Seismological and geological aspects of the Mantaro landslide in Peru

Abstract

ON the evening of April 25 1974 one of the largest landslides known took place in the Mayunmarca valley (12.6°S, 74.6°W), adjacent to the Mantaro River, in the Peruvian Andes (Fig. 1). A total of 1,000 to 1,300 × 10⁶ m³ of soil and rock was displaced in successive slides, which occurred within a few minutes of each other. Part of the slide material reached the Mantaro canyon, where a gigantic natural dam was formed. The dam was 3.8 km long extending along the axis of the canyon, 2.5 km wide across the river (Fig. 2), 170 m high from the river bed to the lowest point on the crest, and had a volume of 800 × 10⁶ m³. At the time of the slide, the Mantaro River was discharging nearly 150 m³ s⁻¹, and a reservoir was quickly impounded upstream from the dam. At its largest, the reservoir was approximately 32 km long and contained over 670 × 10⁶ m³ of water. After 42 days, the reservoir overtopped the natural dam. The high potential of flood damage was minimised by construction of a controlled spillway across the dam.