HR: 14:55h
AN: V13F-06    [Abstracts]
TI: Massive Collapse of Steep Volcanoes Driven by Pore-Fluid Pressures
AU: * Reid, M E
EM: mreid@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 910, Menlo Park, CA 94025 United States
AB: Massive collapse of steep volcanoes, such as at Mount St. Helens in 1980, can initiate as a large flank landslide which then retrogresses rapidly into the core of an edifice via a series of subsequent failures. Over 200 stratovolcanoes around the world have been dramatically sculpted by catastrophic massive collapses; some have occurred with eruption of juvenile magma (e.g. Mount St. Helens), whereas others have not (e.g. Bandai). Both hazard assessment and long-term volcano evolution depend on the volume of rock removed by these retrogressive failures; larger failures commonly travel farther and they greatly modify the edifice. Using a 3-D slope stability model coupled with groundwater and heat flow models, I examine the potential for massive landslide retrogression driven by pore-fluid pressures. The 3-D stability analysis searches digital topography and determines the locations of minimum stability and the volumes of potential failures. Nearly instantaneous removal of rock by a large landslide may produce transiently elevated pore-fluid pressures within the remaining edifice, because insufficient time elapses to allow fluids to drain and equilibrate with the new topography. Such transiently elevated pore pressures further destabilize the oversteepened scar of the initial landslide and instigate failure retrogression. I analyze two potentially destabilizing scenarios, an edifice saturated with cold groundwater when initial failure occurs, and an edifice containing hydrothermally pressurized fluids. For these scenarios, initial edifice failure is triggered by horizontal acceleration representing moderate earthquake shaking. Rapid removal of an initial large landslide volume (about 0.5 km³) from a dry edifice leads to only minor failure retrogression of the exposed steep headscarp. Given an edifice containing cold, topography-driven groundwater flow, instantaneous removal of an initial failure mass can induce modest transiently elevated pore pressures. Here, failure retrogression is limited to relatively small slices in the steep headscarp of the initial failure. The cumulative volume removed by the initial and two subsequent retrogressive failures is about twice the initial failure volume. In contrast, given a volcano flank containing pressurized pore fluids, an initial failure can provoke extensive retrogression. Pressurized fluids within an edifice can result from the heating of pore fluids by shallow magmatic intrusion. Also, a deeper intrusion may cause transiently elevated fluid pressures that propagate outward and upward into the edifice. Given pressurized fluids in the edifice, 3-D simulations show that extensive retrogression following initial failure can remove more than five times the volume of rock as in the cold groundwater scenario. In this case, failure retrogression can remove the summit and create a large amphitheater.
DE: 1810 Debris flow and landslides
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005