Landslides in Central America

Initial Report

A massive landslide occurred in the Las Colinas neighborhood of Santa Tecla, El Salvador, Central America as a result of the M=7.6 earthquake of January 13, 2001. The landslide buried many houses in the neighborhood under tons of earth.

Rescue efforts are still under way at the Las Colinas landslide site, as preliminary reports have as many as 1,200 people missing. Numerous landslides have occurred around other parts of Central America, most notably those which blocked the Pan American Highway, at several points. Electrical and communication lines are also down in many areas, due to landslides.

USGS has dispatched personnel, already working in Central America, to evaluate the earthquake and landslide situation.

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Update #1: added 19 January 2001

Ed Harp, Landslide Emergency Response Coordinator for the USGS Geologic Hazards Team is currently in El Salvador with Jim Vallance from the USGS Cascades Volcano Observatory. Ed, Willie Rodgriguez (USGS Puerto Rico district on special assignment to Guatemala for duration of the Hurricane Mitch Project) and Jim Vallance conducted reconnaissance studies of earthquake effects and landslides Wednesday in San Salvador, surrounding towns, and at Santa Ana volcano, about 60 km WNW of San Salvador. The USGS earthquake, volcano, and landslide reconnaissance team had air support from the U.S. military in the form of a Blackhawk helicopter, allowing coverage of extensive areas in and around the city.

The primary earthquake effect was structural damage to unreinforced masonry residential and business structures. Large buildings and most engineered structures appear to be only lightly damaged. Most buildings had no broken windows and collapse was scattered in the poorly built parts of the city. According to the NEIC, the focal mechanism of the main shock (M7.6, Saturday, January 13, 2001) was 39 km deep, presumably in the subduction zone or lower plate. The earthquake was a tensional event (normal fault dipping about 45 degrees) rather than compressional thrust. The largest aftershock was a strike slip event.

Earthquake-induced landslides were scattered throughout the region. In areas of volcanic tephra deposits the landslides were big and numerous. The team noted widely scattered evidence of liquefaction in the form of sand blows and flows. Landslides have blocked several roads including the Pan American Highway. The team has heard apparently reliable but unconfirmed reports of landslide dams on streams and rivers.

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Field Report

Photo of Volcan Usulutan

Rock and debris slides in gullies on the slopes of Volcan Usulutan.

Photo of Colonia Las Colinas

Oblique aerial view of landslide that buried Colonia Las Colinas.

Photo of scarp of Las Colinas

View of headscarp of Las Colinas landslide showing fractures in Balsamo ridge behind the scarp. Note the white tephra at the base of the steepest part of the headscarp.

Aerial view of ash deposits

Rock falls in rhyolitic Tierra Blanca ash deposits near Lago Ilopongo showing location of structures near steep slopes in the ash.

Ed Harp, Geologist with the Geologic Hazards Team, U.S. Geological Survey, has received flight support from the US military for 2 days and from the El Salvadoran military for 1 day. He spent part of his last day in the country (Saturday, 20 Jan.) on the ground at Santa Tecla (Las Colinas).

The Santa Tecla (Las Colinas) landslide

Site Amplification

Areas of concentrated landslides

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