LANDSLIDES INDUCED BY THE 13 FEBRUARY 2001, MW 6.6, EL SALVADOR EARTHQUAKE

On Feb. 13, 2001, a M_w 6.6 earthquake in central El Salvador caused extensive damage, hundreds of fatalities, and thousands of landslides. The landslides were concentrated in an area about 40 km east of the capital city of San Salvador, where deposits of poorly consolidated late Pleistocene and Holocene rhyolitic tephra are tens of meters thick. Steep walls of narrow valleys incised into the volcanic deposits failed and locally filled valley bottoms with tens of meters of loose debris that is prone to rapid erosion.

Large landslides dammed two major rivers, the Río El Desagüe and Río Jiboa, and two large slides occurred on Volcán San Vicente. The ±1.5-million-m³ Río El Desagüe landslide temporarily dammed the river and formed a shallow, 1.5-km-long lake, but the dam has been overtopped and is stable. The ±12-million-m³ Río Jiboa landslide blocked about 700 m of the valley with debris composed of poorly consolidated tephra, and the upstream lake could potentially have been as deep as 60 m and about 2 km long. A 20-m-deep spillway was excavated to decrease the maximum lake volume and reduce the possible catastrophic failure of the unstable landslide dam. On the northern flank of Volcán San Vicente, ±250,000 m³ of loose landslide debris filled the upper part of Quebrada El Blanco; remobilization of this material in debris flows could inundate part of a downstream village. On the volcano's northwest flank, ±200,000 m³ of lithified andesite blocks slid in the upper part of Quebrada Del Muerto, but this material will not likely remobilize and threaten downstream settlements. Along the shores of Lago de Ilopango, local liquefaction and lateral spreading caused damage to homes and structures. Most of the landslides were caused by strong shaking that destabilized slopes in poorly consolidated volcanic bedrock and alluvium.