HR: 1340h
AN: V53D-1592    [Abstracts]
TI: Frictional properties of gouge generated during the 2004-2005 lava dome extrusion at Mount St. Helens and implications for seismicity
AU: * Moore, P L
EM: pmoore@iastate.edu
AF: Dept. Geological and Atmospheric Sciences, Iowa State University, 253 Science 1, Ames, IA 50011 United States
AU: Iverson, N R
EM: niverson@iastate.edu
AF: Dept. Geological and Atmospheric Sciences, Iowa State University, 253 Science 1, Ames, IA 50011 United States
AU: Iverson, R M
EM: riverson@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Ct. #100, Vancouver, WA 98683 United States
AB: Lava dome extrusion during the 2004-2005 eruption of Mount St. Helens has been accompanied by abundant, nearly periodic, shallow-focus seismicity. This seismic activity is hypothesized to result from incremental uplift of a nearly crystalline magma plug driven by magma ascent from below. Wear along the margin of the uplifting plug has formed a layer of striated, crushed rock, or gouge, which accommodates the relative displacement. Interpretation of the seismicity therefore requires some knowledge of the frictional properties of this gouge. Laboratory experiments were performed in a large ring-shear device to test the dependence of the gouge's peak and steady-state frictional strength on shearing rate and hold time. The sample (0.012 m³ in volume) was sheared under constant normal stresses ranging from 4 kPa to nearly 0.2 MPa and at rates ranging from 10^-6 to 10^-3 m s^-1. At all normal stresses, the gouge exhibited rate-weakening behavior when sheared at rates slower than 10^-4 m s^-1, but at faster rates there was a transition to rate-strengthening. In a series of slide-hold-slide tests (hold time ranging from 3 to almost 10⁵ seconds) performed under constant normal stress and shearing rate, the peak strength of the gouge was found to increase logarithmically with hold time. These results have several implications for the ongoing seismicity at Mount St. Helens. The rate-weakening behavior at low slip rates indicates that the gouge is susceptible to stick-slip behavior and thus may account for observed seismicity. Indeed, regular stick-slip oscillations were observed in two experiments under the highest load and lowest rates of shear. However, because there is a transition to rate-strengthening at higher slip rates, the gouge properties may impose a limit on the size of seismogenic slip events. Additionally, the dependence of peak strength on hold time suggests that slip history may also influence the magnitudes of seismic events.
DE: 8034 Rheology and friction of fault zones (8163)
DE: 8419 Volcano monitoring (7280)
DE: 8434 Magma migration and fragmentation
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005