HR: 1340h
AN: V53C-1413    [Abstracts]
TI: Tremor Source Location at Okmok Volcano
AU: * Reyes, C G
EM: celso@gi.alaska.edu
AF: Alaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks, 907 Koyukuk Drive, Fairbanks, AK 99701, United States
AU: McNutt, S R
EM: steve@giseis.alaska.edu
AF: Alaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks, 907 Koyukuk Drive, Fairbanks, AK 99701, United States
AB: Initial results using an amplitude-based tremor location program have located several active tremor episodes under Cone A, a vent within Okmok volcano's 10 km caldera. Okmok is an andesite volcano occupying the north-eastern half of Umnak Island, in the Aleutian islands. Okmok is defined by a ~2000 y.b.p. caldera that contains multiple cinder cones. Cone A, the youngest of these, extruded lava in 1997 covering the caldera floor. Since April 2003, continuous seismic data have been recorded from eight vertical short-period stations (L4-C's) installed at distances from Cone A ranging from 2 km to 31 km. In 2004 four additional 3- component broadband stations were added, co-located with continuous GPS stations. InSAR and GPS measurements of post-eruption deformation show that Okmok experienced several periods of rapid inflation (Mann and Freymueller, 2002), from the center of the 10 km diameter caldera. While there are few locatable VT earthquakes, there has been nearly continuous low-level tremor with stronger amplitude bursts occurring at variable rates and durations. The character of occurrence remained relatively constant over the course of days to weeks until the signal ceased in mid 2005. Within any day, tremor behavior remains fairly consistent, with bursts closely resembling each other, suggesting a single main process or source location. The tremor is composed of irregular waves with a broad range of frequencies, though most energy resides between ~2 Hz and 6 Hz. Attempts to locate the tremor using traditional arrival time methods fail because the signal is emergent, with envelopes too ragged to correlate on time scales that hold much hope for a location. Instead, focus was shifted to the amplitude ratios at various stations. Candidates for the tremor source include the center of inflation and Cone A, 3 km to the south-west. For all dates on record, data were band pass filtered between 1 and 5 Hz, then evaluated in 20.48 second windows (N=2048, sampling rate=100 Hz), at 20 second intervals. Root-mean- square (rms) values were then calculated for each window of data. The ratios of these RMS amplitudes were used to investigate the tremor behavior. The ratio changes between tremor and non-tremor events suggest that the sources for episodes were closer to Cone A (and station OKCF) than they were to other locales in the caldera. Methods from Battaglia's PhD thesis (2001) were used as guidelines for a tremor location program based on amplitude decay. Written in MATLAB®, this program can be run in near-real time to estimate the tremor source location and strength. Further refinement is underway, as is an examination of all other days that have suitable data .
DE: 7215 Earthquake source observations (1240)
DE: 7280 Volcano seismology (8419)
DE: 7290 Computational seismology
DE: 8419 Volcano monitoring (7280)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting