An integrated field-numerical approach to assess slope stability hazards at volcanoes: the example of Pacaya, Guatemala

摘要

Pacaya is an active stratovolcano located 30 km south of Guatemala City, Guatemala. A large (0.65 km3) sector collapse of the volcano occurred 0.6–1.6 ka B.P., producing a debris avalanche that traveled 25 km SW of the edifice. The current cone has since rebuilt within the scarp of this ancestral collapse. The structural setting of the volcano, along with two recent smaller-volume collapses in 1962 and 2010, suggests gravitational instability of this volcano. To assess Pacaya’s stability and potential for another large lateral collapse of the active cone, standard engineering methodologies for studying non-volcanic slopes were used to examine the SW flank of the edifice. A geomechanical model was developed based on the physical–mechanical material properties of Pacaya’s intact rocks and rock mass characteristics found through field observations and laboratory tests. Slope stability was analyzed in several scenarios with the Limit Equilibrium Method (LEM) and Finite Element Method (FEM), including static conditions (i.e., under gravity forces only), and considering the application of magma pressure and seismic force as triggering mechanisms for slope failure. Results show that the edifice remains stable under gravity alone; however, a large-scale collapse could be triggered by reasonable ranges of magma pressure (≥7.7 MPa if constant along a dyke) or peak ground acceleration (≥460 cm/s2). Results also suggest that a layer of pyroclastics beneath the edifice could have controlled the ancestral sector collapse. Structural analysis shows that a transtensional stress regime is causing a NW–SE orientation of aligned features at the surface, and may be a controlling mechanism for the direction of a future collapse. FEM results are concordant with those from LEM and reveal that maximum shear strain patterns within the edifice may account for long lava flows erupted from lower vent elevations.