Massive tabular icebergs break off the Antarctic ice shelf about every 50 years. Soon after the last calving event in 2000, unusual harmonic tremors were picked up by underwater hydrophones as far as Tahiti. Doug MacAyeal at the University of Chicago and colleagues constructed a network of seismographs on an iceberg called C16, which at the time was aground in the Ross Sea and adjoining another huge iceberg called B15A.
They monitored the tremors (which are inaudible to the human ear, but can be heard if the recordings are speeded) in 2003and found that one particular song was repeated by the giants daily, and the timing matched that of the tides in the Ross Sea. It began vigorously on the first surge, then slowly ground to a halt and began again when the tide reversed.
The researchers discovered that the tremors came from an area where C16 was rammed by B15A.. Pushed by the daily tides, the bergs scraped past each other in many brief, jerky movements. Each of these movements is only 0.6 millimetres, but they send booming broadcasts into the world oceans, MacAyeal told New Scientist. A seismometer at the South Pole records each of these tremors as if an earthquake of magnitude 3.5 was occurring underneath the iceberg, he added.
The icebergs could allow them to study earthquakes in a laboratory-like setting. They are perfect analogies of plate tectonics. They float on the ocean like surface plates float on magma; and just like them, they occasionally collide and slide against each other, said MacAyeal.The iceberg tremors could also lead to ways to predict the strength of aftershocks, he added.