Stratovolcano stability assessment methods and results from Citlalt�petl, Mexico

Abstract

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.

Pyroclastic density currents resulting from the interaction of basaltic magma with hydrothermally altered rock: an example from the 2006 summit eruptions of Mount Etna, Italy

“…Such a condition has been recognized as capable of rendering a volcano more prone to collapse (Lopez and Williams 1993;Reid et al 2001;Zimbelman et al 2004;Opfergelt et al 2006), and indeed, it played a determining role in the events described in the following sections.…”

Geomechanical rock properties of a basaltic volcano

“…These include applying Limit Equilibrium Methods (LEM) (Voight et al, 1983;Iverson, 1995;Elsworth and Voight, 1996;Hürlimann et al, 2000;Reid et al, 2000;Donnadieu et al, 2001;Okubo, 2004;Apuani and Corazzato, 2010) and numerical modeling by Distinct Element Methods (DEM) (Zimbelman et al, 2004), Finite Difference Methods (FDM) (Hürlimann et al, 2000;Apuani et al, 2005aApuani et al, ,b, 2007Apuani and Corazzato, 2010) and Finite Element Methods (FEM) (Sousa and Voight, 1995;Apuani and Corazzato, 2010;del Potro et al, 2013;Schaefer et al, 2013). However, numerical data on the relevant mechanical properties for volcanic rocks and rock masses used in these types of studies remain a major source of uncertainty.…”

Late-Pleistocene flank collapse triggered by dome growth at Tacaná volcano, México-Guatemala, and its relationship to the regional stress regime

“…The largest areas affected are located at the northwestern edge of the horseshoe-shaped crater and on the northwestern flanks of San Antonio volcano. Considering that this region has 4,000 mm of rain per year, these zones may be prone to future rock slides and major landslides, as have been observed elsewhere (Zimbelman et al 2004). …”

“…Collapse potential can be enhanced by structural factors such as interbedded lavas and pyroclastic deposits (Leonov 1995;Vallance et al 1995;Carrasco-Núñez et al 2006), the intrusion of dikes (Siebert 1984;Siebert et al 1987;Elsworth and Voight 1995), intersecting rift zones (Carracedo 1996), gravitational spreading of the volcano substrate (van Wyk de Vries and Francis 1997), faults (Hall et al 1999;Ventura et al 1999;van Wyk de Vries et al 2003), tectonic movement (Francis and Self 1987;Carracedo 1996;Tibaldi 1995), and hydrothermal alteration of the summit (López and Williams 1993;Zimbelman et al 2004). Siebert (1984) concluded that failure of semiconical volcanoes takes place perpendicular to the maximum principal stress in the region.…”