Edifice collapses commonly produce debris avalanches and can also form lahars or lateral blasts. Although rare (about 4 per century worldwide; (Siebert et al., 1987)) these events are extremely dangerous when they occur because they move at 10's to more than 100 m/s and cause total destruction of everything in their paths. It is difficult to predict when or where a collapse event might next occur in Guatemala; however, the features of the Guatemalan volcanic chain suggest that the probability of a collapse event is greater in some areas than others.
A slope failure occurs when the forces acting along a surface exceed the forces resisting failure. Forces that favor failure at a volcano include gravity, earthquakes, and explosions. The forces resisting failure depend on the strength of the rock within the volcano. The probability of slope failure increases with the presence of less competent beds such as pyroclastic beds and zones of hydrothermal alterations. Acatenango is more strongly affected by hydrothermal processes than the other volcanoes. Distruption of the structure of a volcano by the intrusion of a shallow magma body could increase the probability of an edifice collapse and Pacaya, where at least one Holocene edifice collapse has occurred, is thought to have a shallow magma body (Eggers, 1971). Moreover, an avalanche inthe presence of a shallow magma body could cause explosive decompression and a laterally directed blast.
Four sets of paired volcanoes exist in Guatemala, and Halsor and Rose (1988) suggest that the younger of the two volcanoes is invariably to the south toward the subduction one. A younger volcano like Fuego perched on teh flank of an older volcano like Acatenango could form a plane of weakness. Moreover, because of the general north to south sloping topgraphy, Fuego volcano is more likely to collapse away from its pair, Acatenango.
Another influence on the direction of a possible avalanche is the general slope of the basement rock. In Guatemala the general slope between the highlands and the coastal plain is north to south and collapse is more likely to occur in the direction of regional slope. Several notable exceptions occur, however. Both San Pedro and Toliman are situated on regional south-to-north slopes. This means that possible collapses occurring at these two volcanoes would flow north into Lake Atitlán and could cause extensive tidal waves in the lake. During historic time, tidal waves from avalanches moving into water have caused more fatalitites worldwide than the avalanches (Siebert et al., 1987; Kienle et al., 1986). An avalanche flowing into Lake Atitlán would probably cause a tidal wave that would devastate everything within 30 m of the lake level.
Some places in Guatemala could be affected by large volcanic events like debris avalanches from any one of several sources. For example, debris avalanches originatin gfrom any one of Agua, Fuego, Acatenango, or Pacaya Volcanoes could affect the Escuintla area. Such node areas are at greater risk from volcanic debris avalanches than other areas.
The probable travel distance (l) of debris avalanches can be estimated by considering the vertical drop (H) and setting the ratio (H/L) equal to a constant known as the coefficient of friction (Schuster and Crandell, 1984). Siebert et al. (1987) have examined about 200 examples and found that H/L varies between 0.09 and 0.19 and averages 0.13 for debris avalanches having volumes between 0.1 and 1.0 km3. For avalanches greater in volume than 1.0 km3, H/L varies between 0.05 and 0.13. Assuming that pre-avalanche Pacaya was approximately 100 m higher than it is now, H/L is 0.1 for Pagaya debris avalanche. Assuming a source near Fuego and a volcano having about the same height as Fuego, the Escuintla and La Democracia debris avalanches have H/L's of 0.074 and 0.079. Volcanoes of Guatemala have a maxiumum height of 4,220 m and generally don't greatly exceed 3,800 m. Using 3,800 m as a maximum value for H and 0.075 as a minimum value for H/L, travel distances for future Guatemalan debris avalanches shouldn't exceed that of the Escuintla debris avalanche--about 50 km. Future debris avalanches having volumes less than 1 km3 probably won't have travel distances greater than that of the Pacaya avalanche--about 25 km. Future lateral blast events could travel from about 10 km to a maximum of about 25 km from source.
Volcanic collapse events that are accompanied by magmatic eruption like those of Mount St. Helens and Bezymianny will probably be preceded by large-scale deformation and intense seismicity that will indicate the probably direction of slope failure and potential lateral blast. Events like those at bandai-san in 1888 and Unzen in 1792 could occur little or no premonitory activity. If precursors suggest an edifice collapse event at a Guatemalan volcano, evacuation is the only effective mitigative measure.
Collapse events should be considered possible at all steep volcanoes. Our work has shown that at least three such events have occurred in Guatemala in the past 85,000 years. Although rare, edifice collapse events represent devastating hazards, and volcanic observers should be sensitive to changes that could be precursors.
(Vallance et al., 1988)