MASS-WASTING TRIGGERED BY THE 5 MARCH 1987 ECUADOR EARTHQUAKES

On 5 March 1987, two earthquakes (M(s) = 6.1 and M(s) = 6.9) occurred about 25 km north of Reventador Volcano, along the eastern slopes of t he Andes Mountains in northeastern Ecuador. Although the shaking damag ed structures in towns and villages near the epicentral area, the econ omic and social losses directly due to earthquake shaking were small c ompared to the effects of catastrophic earthquake-triggered mass wasti ng and flooding. About 600 mm of rain fell in the region in the month preceding the earthquakes; thus, the surficial soils had high moisture contents. Slope failures commonly started as thin slides, which rapid ly turned into fluid debris avalanches and debris flows. The surficial soils and thick vegetation covering them flowed down the slopes into minor tributaries and then were carried into major rivers. Rock and ea rth slides, debris avalanches, debris and mud flows, and resulting flo ods destroyed about 40 km of the Trans-Ecuadorian oil pipeline and the only highway from Quite to Ecuador's northeastern rain forests and oi l fields. Estimates of total volume of earthquake-induced mass wastage ranged from 75-110 million m(3). Economic losses were about US$ 1 bil lion. Nearly all of the approximately 1000 deaths from the earthquakes were a consequence of mass wasting and/or flooding.