HR: 0800h
AN: V11B-1428    [Abstracts]
TI: Seismic recordings of debris avalanches on Iliamna Volcano, Alaska
AU: Caplan-Auerbach, J
EM: jca@usgs.gov
AF: U.S. Geological Survey, Alaska Volcano Observatory, 4200 University Dr., Anchorage, AK 99508 United States
AU: * Prejean, S G
EM: sprejean@usgs.gov
AF: U.S. Geological Survey, Alaska Volcano Observatory, 4200 University Dr., Anchorage, AK 99508 United States
AU: Power, J A
EM: jpower@usgs.gov
AF: U.S. Geological Survey, Alaska Volcano Observatory, 4200 University Dr., Anchorage, AK 99508 United States
AB: Seismic recordings of three debris avalanches on Iliamna Volcano show a prolonged precursory phase of activity that may represent the initial stages of failure before the onset of the avalanche. Iliamna is an andesitic stratovolcano in the Cook Inlet region of Alaska with no documented eruptions in historic times. The upper flanks of the volcano are hydrothermally altered and prone to failure. In 1994 and 1997, rock and ice debris avalanches occurred on Iliamna's eastern flank, below the main fumarole field, each with a volume of $\approx$15 x 10$^{6}$ m$^{3}$. A similar event on February 9 2004 occurred on the western side of the edifice, also near a hydrothermally altered zone. All three avalanches were detected by a short-period seismic network operated by the Alaska Volcano Observatory. Although the avalanches differ in location and volume, they produced similar seismic signals, suggesting that the events share common failure mechanics. Each signal begins with a sequence of earthquakes lasting 1-2 hours with spectral peaks between 2-8 Hz. The discrete events associated with each avalanche are similar in time series, suggesting they share a common source location and mechanism. High-resolution relocation of these events confirms that for each avalanche, the earthquakes occur at shallow depth beneath the associated avalanche headwall. In all cases the discrete events continue for up to two hours before the avalanche onset, occurring at shorter and shorter intervals, as event amplitudes decline. Eventually, individual shocks cannot be resolved and the signal degrades to diffuse continuous ground shaking. Each sequence ends with a signal comprising several broadband (2-30 Hz) pulses lasting 1-10 minutes that saturate nearby ($<$ 5 km) stations. The wide frequency band and spindle shape of the final period resembles other rockfall and avalanche signals. The diffuse and broadband periods are thus proposed to represent the avalanche, while the discrete earthquakes represent the initial stages of failure. Identification and recognition of avalanche events is critical for identifying avalanche hazards as well as discriminating between avalanche and eruption signals. The long duration of the seismic sequence suggests that some warning may be possible for future avalanche events.
DE: 8419 Eruption monitoring (7280)
DE: 7280 Volcano seismology (8419)
DE: 7209 Earthquake dynamics and mechanics
DE: 1827 Glaciology (1863)
DE: 1863 Snow and ice (1827)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting