On a Sunday afternoon, 31 May 1970, at 15:23 local time, a magnitude 7.9 earthquake struck near the coast of northern Peru. The shaking lasted around 45 seconds. What happened in the minutes after those 45 seconds is why this earthquake is still studied more than half a century later: the shaking loosened the north peak of Huascarán, Peru’s highest mountain, and sent a catastrophic debris avalanche down onto the town of Yungay.
USGS counts around 70,000 deaths in total — about 50,000 people killed and 20,000 missing and presumed dead — and around 150,000 injured in the Ancash and La Libertad departments, from the earthquake and the avalanche combined. USGS calls it one of the world’s most destructive earthquakes. It remains the most catastrophic natural disaster in Peru’s recorded history, and the deadliest part of it was not the shaking itself. It was what the shaking set in motion on the mountain above the valley.
The epicenter lay offshore, near the coast of northern Peru, where the Nazca Plate is being subducted beneath the South American Plate. Reported maximum intensity reached VIII — “severe” on the Mercalli scale. Geological effects spread over roughly 65,000 square kilometres of west-central Peru, according to the USGS geological study of the event: slope failures of all types damaged roads and irrigation canals and temporarily dammed some rivers and lakes.
That wide footprint matters for understanding the toll. This was not a disaster in one town. Shaking damage was reported across the Ancash and La Libertad departments, in coastal cities and in the high Andes valleys inland — and then, on top of the shaking, came the slope failures, the largest of which fell on a valley that had almost no time to react.
The earthquake destabilised the north peak of Huascarán. From between about 5,500 and 6,400 metres altitude, a mass the USGS study estimates at 50–100 million cubic metres of rock, ice, snow, and soil broke away and travelled 14.5 kilometres to Yungay, at an average velocity estimated between 280 and 335 kilometres per hour.
Read those numbers slowly. At that speed, the distance from the mountain to the town was covered in minutes. The debris avalanche buried the city of Yungay and part of nearby Ranrahirca, with a loss of more than about 18,000 lives in that flow alone, according to the USGS study. Yungay had a population of about 20,000. A pulse of muddy water from the avalanche then swept down the Río Santa some 160 kilometres to the sea, inundating farms and small settlements along the way.
What remained above the buried town became its memorial image: Cemetery Hill, with its statue of Christ and four palm trees, stood above the flow. Almost everything below it was gone — not knocked down, but buried, which is a different kind of loss. There was, in much of Yungay, nothing left to dig through and rebuild on.
The first reason is the pairing of hazards. A magnitude 7.9 is a major earthquake on its own. But Ancash is the classic case of a secondary hazard outkilling the primary one: the earthquake’s deadliest act was triggering a slope failure above a populated valley. Preparedness that only pictures shaking — drop, cover, hold on — does not capture a hazard that arrives after the shaking, from the hillside, at highway speed multiplied several times over. For the science of ground that loses its strength, see What Is Liquefaction?, and for the plate system behind Peru’s quakes, Tectonic Plates Explained.
The second reason is geography. The towns of the Callejón de Huaylas valley sit between high ranges, below steep, glaciated peaks. That terrain is beautiful and, in an earthquake, it is loaded: slopes that are stable for decades can be one strong shaking away from failure, and the valley floor — where people live, farm, and build — is exactly where the debris goes. The USGS study catalogued slope failures throughout the mountainous region, not just at Huascarán.
The third reason is time. An avalanche averaging between 280 and 335 kilometres per hour does not leave an evacuation window measured in the minutes a tsunami warning might give a coast. Between the end of the shaking and the arrival of the debris, there was effectively no decision to make. That is why the Ancash lesson is mostly a before lesson: hazard maps, land-use choices, and knowing which slopes above a town are capable of this — long before any given Sunday afternoon.
Every mountainous earthquake region has its version of this question: what will the shaking shake loose? Landslides, rockfalls, and debris flows are a routine part of large earthquakes in steep terrain, and the Ancash avalanche is the extreme end of that spectrum — the case scientists point to when they argue that secondary hazards belong in the headline risk assessment, not in a footnote.
For readers in gentler terrain, the transferable habit is simpler: after any strong shaking near hills, slopes, or cliffs, treat the slopes themselves as part of the hazard. Stay away from steep ground that has just been shaken, expect road-blocking slides, and do not assume the danger ended when the ground stopped moving. The general guide is What to Do After an Earthquake.
The 1970 Ancash earthquake is remembered, correctly, as two disasters in one: a magnitude 7.9 that damaged a region of roughly 65,000 square kilometres, and an avalanche that erased a town of about 20,000 people in minutes. Around 70,000 people died in total, by the USGS count. Half a century on, it is still the clearest warning in the earthquake record that the ground moving is only the first act — and that what stands above a town can matter as much as what lies beneath it.