EDIFICE COLLAPSE AND RELATED HAZARDS IN GUATEMALA

Guatemalan volcanoes have at least seven debris-avalanche deposits, as sociated with Cerro Quemado, Fuego, Pacaya, Tecuamburro and an unident ified volcano. The deposits range in size from less than 0.1 to in exc ess of 9 km(3) and from 2.5 to in excess of 300 km(2). The avalanches traveled 3 to 50 km from their sources in the Guatemalan highlands. Th ree of the avalanches occurred in Late Pleistocene time and four in Ho locene time-two of them within the last 2000 years, The avalanches occ urred at both andesitic and basaltic stratovolcanoes and at dacitic do me complexes, Laterally directed phreatic or magmatic pyroclastic expl osions were associated with two of the debris avalanches. An evaluatio n of factors that might lead to an edifice collapse in Guatemala is ba sed on the case studies presented in this report and a survey of the l iterature. Edifice collapses are more apt to occur if zones of weaknes s exist within the volcanic edifices, such as unwelded pyroclastic roc ks and pervasively altered rocks, Further, the trench-ward side of vol cano pairs like Fuego and Atitlan may be more likely to fail because i t may have weak zones along the contact with the older back-are volcan o. The direction of failure may be influenced by regional slopes, whic h in Guatemala generally trend southward toward the oceanic trench, an d by such structural factors as multiple vents or overly steep slopes reflecting previous activity or erosion, Debris avalanches are more li kely to occur in drainages which have headwaters at two or more volcan oes, Domes ate especially apt to produce small- to moderate-sized debr is avalanches, and, further, if the domes form a coalescing chain, are most likely to fail in a direction normal to the chain. These factors are used at seventeen major volcanic centers in Guatemala to assess t heir potential for edifice collapse and most probable direction of fai lure.