Iceberg tremor and other curiosities of Antarctica's large tabular icebergs

by Douglas R. MacAyeal
University of Chicago

ABSTRACT
In 2000, several titanic icebergs calved from the Ross Ice Shelf and were "witnessed" in more ways than the obvious: Seismometer stations located on tropical islands in the equatorial Pacific began to detect (as T-phase) single-tone tremors with variable pitch that emanated from the icebergs as they drifted within the Ross Sea. This curious and unexpected observation motivated a field study of iceberg drift and seismicity in an effort to understand the source of these mysterious far-field tones. After four years of effort, it is now reasonably clear that iceberg-on-iceberg collisions are responsible for the tremor. Other mysteries remain however. For example, no explanation yet exists for the fact that the tremor changes pitch during the course of a given event, and it is unclear what aspects of the iceberg actually "resonate" in response to the collision. Other curious factors related to iceberg drift also remain, including several unexpected forces that govern the motion of icebergs; including the possibility that the arrival of surface waves (Rayleigh and Love waves) arriving from distant earthquakes cause icebergs to "bob"-- allowing them to break free of submarine shoals on which they are grounded.