HR: 0800h
AN: G51A-0069    [Abstracts]
TI: Volcano-tectonic deformation at Mount Shasta and Medicine Lake volcanoes, northern California, from GPS: 1996-2004
AU: * Lisowski, M
EM: mlisowski@usgs.gov
AF: USGS - Cascades Volcano Observatory, 1300 SE Cardinal Ct., Suite 100, Vancouver, WA 98683-9589 United States
AU: Poland, M
EM: mpoland@usgs.gov
AF: USGS - Cascades Volcano Observatory, 1300 SE Cardinal Ct., Suite 100, Vancouver, WA 98683-9589 United States
AU: Dzurisin, D
EM: dzurisin@usgs.gov
AF: USGS - Cascades Volcano Observatory, 1300 SE Cardinal Ct., Suite 100, Vancouver, WA 98683-9589 United States
AU: Owen, S
EM: owen@usc.edu
AF: University of Southern California, Zumberge Hall, 3651 Trousdale Parkway, Los Angeles, CA 90089 United States
AB: Mount Shasta and Medicine Lake volcanoes are two of the three Cascade volcanoes targeted for dense GPS and strainmeter deployments by the magmatic systems component of Earthscope's Plate Boundary Observatory (PBO). Leveling surveys indicate an average subsidence rate of $\sim$9 mm/yr at Medicine Lake volcano since at least 1954, which could result from draining of a magma reservoir, cooling/crystallization of a subsurface body of magma or hot rock, loading by the volcano and dense intrusions, crustal thinning due to regional extension, or some combination of these mechanisms. Displacements from GPS surveys in 1996 and 1999 revealed regional block rotation and contraction across the summit of the volcano, but the time interval was too short to distinguish between possible mechanisms. On Mount Shasta, a 21-line, 12-km aperture EDM network was measured in 1981, 1982, and 1984 with no significant deformation detected, nor was there significant length change in three EDM lines recovered with GPS in 2000. We present results from GPS surveys completed in June and July 2004 of the region surrounding both Mount Shasta and Medicine Lake volcanoes. We find regional deformation to be dominated by a block rotation about a pole in southeast Oregon, similar to but generally south of poles determined by other workers using GPS in western Oregon and Washington. No significant residual deformation remains in the four GPS stations located on Mount Shasta, which were previously measured in 2000. In contrast, GPS results from six stations on the upper flanks of Medicine Lake volcano confirm the known subsidence and are consistent with elastic half-space models of volume loss that fit the leveling data. No significant residual regional strain was detected. As a result, we believe that subsidence at Medicine Lake does not likely result from crustal thinning due to regional extension. A more detailed examination of Medicine Lake subsidence sources, Mount Shasta edifice deformation, and possible local and regional temporal deformation changes will be available after installation of continuous GPS stations and strainmeters by the Plate Boundary Observatory. In addition, we have begun annual microgravity measurements that in the future should help to distinguish between possible deformation mechanisms for Medicine Lake volcano.
DE: 8424 Hydrothermal systems (8135)
DE: 8434 Magma migration
DE: 1243 Space geodetic surveys
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting