Mass movements in tropical volcanic terrains: the case of Teziutlan (Mexico)

During the last decade, soil degradation coupled with global climate changes has increased hydrogeological hazards in Mexico. In tropical volcanic terrains, alteration processes have enhanced the formation of clay minerals that promote water retention and result in soil/rock weakness. Intense seasonal rainfall can trigger the liquefaction and remobilization of these low-resistance terrains. During the first week of October 1999, heavy rains affected eastern Mexico, including Puebla State. As a consequence, approximately 3000 mass movements, consisting of rock and soil slides and slips, debris flows and avalanches were generated in this area. In the town of Teziutlan (Puebla), which is located on volcanic deposits, a single mass-movement event caused approximately 150 deaths. In the present work we identified two types of mass movements in the Teziutlan area-Type 1: superficial erosion of an unwelded ignimbritic sequence forming small detrital fans, and Type 2: thin soil slide/debris flow from the remobilization of a volcanic sequence composed of clay-rich paleosols interbedded with ashfall horizons. The clay-rich volcanic paleosols favored the formation of perched water tables on a hydraulic aquiclude. Positive pore-water pressures triggered the failure. Based on these results, the principal human settlement in the Teziutlan area may be threatened by future debris flows, which could cause serious harm to the dense population and severe damage to its infrastructure. It is necessary to prevent future deaths and damage by installation of mitigative measures based on detailed studies. Without any further study, it will not be possible to prevent and mitigate a natural disaster with the same magnitude as the 1999 catastrophic hydrogeological phenomena. (C) 2003 Elsevier Science B.V. All rights reserved.