HR: 17:00h
AN: V54B-05 INVITED    [Abstracts]
TI: The SPASM Model of Seismogenic Solid-state Extrusion at Mount St. Helens
AU: * Iverson, R M
EM: riverson@usgs.gov
AF: U.S. Geological Survey, 1300 SE Cardinal Ct., Vancouver, WA 98683, United States
AB: The 2004-2007 eruption of Mount St. Helens (MSH) has been characterized by nearly steady extrusion of a gouge- coated plug of solid dacite and by millions of small (<M2) earthquakes with foci <1 km beneath the extrusion site. For more than a year these earthquakes occurred so regularly (roughly once a minute) that they were dubbed "drumbeats." A recently derived mathematical model I call SPASM (for Seismogenic Plug of Ascending, Solidifying Magma) formalizes the hypothesis that the drumbeat earthquakes resulted from repetitive stick-slip motion along the subsurface margins of the extruding plug (Nature 244, 439-443, 2006). The model assumes that extrusion is driven by steady discharge of magma from a deep reservoir, and it assesses whether this steady discharge favors stable, steady extrusion at the surface, jerky extrusion characterized by stick-slip cycles, or some alternative behavior such as runaway acceleration or progressive plugging of the conduit. Analysis of the model equations shows that the basic requirements for sustained stick-slip behavior are simple and few: a magma ascent rate comparable to the plug accretion rate, a magma compressibility much greater than that of solid rock, and extrusion resistance provided by gravity and rate-weakening friction along the lateral margins of the solidifying plug. Data show that these requirements were likely met during the first 15 months of the 2004- 2007 MSH eruption. Indeed, with use of reasonable parameter values, the SPASM model yields predictions of repetitive stick-slip cycles with periods and amplitudes commensurate with occurrence of drumbeat earthquakes at MSH. The model also predicts, however, that several phenomena could gradually reduce the tendency for stick-slip behavior (as implied, for example, by diminishing drumbeats observed at MSH in 2006-2007). These phenomena include a decline in magma ascent rate, increase in plug-mass growth rate, and decline of plug- boundary friction or the rate-weakening effect. Changes in plug or conduit geometry could also affect extrusion dynamics, but such changes are inherently multidimensional and cannot be assessed with the current version of the SPASM model.
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics (1242)
DE: 7280 Volcano seismology (8419)
DE: 8425 Effusive volcanism
DE: 8434 Magma migration and fragmentation
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting