Calving icebergs: A source of low-frequency seismic signals from Columbia Glacier, Alaska

Low-frequency icequakes produced by Columbia Glacier, Alaska, are caused by large icebergs which break off the glacier terminus into Prince William Sound. Occasionally, many ice seracs fall in rapid succession, producing complex, low-frequency icequake signals with durations sometimes exceeding 10 min. An empirical relationship V = 4000 T between the volume V of an iceberg in cubic meters and the duration T of the seismic signal in seconds, can be used to estimate the calving rate of Columbia Glacier from seismological data alone. During 1984 and 1985, a period of rapid retreat of the glacier, the calculated number of icequakes per month showed seasonal fluctuations that corresponded to variations in calving rate determined by photogrammetry. It is likely that most previously reported low-frequency icequakes in southern Alaska are produced by calving, tidewater glaciers.