Seismic and hydroacoustic tremor generated by colliding icebergs

Iceberg harmonic tremor (IHT) emanating from tabular icebergs in the Southern Ocean and along calving margins of the Antarctic Ice Sheet is a complex, evolving signal at frequencies above approximately 0.5 Hz. IHT has been observed as T phases on islands in the equatorial Pacific, as hydroacoustic signals in the Indian Ocean, and by local and regional Antarctic seismic networks. To identify the IHT source mechanism and to understand its relevance to iceberg calving, evolution, and breakup, we deployed seismometers on a giant (25 by 50 km) tabular iceberg called C16 in the Ross Sea, Antarctica, during a uniquely accessible period (austral summer, 2003-2004) when it was aground against the northern shore of Ross Island. During the deployment period, C16 was in sporadic contact with another giant tabular iceberg, B15A, that was moving under the influence of local ocean currents. This study reveals that the C16-associated IHT was a manifestation of extended episodes of discrete, repeating stick-slip icequakes (typically thousands of individual subevents per hour) produced when the cliff-like edges of the tabular icebergs underwent glancing, strike-slip collisions. The IHT signal that we observed is thus not a phenomenon associated with iceberg elastic or fluid resonance modes, but is instead the consequence of long sequences of very regularly spaced and similar pulses of seismic radiation from these constituent stick-slip subevents. IHT represents a newly identified glaciogenic source of seismicity that can be used to improve our understanding of iceberg dynamics and possibly of ice shelf disintegration processes.