HR: 1340h
AN: S43A-1038    [Abstracts]
TI: Rockfalls at Augustine Volcano, Alaska: 2003-2006
AU: * DeRoin, N
EM: fsnd3@uaf.edu
AF: University of Alaska Fairbanks, Geophysical Institute 903 Koyukuk Dr. PO Box 757320, Fairbanks, AK 99775, United States
AU: McNutt, S R
EM: steve@giseis.alaska.edu
AF: University of Alaska Fairbanks, Geophysical Institute 903 Koyukuk Dr. PO Box 757320, Fairbanks, AK 99775, United States
AU: Reyes, C
EM: celso@giseis.alaska.edu
AF: University of Alaska Fairbanks, Geophysical Institute 903 Koyukuk Dr. PO Box 757320, Fairbanks, AK 99775, United States
AU: Sentman, D
EM: dsentman@gi.alaska.edu
AF: University of Alaska Fairbanks, Geophysical Institute 903 Koyukuk Dr. PO Box 757320, Fairbanks, AK 99775, United States
AB: Rockfalls, avalanches and landslides have been frequently recorded in seismic data at Augustine Volcano for many years. Typical years such as 2003 or 2004 had several dozen such events that were strong enough to trigger the automatic event detection system. Typical events lasted about 30 sec, had frequencies >6 Hz, and were strongest on summit stations, suggesting that they were rockfalls from the steep summit dome into the adjacent moat area. In 2005 both the rate and the occurrence pattern changed. Rockfall activity began in April 2006 and peaked in May and June, then continued through the fall and early winter. Overall there were more than 340 rockfalls in 2005, with both small and large events occurring. The 2005 rockfall activity increased at nearly the same time as earthquake activity and heating of the ground, suggesting that higher temperatures and steaming contributed to mechanical instabilities of the surface dome rocks. We examined relative amplitudes at station pairs and frequency contents to determine relative locations of the rockfalls by assuming that both higher amplitudes and higher frequencies are associated with events closer to a given station. When a low-light camera was installed at Augustine in January 2006 we were able to confirm these relations because there was a clear correlation between rockfalls, debris flows, and pyroclasic flows to the east (towards the camera) and high amplitudes and frequencies at east station AUE. Other events had high amplitudes and higher frequencies at west station AUW and no material was seen moving to the east. Still other events moved to the north and amplitudes were nearly the same at AUE and AUW. The systematic patterns in amplitude and frequency, verified by data from the low-light camera, make it possible to estimate mass flow in various directions using seismic data. Energy estimates of the rockfalls made from video images can be compared with energy estimates from magnitude-energy equations. The observer stations AUE and AUW show shifts in the frequency depending on whether the rockfalls are moving toward or away from them. Estimates of the seismic wave speeds from the rockfalls can be estimated using the Doppler equation, since the rockfalls are a moving frequency source. Also in progress is a program to estimate mass flow around the flanks of the volcano, using the amplitude ratios from stations around the volcano. The results from this work can be compared with geologic maps of deposits from the 2006 eruptions. The high rate of rockfalls in 2005 was also a new class of precursory signal that may be incorporated into long-term monitoring strategies at Augustine and elsewhere.
DE: 7280 Volcano seismology (8419)
SC: Seismology [S]
MN: 2007 Fall Meeting