HR: 0800h
AN: C31B-0316    [Abstracts]
TI: Hydroacoustic Observations of Antarctic-Derived Ice-Generated Tremor in the Indian Ocean
AU: * Chapp, E
EM: chapp@soest.hawaii.edu
AF: Department of Geology and Geophysics, University of Hawaii, 1680 East-West Road, Honolulu, HI 96822 United States
AU: Bohnenstiehl, D
EM: del@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, P.O. Box 1000, Palisades, NY 10964 United States
AU: Tolstoy, M
EM: tolstoy@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, P.O. Box 1000, Palisades, NY 10964 United States
AB: Hydroacoustic data from two sets of moored hydrophones within the Indian Ocean Basin exhibit two distinct tremor signals that can be associated with drifting icebergs and glacial features along the Wilkesland coast of eastern Antarctica. The first type of signal is a variable harmonic tremor (VHT) with spectral peaks (4-100 Hz) that fluctuate through time and can persist from several minutes to over an hour. The second signal is a cusped pulsed tremor (CPT) of regularly spaced packets of curved harmonic spectral bands (4-40 Hz) with the duration of a single pulse ranging from 25-60 seconds, the interval between pulses ranging from 30-90 seconds and a series of pulses lasting anywhere from 10 minutes to over an hour. Sound-channel based observations indicate fundamental frequencies for both signals are typically within the 4-10 Hz range, but can reach as high as 30 Hz in some instances. Received levels at two hydrophone stations ($34\deg$ S $114\deg$ E and $7\deg$ S $72\deg$ E) have maximum values of 142 and 133 dB re $1\mu$Pa (peak-to-peak), respectively, with propagation as a surface-reflected phase at lower latitudes shifting to a sound channel phase after passing through the convergence zone. While VHT signal source locations appear to include most of the Wilkesland coast ($85\deg$ E - $149\deg$ E longitude), we have been able to successfully correlate a subset of the loudest VHT signals, for which the signal azimuth could be constrained at two hydrophone stations, with the westerly-migrating iceberg B-15D from early 2002 through 2003. Most of the VHT signals, however, cannot be correlated with large icebergs, suggesting that these signals also may be sourced from smaller unnamed icebergs and coastal outlet glaciers. The CPT signal appears to source from five specific locations along the Antarctic coast where satellite imagery show active ice movement from ice streams and outlet glaciers. Because of the harmonic nature of both signals, resonance initiated by excitation of an ice mass or fluid-filled cavity within the ice could be a viable source mechanism for the tremor signals.
DE: 4259 Ocean acoustics
DE: 3099 General or miscellaneous
DE: 1827 Glaciology (1863)
DE: 1863 Snow and ice (1827)
SC: Cryosphere [C]
MN: 2004 AGU Fall Meeting