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Landslides: Risky business

The Cocos and Caribbean plates played tug-of-war on Jan. 13 with El Salvador on the rope. When the tension broke, the resulting magnitude-7.6 earthquake jolted the ground beneath Balsamo Ridge along the Pan American Highway. Loosened debris flowed down the ridge taking parts of the forest with it. At least 450 people were reported dead and 1,200 missing after one landslide obliterated a section of the San Salvador suburb of Las Colinas. Another landslide trapped and killed 10 people traveling by bus along the Pan American Highway near San Vicente. Numerous other slides blocked rescue efforts along the roads.

Almost a year earlier in Taiwan, a break in the Chelungpu fault created a magnitude-7.6 earthquake that toppled buildings in Taipei, killing 1,800 people. Like the recent El Salvador earthquake, the Chi-Chi, Taiwan, quake triggered landslides from the country’s topographical highs. Across the Central Mountain Range of Taiwan, at least 7,000 landslides hit an area of several thousand square kilometers. Twenty-nine of the 36 people living under the fall zone of one rock avalanche in Tsaoling died and more than 20 people died from a landslide in Jiu-Feng-Erh-Shan, according to U.S. Geological Survey (USGS) reports.

[At right: The Jan. 13 earthquake that hit El Salvador triggered this landslide from the Balsamo Ridge over Las Colinas. Edwin Harp.]
Both earthquakes induced landslides that were inevitable. “In those cases — where you have tectonically very active areas, high steeped slopes and deeply weathered material — you are going to have earthquake triggered landslides on a frequent basis,” says Randall Jibson of the USGS.
The landslide that buried Las Colinas did not occur, as some environmental groups asserted, because of deforestation. Tropical flora on the ridge could not have stopped the approximately 500,000 cubic meters of earth that inundated Las Colinas. Forest shade trees typically have shallow roots, about 50 inches deep, that grow where oxygen is plentiful. The quake shook free a precarious layer of volcanic deposits reaching 50 to 70 feet in depth. “It probably failed due to liquefaction in ash deposits near its basal shear surface,” says Ed Harp of the USGS, who visited the area after the earthquake.
Before the earthquake struck, Holocene ash-fall deposits at the base of the Las Colinas landslide had probably collected some water, enough to liquefy the land when the shaking started, Harp says. Underlying these deposits is an ancient, impermeable soil layer that acts as an aquiclude, holding the water and creating a perched water table sitting within the ash above. In such a situation, the ash deposit can liquefy during seismic shaking while much of the overlying deposits remain dry. In fact, Harp observed that the tip of the landslide’s tongue was wet and “soupy,” while landslide material from the top of the ridge was dry.

Past volcanic eruptions had left angular shards of ash and glass strewn across the top of Balsamo Ridge, creating deposits about 25 meters thick of andesitic cinders and some interbedded rhyolitic tephra. Two kilometers west of Las Colinas is another housing development, Pinare de Suiza, that is built even closer to the ridge, Harp says. A spider web of cracks along the ridge crest outlines potential slides that may fail in the future. “Come the rainy season I’d be surprised if we didn’t have more landslides,” Harp says.

[At left: Head scarp from the Los Colinas landslide. Edwin Harp.]
In Taiwan, the landslides that followed the September 1999 earthquake were shallow and fast, wrote Hsi-Ping Liu and colleagues in a USGS report. Valley-cut steep slopes and other highway cuts through the Central Range were hit the most. Uplifted, folded and faulted sandstone, siltstone, shale and weakly cemented Pleistocene conglomerate litter the mountain front and western part of the Central Range. The landslide in Jiu-Feng-Erh-Shan brought an estimated 72 to 90 million cubic meters of rock and soil from the slopes. In Tsaoling as much as 140 million cubic meters fell into a valley cut by the Ching-Shui River. “A significant amount of landslide material moved across the river and some distance up the opposite bank,” the report says.
Three months after the earthquake, the dam from the Tsaoling landslide had created a popular tourist attraction. “A sizable lake had built up behind the landslide dam and they had concessions running boat trips out on the lake for sightseeing and fishing,” says David Keefer of the USGS. In the past, at least two other landslides are thought to have dammed the river: one possibly in 1862 that breached in 1898, and another in 1941 that lasted 10 years until the lake overtopped the dam in a flood.
In November 2000, Clarence Duster of the U.S. Bureau of Reclamation visited Taiwan and learned from the Water Conservancy Agency that to protect the dam in Tsaoling, the agency had built a channel through the top of the dam and lowered a portion of the reservoir. The channel was trickling out a flow of a few cubic meters per second. Is the dam at risk from another flood? “There’s always that possibility,” Duster says. For now, Tsaoling has a lake.
In El Salvador, loose earth is still hanging from the edge of the hilltop above Santa Tecla. Ed Harp told Salvadorian President Francisco Flores that hazards from additional landslides would present a risk to the residents of Santa Tecla if another large earthquake approaching a magnitude of 7.0 hit.
In the absence of such an earthquake, aftershock- and rainfall-triggered landslides will probably be relatively small and not present a high hazard to people living in the city below the ridge, he said. However, even if no large earthquake occurs in the near future, “15 to 20 years from now there could be another large earthquake and accompanying landslide similar to the Las Colinas landslide triggered from Balsamo Ridge.”

Christina Reed

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