The Las Colinas landslide, Santa Tecla: A highly destructive flowslide triggered by the January 13, 2001, El Salvador earthquake

The Las Colinas landslide was one of thousands of landslides triggered by the January 13th El Salvador earthquake (MW 7.6) in early 2001. The landslide was highly destructive. It led to the death of ∼585 people when it swept into a residential area of Santa Tecla, a suburb of San Salvador. The landslide originated from the top of a steep escarpment and involved pyroclastic deposits (silty sands and sandy silts) interbedded with paleosol horizons. The initial volume of the landslide was only ∼130, 000 m3. The runout distance of the landslide, which developed into a rapid flowslide, was 735 m over a vertical distance of 166 m giving a H/L ratio of 0.23. The flowslide ran its final 460 m over a slope of only 3°. The flowslide debris was mainly dry but may have been partially saturated. It is postulated that strong seismic shaking amplified by topographic effects led to tensile stripping of the initial failure mass, which then lost strength very rapidly as it moved downslope and disintegrated into cohesionless debris. Urban topography consisting of buildings and streets may have inhibited debris spreading and channelized debris resulting in a long runout. The Las Colinas flowslide illustrates that runout behavior determines the landslide hazard at the base of the source slope and raises the question of landslide risk at the base of the Balsamo Escarpment, where existing residential developments are located within the runout distance of similar flowslide events that could occur in the future. © 2004 Geological Society of America.