NUMERICAL MODELING OF VOLCANIC FLANK INSTABILITY: A CASE STUDY OF PACAYA VOLCANO, GUATEMALA

Located 30 km south of Guatemala City, the Pacaya Volcano is one of Central America’s most active volcanoes having ended a period of dormancy in 1961 and erupting as recently as May 2010. The volcano is composed of the modern MacKenney cone within a S-SW facing horseshoe-shaped avalanche scarp of an ancestral basaltic stratovolcano- the result of a large sector collapse that occurred between 400 and 2,000 years B.P. The slope stability analysis of the Pacaya Volcano is significant given its continual eruptive activity, history of recent collapse and inherent geological features. This study combines geological and mechanical properties of the volcano to model future scenarios of collapse using both limit equilibrium and eventually stress-strain numerical modeling. This study summarizes the physical-mechanical material properties of Pacaya’s intact rocks and rock masses used as input data for numerical modeling based on International Society for Rock Mechanics procedures and suggestions from previous studies. The lithotechnical units at Pacaya have been defined as Lava, Lava-Breccia, Breccia-Lava and Pyroclastic deposits, and the ranges for each of these unit’s intact rock compressive strength, geological strength index, bulk volume and porosity are based on laboratory tests and field surveys. The Hoek and Brown failure criterion was used to calculate the rock mass friction angle, apparent cohesion, and rock mass parameters in a specified stress range. Initial evaluation of local and regional stress fields suggests the south-western flank to be the weakest and therefore most likely to collapse. The lowest factor of safety values found in preliminary limit equilibrium analyses suggests that an external triggering mechanism is required to destabilize the edifice as it remains stable under gravity alone. Studies such as these are important for determining the main destabilizing features and mechanisms of volcanic slopes, and at what point the destabilizing features will result in flank collapse.