Seismic Stability Assessment of Volcanic Slopes: A Case Study from São Miguel Island, Azores

The Azores Archipelago, prone to multiple geohazards such as earthquakes, volcanic eruptions, and extreme weather events, faces significant risks of landslides, particularly due to the complex geological characteristics of its volcanic formations. This study focuses on assessing the seismic stability of a slope near Porto Formoso on São Miguel Island, composed of trachytic volcanic rock overlain by a loose pumice deposit. Using the Volcanic Rock Stability (VRS) empirical system, tailored for volcanic rock environments, this study provides a geotechnical evaluation that considers the unique heterogeneities of these formations. The VRS system's application yielded classifications ranging from Class III (Reasonable) to Class IV (Regular), reflecting varying degrees of stability under seismic conditions. A 3D numerical model, using the Material Point Method (MPM), was developed to simulate the slope's behavior under seismic loading as defined by Portuguese regulations. The analysis identified a Factor of Safety (FS) of 3.85, indicating stability under the specified conditions but highlighted potential risks under more extreme seismic events. A run-out analysis further indicated that slope failure could result in significant material deposition, potentially obstructing critical access routes and posing risks to local infrastructure. These results underscore the need for targeted mitigation strategies to enhance the resilience of volcanic slopes in seismic-prone regions.