Kinematic and seismic analysis of giant tabular iceberg breakup at Cape Adare, Antarctica

摘要

Satellite imagery reveals that a series of large icebergs (B 15B in April 2001, C19 in June 2003, and B15A in October 2005) broke up or fractured while exiting the Ross Sea in a narrowly defined area off Cape Adare, Antarctica. Examination of recent swath-mapped bathymetric observations revealed that the principle agent of these breakups is a previously unknown 9 km long ridge with minimum depths of 215 m that we call Davey Shoal. Satellite imagery shows that the icebergs are driven into the shoal by coastal currents that converge over the narrow continental shelf. One of the icebergs, the 120 km by 30 km B 15A, was instrumented with a seismograph, GPS, and fluxgate compass. This instrumentation provided a unique opportunity to establish the details of the iceberg kinematics that were not revealed by satellite imagery alone and to correlate seismic events observed both on the iceberg and in the far field during breakup. B15A fractured from multiple strikes against Davey Shoal and the adjacent Possession Islands; these strikes were driven by the combination of tidal currents and the coastal mean flow. The periods of iceberg-sourced seismic radiation were correlated with the strikes. The iceberg- and land-based seismic signals showed that the iceberg fracture, its sliding across the shoals, and the ice-on-ice stick-slip contacts among the postbreakup iceberg fragments generated the strong chaotic and harmonic tremor episodes that were observed at distances as far as the South Pole, where these signals propagated as seismically coupled hydroacoustic T phases.