Analysis and modeling of the Tancitaro debris avalanche (Mexico)

The Tancitaro (3840 m asl) is one of the highest volcanoes of the central portion of the Mexican Volcanic Belt. It is located in the SW sector of the Tarascan corridor, where two important types of volcanoes of quaternary age are observed: a single composite stratovolcano (Tancitaro) surrounded by a field of monogenetic cinder cones the distribution of which is mainly controlled by NE-SW trending faults. The Tancitaro is located at the intersection of the above-mentioned faults with the NW-SE trending structures of the Chapala-Oaxaca fault zone. Morphologically the volcano is characterized by a large east-facing horseshoe-shaped crater (4 km wide and 5.3 km long). This collapse structure can be related to a large deposit that occupies the Tepalcatepec tectonic depression starting from 7 km below the crater. The deposit, which also comprises a fan, appears to have been primarily formed by a huge debris avalanche (~18 km3) and by fluvial and debris flow deposits. From the head of the displaced material to the toe of the fan the deposit is 66 km long and covers an area of approximately 1155 km2. Field investigations on the fan area highlighted that the debris avalanche body is composed of unstratified and unsorted rubble with angular clasts ranging in size from millimetric to metric. It contains megablocks many tens of meters wide that create typical debris avalanche hummock morphologies in the proximal area. The hummocks are composed of structures ascribable to the so-called "block facies" seen in many other debris avalanche deposits (e.g. Capra et al., 2001). In the distal zones, blocks and mega-blocks, some of which are characterized by a "jigsaw" texture, gradually decrease in size until they disappear entirely in the most distal reaches; this portion of the deposit corresponds to the "mixed block and matrix facies" described by Glicken (1996). Recent studies (Ownby, et al., 2007) have used radiocarbon dating techniques that indicate that the landslide occurred between 261 and 238 ka BP. The comparison between the debris avalanche of the Tancitaro and other great collapses with similar morphometric characteristics (vertical relief during runout, travel distance, volume and area of the deposit) indicate that the collapse was most likely not primed by any type of eruption, but rather triggered by an intense seismic shock that could have induced the failure of a portion of the edifice, already deeply altered by intense hydrothermal fluid circulation. It is also possible to hypothesize that mechanical fluidization (Hungr, 1990) may have been the mechanism controlling the long runout of the avalanche. Numerical modeling was performed to study the behavior of the Tancitaro debris avalanche using DAN, a code developed by Hungr (1995). In this software it is possible to select the most appropriate rheological kernel for the event being studied. Trial-and-error analyses indicated that the two-parameter "Voellmy model" provides the best approximation of the Tancitaro avalanche movement. This model produces the most realistic results in terms of runout distance, velocity and spatial distribution of the failed mass. Approximate velocities and overall dynamic behavior of the debris avalanche were inferred from comparisons with similar events, mainly the Mt. St. Helens debris avalanche of 18 May 1980.