HR: 0800h
AN: V21C-0616    [Abstracts]
TI: Eruption and earthquake activity at Mt. Veniaminof, Alaska, during January-February 2005.
AU: De Angelis, S
EM: silvio@giseis.alaska.edu
AF: Geophysical Institute - Alaska Volcano Observatory, University of Alaska Fairbanks, 903 Koyukuk Drive, PO BOX 757320, Fairbanks, AK 99775-7320 United States
AU: * McNutt, S R
EM: steve@giseis.alaska.edu
AF: Geophysical Institute - Alaska Volcano Observatory, University of Alaska Fairbanks, 903 Koyukuk Drive, PO BOX 757320, Fairbanks, AK 99775-7320 United States
AB: Mt. Veniaminof (MV), Alaska, is a 2507 m high strato-volcano with a 10 km summit ice-filled caldera that formed about 3700 years ago. An intra-caldera cone (2156 m) is the site of all historical activity at MV. From January 2 to February 21, 2005, Mt. Veniaminof erupted. The eruption was characterized by numerous ash emissions and accompanied by nearly continuous seismic activity. During the first stages of the eruption low-frequency (LF) earthquakes occurred at rates of about 1 per minute with durations of 10-20 seconds along with weak volcanic tremor. Ash outbursts were observed at the surface suggesting the presence of an explosive source at shallow depths. Amplitude and occurrence rates of volcanic tremor and LF events increased over the month of January, while surface activity evolved from discrete and relatively small ash explosions into more continuous emissions forming ash clouds and ash fall reaching outside the caldera boundary. On February 4, incandescence was clearly visible on the night-time webcam images of MV indicating mild Strombolian activity with ejection of hot blocks and lava from the intra-caldera cone. The activity was accompanied by an increase in the amplitude and occurrence rate of LF earthquakes; a notable feature of the seismic events recorded during this phase was the striking similarity of their waveforms. Between February 5 and February 21, seismicity was dominated by volcanic tremor ranging from bursts with durations of few tens of seconds to continuous tremor lasting hours. Continuous tremor showed energy spread in the band 0.5-4.0 Hz and spectra characterized by narrow peaks either randomly (non-harmonic tremor) or evenly spaced (harmonic tremor). Spectra of non-harmonic tremor resembled those of low frequency events recorded during periods of Strombolian activity at the volcano; we infer that non-harmonic tremor may result from continuous bubble bursting activity through the surface of a magma column within the volcanic conduit. Harmonic tremor was observed between February 17 and February 21 and was characterized by regular, quasi-sinusoidal waveforms lasting up to 10 hours; spectra, containing up to five harmonics, suggest the presence of a resonant source, such as a gas-filled cavity vibrating in a longitudinal acoustic mode. Harmonic frequencies were observed to change over time; spectral lines moved towards higher or lower values while the harmonic structure of the spectra was maintained. Factors controlling the non-stationary features of harmonic tremor may include changes in acoustic velocity at the source and variations of the effective size of the resonator.
DE: 8419 Volcano monitoring (7280)
DE: 8428 Explosive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005