The earthquake in west-central Peru triggered a massive avalanche that swept downhill from a height of 18,000 feet.
On the afternoon of May 31, 1970, an earthquake of magnitude 7.9 struck an area of more than 30,000 square miles in west-central Peru near the town of Chimbote about three hundred miles north of the capital, Lima. Earthquake-triggered slope failures of all types in the white and black mountain ranges of the Ancash region extensively damaged transportation routes and irrigation canals and temporarily dammed some rivers and lakes. The geologically most important and most spectacular disruption was a massive debris avalanche from an altitude of 18,000 feet on the north peak of Mount Huascaran that overwhelmed and buried the city of Yungay. Altogether 70,000 lives were lost, 140,000 injured, 800,000 lost their homes, and 160,000 buildings were destroyed. Property damage amounted to more than half a billion dollars.
The debris avalanche consisted of one hundred million cubic yards of rock, ice, snow, and soil. The earthquake that lasted less than a minute destabilized the northern wall of Mount Huascaran and created the avalanche, a sliding mass of glacial ice and rock, 3,000 feet wide and one-mile long. This mass of ice and rock traveled downhill for ten miles from source to Yungay at an average speed of two hundred mph. Eyewitness accounts of the event told of topographic obstructions as high as five hundred feet being overridden by the avalanche and boulders weighing several tons being thrown forward as far as 3,000 feet.
The valley between the two mountain ranges experienced the greatest overall damage, especially in the valley of the Santa River. Coastal towns were hit very hard with Chimbote having 90 percent of its buildings destroyed. The Pan-American Highway was damaged and that made the provision of humanitarian aid difficult. Most of the fatalities from the earthquake were caused by the collapse of buildings. The buildings, unfortunately, were often located on alluvial fill, and they were made of adobe construction.
This earthquake, the worst natural disaster in the history of the Western Hemisphere up to that time, had an epicenter that was located twenty miles offshore and at a depth of twenty-five miles below the ocean surface. Peru is no stranger to violent earthquakes. There have been many in the past and there will be more in the future. The great heights of the Andes Mountains are a consequence of these earthquakes as the Nazca Tectonic Plate subducts beneath the South American Plate. This movement between the two plates is continuous and the additional continuous westward movement of the continental block of South America, at the same time, speeds up the rate of interaction between these two plates.
The Nazca at Peru dips under the continent at an angle of sixty degrees, and earthquakes occur at increasingly greater depths toward the east, reaching a maximum of 380 miles near the Peru–Brazil border. The rupture that created this 1970 earthquake extended one hundred miles to the south. The absence of surface tectonic displacements or of a significant tsunami and the spatial distribution of the main shock and aftershocks suggest that the earthquake originated by movement on a fault, or faults, beneath the continental shelf.
The River Santa flows through a narrow valley, 125 miles in length, one-mile-wide at its widest point, and all of it at an elevation of 14,000 feet. It lies between the black and white mountain ranges. For centuries the lifestyle of the people who lived in this valley changed very little. The farmland is rich, regularly irrigated with water from the neighboring mountains, providing pasture for rearing sheep and soil for growing corn, wheat, and barley.
The people of the valley frequently speak both Spanish and the ancient Inca language, Quechua. Some own land and many others are workers employed by large landowners. All of this traditional way of life changed as a result of the earthquake. New communities and new opportunities took the place of the old. The Peruvian government has forbidden excavation in the area where the town of Yungay is buried, declaring it a national cemetery. The few survivors from the Santa River valley were resettled and the government declared May 31 as Natural Disaster Education and Reflection Day, in memory of the deadliest seismic disaster in the history of Latin America.