HR: 15:25h
AN: G32B-08    [PDF]
TI: Before PBO: What Do We Know About Ground Deformation in the Cascade Range?
AU: * Dzurisin, D
EM: dzurisin@usgs.gov
AF: U.S. Geological Survey, David A. Johnston Cascades Volcano Observatory, 1300 S.E. Cardinal Court, Vancouver, WA 98683-9589 United States
AU: Lisowski, M
EM: mlisowski@usgs.gov
AF: U.S. Geological Survey, David A. Johnston Cascades Volcano Observatory, 1300 S.E. Cardinal Court, Vancouver, WA 98683-9589 United States
AU: Poland, M
EM: mpoland@usgs.gov
AF: U.S. Geological Survey, David A. Johnston Cascades Volcano Observatory, 1300 S.E. Cardinal Court, Vancouver, WA 98683-9589 United States
AU: Wicks, C W
EM: cwicks@usgs.gov
AF: U.S. Geological Survey, MS 977, Menlo Park, CA 94025 United States
AU: Diefenbach, A K
EM: angiedbach@hotmail.com
AF: Central Washington University, 309 N. Anderson 1A, Ellensburg, WA 98926 United States
AB: Three of thirteen active volcanic centers in the Cascade Range are known to be deforming and therefore are among the candidates for PBO instrument clusters ({\it i.e.}, concentrations of continuous GPS stations and borehole strainmeters to be installed as part of the Plate Boundary Observatory). At Three Sisters in central Oregon, a broad area of uplift centered 5 km west of South Sister volcano was discovered in April 2001 using satellite radar interferometry (InSAR). Subsequent InSAR, campaign GPS, continuous GPS, and tilt-leveling observations indicate that the inflation is centered near 6 km depth, began in 1997 or 1998, and has continued at a constant rate of 0.005$\pm$0.001 km$^{3}$/yr. The total volume increase through September 2002 was 0.027$\pm$0.005 km$^{3}$. Results of the most recent (September 2003) campaign GPS and tilt-leveling surveys will be reported at the meeting. Medicine Lake Volcano in northeast California is known from repeated geodetic leveling, campaign GPS, and InSAR observations to be subsiding 8$\pm$1 mm/yr at the summit. A leveling survey in June 2003 between Tionesta and Alturas east of the volcano suggests fault displacements of several centimeters at three sites along the route since the previous survey in 1940. At Mount St. Helens in southwest Washington, a 14-station Geodolite EDM network centered near the volcano and extending 30 km across and 90 km along the volcanic arc was established in 1982. All 26 line lengths were remeasured in 1991, and 10 line lengths between 7 central stations were recovered by GPS in 2000. Strain accumulation between 1982 and 1991 was dominated by areal dilatation (0.15$\pm$0.06 $\mu$strain/yr) in the central part of the network around Mount St. Helens. No significant strain accumulation was observed in the same area between 1991 and 2000. A continuous GPS station (JRO1) located 9 km north of the volcano is moving 8$\pm$1 mm/yr north-northeast relative to interior North America, which is generally consistent with rigid rotation of coastal Oregon as determined from modeling of regional GPS-measured deformation. A dense 40-station campaign GPS network around Mount St. Helens was established in 2000 and remeasured in 2003. Preliminary results indicate no significant deformation of the volcano beyond the 1980-86 lava dome, part of which is subsiding 8 cm/yr. The source of apparent far-field deformation is not yet clear. InSAR observations show that thick 1980 deposits in the North Fork Toutle River valley north of the volcano are subsiding and presumably compacting a few cm/yr.
DE: 1208 Crustal movements--intraplate (8110)
DE: 1299 General or miscellaneous
DE: 8499 General or miscellaneous
SC: Geodesy [G]
MN: 2003 Fall Meeting