Deadliest landslide

The deadliest single landslide occurred on 31 May 1970 in Yungay Province, Peru. A 7.8-magnitude earthquake triggered a major rockfall from the upper slopes of mount Huascarán, which tumbled down a glacier, gathering mass and momentum before sweeping away several towns and villages in the valleys below. The numbers of dead and missing were initially estimated at 22,000, though more recent research has seen this figure revised down to around 6,000.

The valleys around Huascarán have witnessed many catastrophic landslides, some of which made their way into the historic record, while others are known only from the scars the left in the terrain of the region. Only eight years prior to the 1970 event, on 10 January 1962, a very similar landslide had swept down the same valley, burying the village of Ranrahirca and causing an estimated 4,000 deaths.

The 1970 landslide began at around 3:23 p.m., when the tremors from the 1970 Ancash earthquake dislodged a section of a rock face near the summit of Huascarán. The initial collapse saw an overhanging slab of stone 800 m high, 350 m wide and 50 m thick (2,624 x 1,148 x 164 ft) break away and fall on to the surface of Glacier 511, located at the head of the Ranrahirca valley.

As the initial rock fall tumbled down the mountainside, it set more landslides in motion; by the time it entered the Ranrahirca valley, it had formed a 58-million-cubic-metre (2-billion-cubic-feet) mass of rock, mud, ice and water travelling at between 180 and 300 km/h (111–190 mph).

As it moved down the valley, the mass split into two lobes. The main body of the landslide continued straight down the Ranrahirca valley to the Rio Santo, destroying the villages of Ranrahirca (which had been rebuilt since 1962) and Matacoto. A smaller lobe diverted to the north crossing a flat plain and burying the town of Yungay.

For many years, it was reported that the landslide had killed at least 22,000 people, and this figure is still frequently quoted in articles relating to the disaster. In 2009, however, a paper was published by researchers from Canada, Peru and the USA, led by Stephen Evans of the University of Waterloo, which scrutinized these figures. It was discovered that early casualty estimates mixed up the population of Yungay District (an area of 276 km², population 15,000) with the district's main town, Yungay (population 3,543). Similar errors were made for Ranrahirca District and Matacoto. Using population projections from the 1961 Peruvian census, a revised death toll of 6,000 was produced.

It is possible that an even greater number were killed by one of the many simultaneous landslides that were triggered by the 1920 Haiyuan earthquake in China. However, this event was poorly reported on at the time and the government was preoccupied with the concurrent 1920–21 North China famine. As a result, it is impossible to say what proportion of the reported 234,117 dead were killed by landslides or by the earthquake and its aftermath.