HR: 11:15h
AN: G42A-04 INVITED     [Abstracts]
TI: An Ongoing Episode of Magmatic Inflation at the Three Sisters Volcanic Center, Central Oregon Cascade Range: Inferences from Recent Geodetic and Seismic Observations
AU: * Dzurisin, D
EM: dzurisin@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory 1300 S.E. Cardinal Court Building 10, Suite 100, Vancouver, WA 98683-9589 United States
AU: Lisowski, M
EM: mlisowski@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory 1300 S.E. Cardinal Court Building 10, Suite 100, Vancouver, WA 98683-9589 United States
AU: Moran, S C
EM: smoran@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory 1300 S.E. Cardinal Court Building 10, Suite 100, Vancouver, WA 98683-9589 United States
AU: Wicks, C W
EM: cwicks@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 977, Menlo Park, CA 94025 United States
AU: Poland, M P
EM: mpoland@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory 1300 S.E. Cardinal Court Building 10, Suite 100, Vancouver, WA 98683-9589 United States
AU: Endo, E T
EM: etendo@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory 1300 S.E. Cardinal Court Building 10, Suite 100, Vancouver, WA 98683-9589 United States
AB: Tumescence at the Three Sisters volcanic center began between summer 1996 and summer 1998 and was discovered in April 2001 using interferometric synthetic aperture radar (InSAR). Vertical surface displacement at a maximum rate of 3-4 cm/yr is centered about 5 km west of the summit of South Sister, a composite basaltic-andesite to rhyolite volcano that last erupted between 2,200 and 2,000 years ago. The deforming area is about 20 km in diameter and extends across much of the western part of the Three Sisters Wilderness. The background level of seismic activity is low, suggesting that temperatures in the source region are high enough or the strain rate is low enough to favor plastic deformation over brittle failure. A swarm of over 300 small, shallow (6-10 km) volcano-tectonic earthquakes (Mmax = 1.9) in the northeast quadrant of the deforming area on March 23-25, 2004, was the first notable seismicity in the area for at least two decades. Tilt-leveling and EDM networks were established at South Sister in 1985-1986 and resurveyed in 2001, the latter with GPS. Both networks have since been extended to cover more of the deforming area and 3 continuous GPS stations have been installed. In addition, 1 broadband seismometer and 4 short-period seismometers have been installed since 2001 to improve real-time seismic monitoring and earthquake-location capabilities. InSAR results together with annual GPS and leveling surveys constrain the location, geometry, and inflation rate of the deformation source as a function of time. A best-fit source model derived from simultaneous inversion of all three datasets through 2003 is a shallowly dipping tabular body located 6.5 ñ 2.5 km below the surface with a volume increase of 0.0050 ñ 0.0015 km3/yr. GPS and leveling data acquired in August 2004 will be used to update the model. The most likely cause of tumescence is a pulse of basaltic magma intruding the upper crust along the brittle-ductile interface - a process that must occur repeatedly beneath the Cascade Range but in the past would have escaped detection in the absence InSAR observations, unless accompanied by unusual seismicity. The likelihood that the current episode at Three Sisters will culminate in an eruption in the near term is judged to be low, but the impact of an eruption could be great. An updated volcano hazards assessment has been completed, concerned agencies and the public have been notified, and an emergency coordination and communication plan is being prepared.
DE: 8434 Magma migration
DE: 7280 Volcano seismology (8419)
DE: 1208 Crustal movements--intraplate (8110)
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting