SEISMICITY ASSOCIATED WITH RECURRENT ICE AVALANCHES AT ILIAMNA VOLCANO, ALASKA AND MT. BAKER, WASHINGTON

Mt. Baker, Washington and Iliamna Volcano, Alaska both experience recurrent ice avalanches, some of which exhibit a characteristic seismic signal beginning 0.5-2 hours before failure. This seismic sequence includes a series of repeating earthquakes, followed by a period of continuous ground-shaking, culminating in a spindle-shaped signal representative of avalanche propagation. The precursory portion of the sequence is thought to be due to slip at the ice-rock interface, accommodated by ice deformation prior to failure. At least ten major (volume > 10⁶ m³) ice avalanches have occurred at Iliamna since 1980, five of which share a common failure zone and travel path. Avalanches in 1960, 1980, 1994, 1997 and 2003 initiated near the summit of the volcano, just below the volcano's main fumarole field, and propagated 5.5-10 km along Red Glacier. Similarly, Mt. Baker avalanches occur every 2-4 years, initiating at Sherman Crater and traveling 2-3 km down Boulder Glacier. At least seven such events have occurred since 1958 which, like the Iliamna events, share identical failure zones and propagation paths. Mt. Baker has rarely had a seismic network capable of locating small earthquakes on the volcano, so the extent to which these events may have exhibited seismicity similar to Iliamna's is uncertain. Furthermore, Baker avalanches are 1-2 orders of magnitude smaller than those at Iliamna, and their associated earthquakes may be small. However, at least one avalanche, a glacial failure that entered Sherman Crater in 1977 following a period of thermal unrest, exhibited the same precursory signal identified at Iliamna. The fact that some avalanches exhibit precursory seismicity while others do not may be due to differences in failure material or mechanism. Precursory seismicity may be a characteristic common to glacial ramp failures, in which slip is promoted by a decrease in basal drag. The proximity of Iliamna's and Baker's avalanches to fumarole fields strongly suggests that reductions in basal friction may be triggered by geothermal meltwater. Our investigations may contribute to an improved understanding of causes and mechanisms of the failure of ice-rock avalanches, eventually leading to enhanced hazard assessment and warning capabilities.