Southern Symphony: Hydroacoustic and other signals from wandering icebergs

Emile A. Okal, Northwestern University

In the early part of the decade, seismic stations in Polynesia detected
hydroacoustic signals emanating from large icebergs parked or slowly drifting
in the Ross Sea, along Western Antarctica's shorelines. These signals are a
form of harmonic tremor, characterized by a prominent frequency (typically a
few Hz), accompanied by overtones, and modulated over the duration of a swarm
(typically tens of minutes). In order to further investigate the origin and
mechanism of these phenomena, we deployed temporary PASSCAL seismic stations on
3 calved and one future ("nascent") iceberg during 2003-2007. We conclude that
the ice tremor arises from a process of stick-and-slip during collisions
between icebergs. The operation of the seismometers on the icebergs also
provided a lot of unexpected signals, such as the recording of distant storms
occurring as far away as the Gulf of Alaska, and of the 2004 Sumatra tsunami.