The Great Peruvian Earthquake


Statue on higher ground in Yungay, Peru [1].

A couple weeks ago I found myself on Wikipedia reading about the largest earthquakes in history. One particularly stuck out given its magnitude in terms of number of deaths, how recently it occurred, and the little science-political twist that resulted in the large number of deaths from a particular event; the Ancash earthquake or “Great Peruvian Earthquake” that happened on May 31, 1970 off the coast of Peru where a total of approximately 74,000-80,000 people died.

So what happened? Well the earthquake was a 7.9 on the moment magnitude scale (Mw). To put things into perspective, the 2004 Indian Ocean and 2011 Tohoku earthquakes were magnitudes of 9.1 and 9.0 Mw, but as we all remember the bulk of the damage and deaths were the result of subsequent tsunamis as a result of subduction of continental plates that caused an upthrust in the ocean. The Ancash earthquake was similar, but the bulk of the damage was done not by a tsunami but a giant avalanche.


Aerial image of the avalanche (left) [2,3] and a photograph of the damage at Yungay (right) [4].

The most devastating and surprising part of this disaster was what happened to the towns of Yungay and Ranrahirca. The northern peak of the HuascarĂ¡n Mountain was destabilized from the earthquake, which resulted in a massive debris avalanche of soil, rocks, ice, and snow that buried the towns killing 20,000 people (88% of the towns’ populations). It has been estimated that the avalanche amounted to 50 million m3, with a spread of 910 m (3,000 ft) wide and 1.6 km (1 mi) long, traveling at speeds upwards of 320 km/h [5]. That’s like a wall nearly 1 km wide hitting your house at the speed of a bullet train.

Here’s the twist. A land slide from the same location occurred eight years prior on Jan 10, 1962 killing 4,000 people across nine towns and destroying parts of Ranrahirca. That same year, two American scientists David Bernays and Charles Sawyer went to investigate and warned that a vertical slab of rock undermined by a glacier could fall at any moment destroying the villages below [5] noting that the resulting avalanche from this could be much larger and that Yungay could be in imminent danger. This was reported in the Espreso local newspaper, however the government ordered them to retract the article or be charged. The two scientists were forced to flee the country and the local government forcibly prevented citizens to speak of the potential danger [6]. Unfortunately, their prediction came true.

Someone visited the town recently where the town area has been declared a national cemetery.

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