On This Day - What happened 31 May 1970 AD

The 1970 Ancash earthquake (also known as the Great Peruvian earthquake) occurred on 31 May off the coast of Peru in the Pacific Ocean at 15:23:29 local time. Combined with a resultant landslide, it is the most catastrophic natural disaster in the history of Peru. Due to the large amounts of snow and ice included in the landslide that caused an estimate of 66,794 to 70,000 casualties, it is also considered to be the world's deadliest avalanche.

The undersea earthquake struck on a Sunday afternoon and lasted about 45 seconds. The shock affected the Peruvian regions of Ancash and La Libertad. The epicenter was located 35 km (22 mi) off the coast of Casma and Chimbote in the Pacific Ocean, where the Nazca Plate is being subducted beneath the South American Plate. It had a moment magnitude of 7.9 and maximum Mercalli intensity of VIII (Severe). The focal mechanism and hypocentral depth of the earthquake show that the earthquake was a result of normal faulting within the subducting slab. The northern wall of Mount Huascarán was destabilized, causing a rock, ice, and snow avalanche and burying the towns of Yungay and Ranrahirca. The avalanche started as a sliding mass of glacial ice and rock about 910 meters (2,990 ft) wide and 1.6 km (1 mile) long. It advanced about 18 kilometers (11 mi) to the village of Yungay at an average speed of 280 to 335 km per hour. The fast-moving mass picked up glacial deposits and by the time it reached Yungay, it is estimated to have consisted of about 80 million m³ of water, mud, rocks, and snow.