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Particle Dynamics Simulations of Gravitational Volcanic Deformation -- (undated)
Julia K. Morgan, (Rice University)
A wide spectrum of gravitationally driven phenomena has been documented on volcanic edifices, including debris avalanches, shallow slumps, deep seated landslides, and volcanic spreading, the latter accommodated by outward displacement of volcano flanks along a basal decollement. While active volcanoes are also subject to dynamic forcing due to magmatic intrusion, eruption, and seismicity, gravitational loading probably plays a determining role in volcano deformation. We have carried out 2D particle dynamics simulations to explore the mechanical conditions that favor each mode of deformation within dry granular piles subject to Navier-Coulomb (frictional) failure criteria. Bulk friction coefficients were parameterized from interparticle friction; internal friction of the pile was fixed at 0.6; basal friction coefficients ranged from 0.1 to 0.3, with an extreme case of cohesive or "welded," non-sliding substrate. Under steady-state conditions, the granular piles grow self-similarly, developing distinctive layer stratigraphies, deformation structures, and morphologies, indicative of the mechanical state of the granular pile. Piles constructed upon the welded substrate primarily exhibit particle avalanching, forming outward dipping strata and angle of repose slopes of ~32. Systematic decreases in basal strength lead to formation of progressively deeper and steeper detachment faults that intersect the substrate and allow outward displacement of the lower flanks. Accordingly, surface slopes decrease with decreasing basal strength ( ~7.5 for friction coefficient of 0.1), producing broadly concave up flank morphologies; depositional layering tilts increasing inward; symmetric flank synclines and an axial high form at intermediate basal strength; a broad axial syncline develops at minimum basal strength due to topographic subsidence. Our results can be readily explained by examining traction shear stresses along the substrate, which increase away from the axis of the pile due to topography. Basal slip occurs when traction stresses meet the Coulomb failure criterion, which occurs closer to the pile axis for lower basal strength conditions. Lateral sliding of the lower flanks enables downward displacement above a favorably oriented detachment, defining a wedge shaped slump. Surface slopes adjust to maintain the equilibrium balance between gravitational driving and basal resisting stresses, defining an extensional critical Coulomb wedge. Spatial and temporal variations in basal strength cause gradual transitions in pile structure, stratigraphy, and morphology, as the slopes readjust to the new equilibrium state. Remarkably, this simplified treatment of volcanoes as Coulomb granular piles reproduces the richness of deformational structures and styles, and slope morphologies found in many volcanic settings. This model also provides us with a useful tool with which to better understand the dynamic behavior of active volcanoes, and the associated geologic hazards.