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Using PFC2D to investigate volcanic debris avalanche emplacement processes and deposit features with special consideration to Mexican volcanic collapse deposits

N. Thompson

ABSTRACT: Volcanic edifice collapse is a catastrophic phenomenon threatening population centers and infrastructure worldwide. Because mass movements of Earth are often modeled as granular flows, this approach translates directly to modeling the flow and emplacement processes of volcanic debris avalanches. PFC2D (Itasca 2004) is employed in this study in an effort to recognize key parameters controlling avalanche emplacement and deposit morphology and, ultimately, to aid in a better understanding of the hazard they present. The focus of the analysis concerns the grain/boundary interactions after the instance of failure; during emplacement to the point of flow termination. A general volcanic edifice geometry has been developed where a systematic sensitivity analysis can be employed to recognize the relative influence of each controlling parameter on avalanche travel distance and deposit morphology. Controlling parameters include avalanche properties (particle material properties), boundary conditions (wall material properties), model geometry (runout area), and PFC controls (damping models, bonding models, etc.). The influence of changes in pore pressures during avalanche emplacement, which potentially includes transformation to highly mobile, fluidized flow, may also be considered by employing the fluid coupling scheme. PFC2D’s potential to replicate more complicated scenarios thought to be common processes in volcanic edifice failures, such as retrogressive failure, debris entrainment, and avalanche particle fragmentation is also investigated. To establish the validity of the avalanche model created in PFC2D, comparisons with characteristic debris avalanche deposits around the world are being performed through field and digital elevation studies. Deposits in central Mexico, in particular, display numerous characteristics (jig-saw fractured blocks, hummocky topography, fluid-injection structures) that potentially indicate avalanche emplacement mechanisms; using PFC2D to recreate the conditions necessary to form these features will aid in our understanding of the key processes controlling the movement of volcanic debris avalanches.

Available in Continuum and Distinct Element Numerical Modeling in Geo-Engineering - 2008

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