Assessing massive flank collapse at stratovolcanoes using 3-D slope stability analysis

Massive rock failures pose one of the greatest hazards at stratovolcanoes; more than 20,000 fatalities have resulted worldwide from historical volcano edifice collapses. Although numerous processes can destabilize an edifice, gravitational instability is strongly influenced by the interplay of topography, variable potential failure surfaces, and the three-dimensional (3-D) distributions of rock strength and pore-fluid pressure. We have developed a 3-D slope stability analysis that can search digital topography and determine the locations of minimum stability and the volumes of potential failures. We use this 3-D method to conduct preliminary stability analyses of three stratovolcanoes that have had large rock failures: Mount St. Helens and Mount Rainier in the USA and Volcan Casita in Nicaragua. For the relatively uniform Mount St. Helens edifice, a 3-D analysis using topography alone provides a good predictor of the location and volume of the catastrophic 1980 collapse. At Mount Rainier, both topography and a 3-D distribution of weaker, hydrothermally altered rocks are needed to adequately characterize future hazard. For Casita, the location of the smaller, yet devastating, 1998 failure is predicted using topography and a reconnaissance interpretation of strength based on the distribution of fumarolic activity.