HR: 1340h
AN: G53B-0889    [Abstracts]
TI: Evidence for Magmatic Intrusion at Mount Spurr Volcano, Alaska, from GPS measurements.
AU: * Cervelli, P F
EM: pcervelli@usgs.gov
AF: U.S. Geological Survey Alaska Science Center Alaska Volcano Observatory, 4200 University Drive, Anchorage, AK 99508 United States
AU: Coombs, M L
EM: mcoombs@usgs.gov
AF: U.S. Geological Survey Alaska Science Center Alaska Volcano Observatory, 4200 University Drive, Anchorage, AK 99508 United States
AU: Freymueller, J T
EM: jeff@giseis.alaska.edu
AF: Geophysical Institute University of Alaska, P.O. Box 757320, Fairbanks, AK 99775 United States
AU: McGee, K A
EM: kenmcgee@usgs.gov
AF: U.S. Geological Survey Cascades Volcano Observatory, 1300 SE Cardinal Ct., Bldg 10, Vancouver, WA 98683 United States
AB: Mount Spurr is a 3400-m high ice- and snow-covered andesitic stratovolcano located ~105 km east of Anchorage, Alaska, USA. Two historical eruptions (1953 and 1992) have occurred. Both were sub-Plinian (VEI 4), the eruption columns reaching ~20 km above sea level, and both deposited several mm of ash over south-central Alaska. In July, 2004, the micro-seismicity rate at Mount Spurr rose markedly. At about the same time, a melt pit appeared at Spurr's summit. Airborne gas measurements, begun in August 2004, showed abnormally high CO₂ flux (~1000 tonnes/day). A plausible interpretation of this unrest is the intrusion of magma at some depth beneath Mount Spurr. As volatiles began to exsolve from the intrusion, they rose into the edifice, raising pore fluid pressure and triggering the increased seismicity. The rising volatiles carried heat convectively to the surface, melting the ice and snow at the summit. In an effort to image the hypothesized magmatic intrusion, three telemetered, continuous Global Positioning System (GPS) receivers were deployed on the flanks of Mount Spurr in September, 2004. Four campaign monuments were also established and then re-occupied in June, 2005. For terrain and logistical reasons, the stations are located predominantly to the south of the summit, though one campaign station does lie slightly to the northwest. Benchmark instability is a concern in this region of alpine permafrost. One station in particular shows evidence of seasonal down-slope creep. We calculated station velocities from the GPS measurements, correcting for obvious benchmark instability where possible. The velocity field stands out prominently from the background regional signal. The southern stations, including all three continuous instruments, show a radial pattern of motion, with the center roughly coincident with Spurr's summit, with a maximum horizontal velocity of 3 to 4 cm/yr. However, the northwest campaign station shows little or no motion, making it inconsistent with the radial pattern of the other stations. We interpret the velocity field as arising from magmatic intrusion, especially in light of the other geophysical measurements and geologic observations, which support the same conclusion. The asymmetric character of the velocity field suggests either a geometrically complex source or the existence of active structures within the edifice that distort an otherwise radial magmatic signal. We explore both possibilities, though the relatively small number of geophysical data points and the poorly known structural geology of the area pose a serious non-uniqueness problem. Provided that the basic interpretation of magmatic intrusion is correct, the magnitude of the velocities suggests an intrusive rate on the order of 10⁷ m³ per year.
DE: 1240 Satellite geodesy: results (6929, 7215, 7230, 7240)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 8419 Volcano monitoring (7280)
DE: 8428 Explosive volcanism
DE: 8430 Volcanic gases
SC: Geodesy [G]
MN: Fall Meeting 2005