HR: 10:50h
AN: V52B-03    [Abstracts]
TI: Ground Deformation Associated with the 2004-2005 Dome-building Eruption of Mount St. Helens, Washington
AU: * Dzurisin, D
EM: dzurisin@usgs.gov
AF: USGS 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: USGS Cascades Volcano Observatory, 1300 S.E. Cardinal Court Building 10, Suite 100, Vancouver, WA 98683-9589 United States
AU: Schilling, S P
EM: sschilli@usgs.gov
AF: USGS Cascades Volcano Observatory, 1300 S.E. Cardinal Court Building 10, Suite 100, Vancouver, WA 98683-9589 United States
AU: LaHusen, R G
EM: rlahusen@usgs.gov
AF: USGS Cascades Volcano Observatory, 1300 S.E. Cardinal Court Building 10, Suite 100, Vancouver, WA 98683-9589 United States
AU: Sherrod, D R
EM: dsherrod@usgs.gov
AF: USGS Cascades Volcano Observatory, 1300 S.E. Cardinal Court Building 10, Suite 100, Vancouver, WA 98683-9589 United States
AU: Iwatsubo, E Y
EM: ewatsu@usgs.gov
AF: USGS Cascades Volcano Observatory, 1300 S.E. Cardinal Court Building 10, Suite 100, Vancouver, WA 98683-9589 United States
AU: Diefenbach, A
EM: angiedb@usgs.gov
AF: USGS Cascades Volcano Observatory, 1300 S.E. Cardinal Court Building 10, Suite 100, Vancouver, WA 98683-9589 United States
AU: Thompson, S K
EM: sthompson@usgs.gov
AF: USGS Cascades Volcano Observatory, 1300 S.E. Cardinal Court Building 10, Suite 100, Vancouver, WA 98683-9589 United States
AB: Following nearly 18 years of eruptive quiescence, a new dacite lava dome began growing in the crater at Mount St. Helens in October 2004. Extrusion was preceded by an intense swarm of shallow earthquakes starting on September 23, by several small explosions starting on October 1, and by remarkable uplift of the south crater floor and glacier. The resulting welt, which was first identified in air photos on September 27, was 450 m wide and 100 m high on October 11 when the first new lava emerged from it. Campaign-style GPS surveys in 2000 and 2003 of a 50-station network concentrated within 10 km of the volcano's summit, but extending more than 30 km radially and covering an area of more than 7400 km², revealed no surface deformation indicative of magmatic inflation or deflation. A single-frequency continuous GPS station on the 1980-1986 lava dome and annual GPS surveys of points on the dome and surrounding crater floor showed only subsidence of the dome at rates of a few cm/yr, which we attribute to cooling and compaction. A continuous GPS station (JRO1) located 9 km NNW of the vent abruptly started moving toward the volcano, suggesting deflation of a deep magma reservoir, concurrent with the onset of seismicity. Southward motion of JRO1, which is distinctly different from the regional trend of clockwise block rotation in SW Washington, gradually slowed from ~0.5 mm/d before emergence of the new dome to an average of ~0.04 mm/d during the first 11 months of continuous extrusion. Meanwhile, the extrusion rate was relatively steady at ~2 m³/s. Taken together, the JRO1 GPS and extrusion-rate results indicate that the crustal magma reservoir feeding the eruption is being replenished. Twelve new continuous GPS stations were installed on or near the volcano starting in October 2004 by the USGS Cascades Volcano Observatory and UNAVCO Inc., the latter representing the Plate Boundary Observatory. Like JRO1, these stations moved mostly toward the volcano during the ensuing 11 months. A best-fit, spherical, elastic source model constrained by the GPS station displacements from October 2004 to May 2005 indicates a source depth of ~13 km and a net volume change (assuming the bulk modulus of surrounding rock is 30 GPa) of ~16 x 10⁶ m³. The volume extruded, calculated by differencing digital elevation models that span the same time period, is ~45 x 10⁶ m³. This discrepancy suggests that the source has been partially replenished since the eruption began, consistent with the inference drawn from JRO1 and extrusion-rate data. Rapid motions of the growing dome and surrounding crater floor are being measured with helicopter-deployable GPS stations ("spiders"). Lineal extrusion rates during the first few months of the eruption ranged from 0.07-0.12 mm/s (6-11 m/d); subsequent estimates from time-lapse photography were relatively steady at 4-5 m/d. In July 2005, a borehole tiltmeter installed at a depth of 2.3 m on the 1980-1986 lava dome, 440 m NNW of the active vent, began recording reversible tilt episodes with periods of tens to hundreds of seconds, some of which were associated with shallow earthquakes. The tilt signals might indicate recurring stick-slip cycles as a solid plug of dacite lava is forced upward along frictional conduit walls by magma pressure from below (Iverson et al., 2005, this session).
DE: 1211 Non-tectonic deformation
DE: 8400 VOLCANOLOGY
DE: 8419 Volcano monitoring (7280)
DE: 8425 Effusive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005