Why do some ice avalanches give warning prior to failure?

Ice avalanches on Iliamna volcano Alaska, Mt. Baker, Washington, and Mt. Steller in the Chugach range of Alaska, exhibit up to several hours of precursory seismicity prior to failure. The precursory sequence includes a series of repeating earthquakes that become progressively more frequent, eventually degrading into continuous ground shaking. The amplitude of ground shaking typically grows until the avalanche suddenly fails. Avalanche propagation is represented by a broadband, spindle-shaped seismic signal. This sequence is interpreted as resulting from slip at the base of a glacier, or within a weak rocky layer beneath the ice. Avalanches with precursory seismicity also share certain physical characteristics, including exposure of the underlying rock surface and evidence of nearby liquid water. In contrast, many other mass-wasting events fail without any kind of seismic warning. These events, however, appear to have initiated in rock or due to glacial calving, rather than at an ice-rock interface. Precursory seismicity may be a characteristic common to glacial ramp failures, in which slip is promoted by a decrease in basal drag. Precursory activity was also not identified in association with avalanches such as the 2002 Kolka or 2005 Monte Rosa events, although this may be due to the large distance between these avalanches and regional seismic stations. The frequent identification of such events on volcanoes may therefore be a consequence of seismic network density, allowing identification of small precursory seismic events. In the case of Iliamna and Mt. Baker volcanoes, avalanches recur in fairly predictable locations at short (1-5 year) intervals. Such frequent failure, as well as the presence of active fumaroles near the failure site, indicates that these events are promoted by geothermal melting. However, the Mt. Steller event confirms that precursory seismicity is not unique to volcanic ice avalanches. Since temperate slab fractures do not fail at predictable intervals (Pralong and Funk, 2006), identification of precursory seismicity could be a critical means by which imminent events could be identified and warning given.