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.
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.
Field Report
Rock and debris slides in gullies on the slopes of Volcan Usulutan.
Oblique aerial view of landslide that buried Colonia 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.
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).
- Earthquake-induced landslides occur in scattered pockets across the country, but the Santa Tecla landslide is one of the largest and is the remaining hazard of greatest concern.
- Ed recommends that Santa Tecla slide and the adjacent area be flown at scale of 1:6000 covering an area of about 5 x10 km. Salvadoran officials want this area mapped in detail to determine what to do about the threat of further slides.
The Santa Tecla (Las Colinas) landslide
- The landslide slid off the northern flank of Balsamo Ridge composed of the Balsamo Fm. The formation is composed of mainly andesitic cinders and some interbedded tephra. Earthquake shaking was clearly amplified in the Balsamo Fm. and on the Ridge. Extensive network of cracks exists on the ridge crest in areas that did not slide; these are cause for additional concern. Liquefaction may have been a factor in the failure of the Las Colinas landslide; possibly related to the tephras resting on less permeable paleosols. The distal end of the landslide showed evidence of being "soupy" even though there didn't appear to be much clay in the deposits.
Site Amplification
- The topographic effects of ridge apparently had pronounced effect of increasing the strength of the shaking intensity. Ed has film copies of 10 strong motion records from the CIG that show max. acceleration of about 0.6 g at the stations but acceleration on ridge top was probably higher because people report being thrown to the ground during the earthquake. This stronger shaking is also supported by the abundant network of cracks along the ridge crest.
- Ministry of the Environment is overseeing effort to do this detailed crack mapping in a 1 x 5 km area. They have the assistance of two consultants, (Spanish and Swiss) and some local technical people.
- Salvadoran officials need answers about how to deal with the landslide immediately and in the longer term: When can people move back in? (The place is still dangerous) What will be effects of aftershocks? Will they induce more slides? Should additional areas be evacuated? Will rainfall in rainy season cause additional problems? Should the area of steep slopes immediately adjacent to the ridge be forbidden for resettlement?
- Landslides elsewhere in the country. Pockets are scattered and not of as large a concern in terms of immediate threats to people, but they have disrupted transportation routes, in particular the PanAmerican Highway at several locations.
- In a general way, the geographic distribution of landslides roughly corresponds to the distribution of young ash, tuff, and tephra deposits on steep slopes incised valley walls and river channels.
Areas of concentrated landslides
- In region to east and north of Lago Ilopango and some near Lago Coatepeque (east of Volcan Santa Ana). Rock slides and soil slides on flanks of Volcan San Vicente; rock slides on slopes of steep valleys incised into volcanic edifices. In rainy season there may be some potential for debris flows that could affect local farms, but probably won't threaten major populations centers. East of Usulutan adjacent to channel of Rio Grande; river cuts through small range and abundant flooding of this part of river during Hurricane Mitch. One strategy to mitigate this flooding potential was to create a diversion route through an abandoned paleochannel through the range; abundant slides occurred in the paleochannel which may affect the mitigation plan. Not many earthquake-induced landslides in the eastern part of the country.
- PanAmerican Highway Closure: slides have blocked highway in area west of San Salvador, but major slide has blocked it east of Ilopango. Here a steeply dipping joint surface has served as a slide plane and a large mass has collapsed onto the highway. Slide is about 150 m deep and about 300-400 m wide. Bulldozers are working on removing the slide material but progress is slow.
