HR: 0800h
AN: S11B-0169    [Abstracts]
TI: Seismic Observations of Westdahl volcano and Western Unimak Island Alaska: 1999-2005
AU: * Dixon, J P
EM: jpdixon@usgs.gov
AF: USGS-Alaska Science Center, 903 Koyukluk Drive , Fairbanks, AK 99775
AU: Power, J A
EM: jpower@usgs.gov
AF: USGS-Alaska Science Center, 4200 University Drive, Anchorage, AK 99508
AU: Stihler, S D
EM: scott@giseis.alaska.edu
AF: Geophysical Instiute-UAF, 903 Koyukluk Drive , Fairbanks, AK 88775
AB: Westdahl volcano is a large basaltic shield volcano on the western end of Unimak Island Alaska in the Aleutian Island Arc. The volcano is topped by three separate vents, Pogromni Volcano, Faris Peak, and Westdahl Peak. The volcano is frequently active with known eruptions from Westdahl Peak in 1964, 1978, and 1991-92 that produced large basaltic lava flows. InSAR measurements indicate that Westdahl Volcano has been inflating at a slowly declining rate since 1992 (Lu et al., 2003). The Alaska Volcano Observatory has operated a network of six short-period seismometers on Westdahl Peak since 1998. Complementing this network are similar networks centered on Shishaldin and Akutan Volcanoes. Since 1999 more than 300 earthquakes have been located within 20 km of Westdahl Volcano. A volcano specific velocity model was determined for the western half of Uminak Island by simultaneously inverting for the velocity model and hypocentral earthquake locations using the program VELEST. Earthquakes located with the new model reveal five clusters of hypocenters: (a) a shallow cluster beneath Westdahl Peak, that largely occurred during a 24-hour period on January 7, 2004, (b) a concentration of 68 earthquakes with hypocenters ranging in depth from zero to eight km beneath Faris Peak occurring continually since 1999, (c) a diffuse cluster of long-period events northwest of Westdahl and Faris Peaks, (d) a cluster of 12 earthquakes near Pinnacle Rock, 12 km southwest of Westdahl Peak in October 2003, and (e) a cluster of 43 hypocenters near Unimak Bight, 20 km east of Westdahl Peak, that occurred between January and April 2004. Focal mechanisms were derived for four earthquakes in the Faris Peak cluster and four additional earthquakes that locate off the volcanic edifice (the four mechanisms are in the Pinnacle Rock cluster, the Unimak Bight cluster, and 20 km southeast and 30 km northeast of the volcano). Focal mechanisms in the Faris Peak cluster showed normal faulting with nodal planes trending north-south to northwest-southeast. Mechanisms of the off-volcano earthquakes are generally characterized by normal faulting with nodal planes trending southwest-northeast. These events are consistent with a stress field dominated by the Aleutian subduction zone. The Faris Peak mechanisms are not consistent with the presumed regional stress field and may reflect volcanic process. Lu et al., (2003) proposed the observed inflation of Westdahl Volcano resulted from a slowly pressurizing magma source at 6 km depth beneath Westdahl Peak. The observed seismicity is consistent with this model. Lu, Z., T. Masterlark, D. Dzurisin, and R. Rykhus, 2003, Magma supply dynamics at Westdahl volcano, Alaska, modeled from satellite radar interferometry, Alaska, J. Geophys. Res. 108, 2354, doi:10.1029/2002JB002311, 2003.
DE: 7280 Volcano seismology (8419)
DE: 8419 Volcano monitoring (7280)
SC: Seismology [S]
MN: Fall Meeting 2005