaks that exhibit frequency fluctuations through time. These signals typically display a 4 to 10 Hz fundamental frequency and may have as many as ten harmonic bands. Individual VHT signal packets have durations of up to similar to 30 min and occur throughout the year in clusters that continue for hours to days. A second, less commonly observed signal is characterized by shorter duration ( 25 to 90 s) pulses with a convex-upward spectrogram appearance. These cusped pulse tremors (CPT) often exhibit a near-uniform pulse spacing, with episodes continuing for minutes to hours. Tremor received levels at hydrophones near 32 degrees S, 114 degrees E and 7 degrees S, 72 degrees E reach as high as 142 and 133 dB re 1 mu Pa ( peak to peak), respectively. Propagation likely occurs as a sea surface - reflected phase at high latitudes and a sound channel phase north of the convergence zone, with low-frequency transmission loss estimates suggesting maximum acoustic source levels of similar to 245 dB re 1 mu Pa at 1 m. Source locations for a subset of the loudest VHT signals correlate with the satellite-derived locations of a large iceberg (B-15D) that migrated westward along the Wilkes Land shelf region during 2002 and early 2003. Most VHT sources, however, cannot be correlated with known iceberg locations, suggesting that these signals also may be sourced from smaller unnamed icebergs and/or associated with outlet glaciers distributed along the Wilkes Land coast. CPT signals have a more limited spatial distribution, originating from five specific regions where ice streams are observed. The harmonic nature of both signal types is consistent with the resonance of an ice layer or fluid-filled cavity within an ice mass.