HR: 0800h
AN: S51A-0143    [Abstracts]
TI: Renewed Seismic Unrest at Mount Spurr Volcano, Alaska in 2004: Evidence for a Magmatic Intrusion
AU: * Power, J A
EM: jpower@usgs.gov
AF: Alaska Volcano Observatory, USGS 4200 University Dr., Anchorage, AK 99508 United States
AU: Stihler, S D
EM: scott@giseis.alaska.edu
AF: Alaska Volcano Observatory, Geophysical Institute University of Alaska, Fairbanks, AK 99775 United States
AU: Dixon, J P
EM: jpdixon@usgs.gov
AF: Alaska Volcano Observatory, USGS at Geophysical Institute University of Alaska, Fairbanks, AK 99775 United States
AU: Moran, S C
EM: smoran@usgs.gov
AF: USGS - CVO, 1300 Cardinal Court, Bld. 10, Suite 100, Vancouver, WA 98683
AU: Caplan-Auerbach, J
EM: jca@usgs.gov
AF: Alaska Volcano Observatory, USGS 4200 University Dr., Anchorage, AK 99508 United States
AU: Prejean, S G
EM: sprejean@usgs.gov
AF: Alaska Volcano Observatory, USGS 4200 University Dr., Anchorage, AK 99508 United States
AU: McGee, K
EM: kenmcgee@usgs.gov
AF: USGS - CVO, 1300 Cardinal Court, Bld. 10, Suite 100, Vancouver, WA 98683
AU: Doukas, M P
EM: mdoukas@usgs.gov
AF: USGS - CVO, 1300 Cardinal Court, Bld. 10, Suite 100, Vancouver, WA 98683
AU: Roman, D C
EM: droman@earth.leeds.ac.uk
AF: School of Earth Sciences, University of Leeds, Leeds, LS2 95T United Kingdom
AB: In early July 2004 the Alaska Volcano Observatory (AVO) detected a pronounced increase in seismic activity beneath the summit of Mount Spurr volcano that continues at present. From 1 July through 31 August 2004, AVO located 1094 Volcano-Tectonic (VT) earthquakes and 177 Long-Period (LP) events within 12 km of the volcano's summit, although many events classified as VT contained mixed frequencies. The largest event has a magnitude of 1.6 and hypocentral depths generally range from 0 to 5 km below sea-level. The cumulative seismic moment for July - August 2004 is 5x10**13 Nm. Focal mechanisms of located events in July and August 2004 are dominated by normal faulting, which is consistent with what has been observed beneath the summit since 1984. This seismicity rate is the highest observed at Mount Spurr since the conclusion of the 1992 eruption sequence. Seismicity in 2004 differs markedly from that observed prior to the eruptions in 1992 in that almost all hypocenters are concentrated beneath the volcano's summit vent and not the historically active Crater Peak vent, site of eruptions in 1953 and 1992. Analysis of AVO earthquake catalogs suggests anomalous seismicity may have begun as early as 20 October 2002 with a prominent swarm of 60 VT earthquakes (Mmax = 2.4) located roughly 2 km west of the volcano's summit. Smaller increases in the shallow seismicity rates were also noted between July and November 2003 and beginning in February 2004. These events ranged in depth between 0 and 4 km below sea-level. A subtle increase of deep LP events was also detected beginning in July 2003 and peaking in June 2004, immediately prior to the onset of strong shallow seismicity. These events concentrate about 4 km to the southeast of Crater Peak, generally range in depth from 20 to 35 km and occur at a rate of 2 to 4 located events per month. Associated with the 2004 seismic activity AVO has also observed anomalous melting and disruption of the summit ice cap that began in late May or early June 2004. On 7 August 2004 an emission rate of 600 metric tons per day of CO2 was measured and H2S was also detected (more than 1 ton per day). The increase in earthquake rates, initiation of both deep and shallow LP events, the melting of the summit ice cap, and the degassing of both CO2 and H2S suggest that new magma intruded beneath the volcano possibly as early as October 2002.
UR: http://www.avo.alaska.edu
DE: 8419 Eruption monitoring (7280)
DE: 8434 Magma migration
DE: 7280 Volcano seismology (8419)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting