LANDSLIDES TRIGGERED BY THE 13 JANUARY 2001, M-7.7 EARTHQUAKE IN EL SALVADOR

The 13 January 2001 earthquake (M 7.7) off the coast of El Salvador triggered widespread damaging landslides in many parts of the El Salvador. Most of the triggered landslides were relatively shallow falls and slides in rock and debris. The greatest number of landslides occurred in soft, weak, late Quaternary pyroclastic deposits; these tended to be highly disrupted masses of rock and earth. Better indurated rocks that originated as lava flows also produced many landslides; they consisted primarily of boulders up to several tens of cubic meters in volume that were shaken loose from steep outcrops and then bounced and rolled down steep slopes. Heavily affected areas included the Cordillera Balsamo region west and south of San Salvador, areas around Lago Ilopongo and Lago Coatepeque, slopes of some volcanoes in the southern part of the country (particularly Usulutan Volcano), and several slopes around San Salvador proper. The most damaging landslide triggered by the earthquake was the Las Colinas landslide from the steep northern flank of Balsamo Ridge. The landslide—a rapid earth flow—originated at an elevation of about 1070 m and traveled northward a distance of 700-800 m into the Las Colinas neighborhood of Santa Tecla. The vertical drop from the landslide source to the terminus was about 160 m. The volume of the landslide material is estimated at about 250,000 cubic meters. The landslide material exposed in the outcrop was somewhat moist but not saturated, but once mobilized it behaved as a semi-liquid mass that allowed slide material to travel an abnormally long distance from the base of the slope. The transformation of slope material from solid to fluid in the absence of large quantities of water (such as in a heavy storm) and the long runout distance of the landslide indicate unusual material properties that require detailed investigation to explain its geotechnical behavior.