CitlaltØpetl volcano is the easternmost stratovolcano in the Trans-Mexican Volcanic Belt. Situated within 110 km of Veracruz, it has experienced two major collapse events and, subsequent to its last collapse, rebuilt a massive, symmetrical summit cone. To enhance hazard mitigation efforts we assess the stability of CitlaltØpetl's summit cone, the area thought most likely to fail during a potential massive collapse event. Through geologic mapping, alteration mineralogy, geotechnical studies, and stability modeling we provide important constraints on the likelihood, location, and size of a potential collapse event. The volcano's summit cone is young, highly fractured, and hydrothermally altered. Fractures are most abundant within 5-20-m wide zones defined by multiple parallel to subparallel fractures. Alteration is most pervasive within the fracture systems and includes acid sulfate, advanced argillic, argillic, and silicification ranks. Fractured and altered rocks both have significantly reduced rock strengths, representing likely bounding surfaces for future collapse events. The fracture systems and altered rock masses occur non-uniformly, as an orthogonal set with N-S and E-W trends. Because these surfaces occur non-uniformly, hazards associated with collapse are unevenly distributed about the volcano.Depending on uncertainties in bounding surfaces, but constrained by detailed field studies, potential failure volumes are estimated to range between 0.04-0.5 km 3 . Stability modeling was used to assess potential edifice failure events. Modeled failure of the outer portion of the cone initially occurs as an "intact block" bounded by steeply dipping joints and outwardly dipping flow contacts. As collapse progresses, more of the inner cone fails and the outer "intact" block transforms into a collection of smaller blocks. Eventually, a steep face develops in the uppermost and central portion of the cone. This modeled failure morphology mimics collapse amphitheaters present at many of the world's stratovolcanoes that have experienced massive failure events.
Mass‐waste events from volcanic edifice collapse constitute one of the world's most catastrophic natural hazards. To date, hazards associated with collapse have generally been considered evenly dispersed about a volcano. However, these events are often referred to as sector collapses because only part of a volcano's summit and flank collapse. This is because volcanoes do not have uniform geology or geotechnical characteristics and, thus, their hazard distributions are also non‐uniform. To assess the non‐uniform distribution of geologic controls on volcano stability, we have developed a method that combines field‐based geologic mapping and sampling with mineralogic, petrologic, stable‐isotope geochemical, and geotechnical laboratory studies. Our results provide methods that help quantify the non‐uniform stability of stratovolcanoes and significantly improve volcanic hazard and risk assessments.
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