On 31 May 1970, a debris avalanche and mudflow triggered by the Ancash earthquake destroyed the town of Yungay, Peru and 10 nearby villages, leaving up to 30,000 people dead. It is the deadliest avalanche or glacier-related disaster in history, surpassing the death toll from the previous deadliest avalanche disaster, the avalanches of White Friday on the Italian front of World War I and the third or fourth most deadly landslide-related disaster of the 20th century, after the Haiyuan landslides (China), the Armero tragedy (Colombia) and by some estimates the Khait landslide (Tajikistan).
The north peak of Huascarán from which the avalanche originated had been considered unstable since 1962, when a smaller collapse wiped out several villages of the Callejón de Huaylas valley near Yungay. However, the provincial government made efforts to prevent the news from spreading and urged people not to panic. The 1970 earthquake destabilized a glacier and snowmass which surged rapidly downhill, becoming a mudflow as it accumulated large volumes of loose dirt, rock and surface water. The death toll was made worse due to the earthquake having occurred on a Sunday, as thousands more people had congregated in Yungay for market when the mudflow struck and leveled the city. The slide then entered the Río Santa and caused extensive damage as it flowed all the way downstream to the Pacific Ocean, a distance of , mostly through a narrow canyon.
Following the disaster, the Peruvian government conducted relief efforts and planned to move the provincial capital from Yungay to a safer location at Tingua. Survivors resisted the resettlement proposal and thousands stayed in a camp just north of the destroyed city known as "Yungay Norte", which would eventually become the present-day town of Yungay. The government has forbidden redevelopment or excavation of the original town site, where a memorial has been established to commemorate the dead. Although most of Yungay was completely leveled by the earthquake and mudflow, some remnants, including the ruined cathedral and cemetery, can still be seen in the area.
Although Yungay was located outside of the direct avalanche path, the slide was so large that it overwhelmed the natural geographic barriers protecting the town, whose location had previously been considered safe. The United States Geological Survey stated that "conceivably, such an event may not occur again for thousands of years."
Background Geography and geology Due to its rugged terrain, high elevation and location in an active seismic zone, the Callejón de Huaylas, or upper Río Santa valley has historically been vulnerable to large landslides, mudslides and avalanches. Huascáran, the highest peak of the Peruvian Andes, rises to an elevation of to the east of the valley. Río Llanguanuco, a tributary of Río Santa, drains a ravine on the northwest flank of Huascáran and cuts a steep southwesterly path to join the larger river, forming an alluvial fan in a wide valley near its mouth. Due to the availability of fertile soils and surface water, the area had been farmed for hundreds of years and was home to many small villages. The original site of Yungay town was an elevated area northeast of the Río Santa and north-west of the Río Llanguanuco; it was outside of the Llanguanuco valley, with a low ridge (Cerro de Aira) separating the two.
Debris-flow deposits of Holocene origin make up the Río Llanganuco alluvial fan and underlie the Río Santa bed for depths of hundreds of metres in places, indicating that such flows have occurred here regularly for thousands of years. Geologic evidence indicates that Yungay itself was "probably built on a flow or avalanche deposit." The name of nearby Ranrahirca village means "pile of thrown stones" in Quechua, suggesting that debris avalanches have been observed in the valley since ancient times. These destructive flows of snow, ice, mud and rock are locally termed huaycos.
The northwest side of Huascarán has been called "one of the most unstable ice-covered areas in the Cordillera Blanca". Among the many glaciers and permanent icefields on Huascarán is "Glacier511" located below the nearly vertical face of Huascarán's north peak. The glacier fills the head of the very steep Quebrada Armapampa ravine, which eventually flows into Río Llanguanuco. Beneath the snout of Glacier511 the terrain slopes down at an average pitch of 23degrees (in some places exceeding 70degrees), with a total vertical drop of before reaching the more gently sloped valleys below, creating ideal conditions for any large falling mass of rock to move unobstructed at high speed.
Yungay before the disaster
In the late 1960s, Yungay's permanent population was around 4,000 or 5,000,Haller, Andreas (2010): Yungay: recent tendencies and spatial perceptions in an Andean risk zone. In Espacio y Desarrollo 22, pp. 65–75 which swelled greatly on market and church days when thousands of visitors arrived from the surrounding villages. With the notable exception of the massive stone cathedral at the center of town, the city consisted predominantly of two or three-story adobe and wood structures with heavy tile roofs, dating from the Spanish colonial period. This fragile housing stock, typical of structures elsewhere in the Río Santa valley, was highly vulnerable to seismic activity. Other prominent features of Yungay included the central Plaza de Armas surrounded by palm trees, which hosted events including the fiesta of Santo Domingo, and the elevated cemetery, which was actually built on top of a pre-Inca burial mound which may be as old as 10,000 years.
In the years leading up to 1970, Yungay – sometimes nicknamed "Pearl of the Huaylas Corridor" – had become one of the most popular tourist destinations in Peru due to its picturesque location, architecture, and proximity to Huascarán and the Llanganuco lakes.
Yungay had been struck by disaster before; it was reported to be "almost completely wiped out by an avalanche" in 1872, implying an avalanche at least equal to the magnitude of the 1970 event.
1962 Ranrahirca avalanche On January10, 1962, part of Glacier 511 collapsed, causing 10millionm3 of rock, ice and snow to crash down into Río Llanganuco. The slide destroyed nine villages and killed more than 4,000 people. The entire town of Ranrahirca, which was built on the Llanguanuco fan at the edge of the slide path, was flattened; some 2,900 people died in Ranrahirca alone. Residents had little to no warning, as less than four minutes passed from the beginning of the slide before the flow reached the Río Santa. Everything in the wave's path was destroyed, and corpses were washed as far as the Pacific Ocean, downstream.
This slide was not caused by an earthquake, nor were seven other large avalanches and debris flows that occurred in the Santa Valley between 1940 and 1970. Rather, the glacier had fractured under its own weight. Due to the steep slope of the underlying rock and lubrication by glacial melt, there was nothing to stop the ice from sliding downhill at high speed. In the wake of this and other glacier-related disasters in the Ashish region, notably glacial lake outburst floods that destroyed parts of Huaraz in the preceding two decades, the national government had tried to push forward "hazard zoning" which would limit development in flood and avalanche zones. However, these efforts were largely unsuccessful due to resistance by locals, who feared the economic and cultural disruption that would result from forced relocation.
Post-1962 observations and controversy In September 1962, the Massachusetts Institute of Technology sponsored a glaciological expedition to assess the aftermath of the Ranrahirca slide. American climbers David Bernays and Charles Sawyer observed, in the aftermath of this slide, "a massive vertical slab of rock being undermined by Glacier511" posing the threat of a much larger collapse. The overhanging cliff was as much as high in places and the rock was described as being "pervasively fractured." Although the risk to Yungay was remote, the town could be threatened if a "sufficiently large avalanche" were to spill over the ridge to its east. The Americans' findings were reported on September27, 1962, in the El Expreso newspaper, under the headline "Dantesque Avalanche Threatens Yungay."
Local authorities ordered this information to be retracted, and discouraged citizens from speaking about the threat, in order to prevent panic: "Anyone who spoke in favor of the Americans' conclusions would be charged under the Penal Code for disrupting public tranquility." The director of the regional Lakes Commission, Miguel Eliás Pizarro, was asked to "provide an expert analysis." Pizarro "dismissed the American mountaineers' statements as outlandish, hurried and uninformed" and stated that Yungay was in no danger, being well out of the path of the 1962 slide. Bernays and Sawyer were reportedly threatened with arrest, and they soon fled the country. Prior to that, Bernays had attended a public meeting in Huaraz where he called the Expreso coverage "exaggerated", though he did not deny the possibility that a large enough avalanche could affect Yungay.
This same glacier would be the origin of the much larger collapse that destroyed Yungay eight years later.
Earthquake and landslide At 3:23 PM on Sunday, May31, 1970, the Ancash earthquake (also known as the Great Peruvian earthquake) struck off the coast of Peru with a moment magnitude of 7.9. The shaking dislodged a slab of rock about wide from the western face of Huascarán's north peak, at an elevation of roughly . The rockfall occurred almost immediately after the shaking began, with witnesses comparing the sound to a gunshot or explosion. The fall triggered an "enormous dark cloud of dust" which rapidly blocked the view from below.
The rock fell some vertically before landing on Glacier511 and sliding nearly down its surface, accumulating a large volume of firn snow as it went. The initial ice-rock avalanche volume was at least 25millionm3 (33millionyd3), and grew rapidly in size as it fell down the steep Llanganuco valley, picking up large volumes of dirt, loose glacial moraine, water and uprooted trees. With area soils saturated near the end of the rainy season, and the large amount of snow and ice scraped off the glacier surface, the avalanche quickly liquefied into a fluid, fast-moving mudflow.
The maximum volume of mudflow was as much as 50-100 millionm3 (130millionyd3), and it reached speeds of up to . Some debris projectiles launched ahead of the main flow may have exceeded . According to a U.S. Geological Survey report published the same year, the mudflow may have achieved its unusually high speed due to "air-cushioned flow", a mixture of snow, ice and entrapped air that allowed the bulk of the material to essentially float over the ground. The initial acceleration of the mass down the low-friction glacial surface was also a major factor, catapulting the material downhill at a much higher speed than if it had slid over bare rock or earth.
Destruction of Yungay In Yungay, people rushed outside after the earthquake hit; many flocked to churches to shelter and pray. Most buildings in the city were damaged or destroyed by the earthquake, though initial casualties were low compared to the mudflow that followed. The mudflow struck Yungay, from the avalanche origin, an estimated 1 minute, 42seconds after the earthquake began. Although the nearly hill of Cerro de Aira stood between Yungay and the Llanganuco valley, the mudflow struck the ridge at a high enough speed – some – that part of it leapt over the crest, first destroying the villages of Shillkop, Aira and Ongo before descending upon Yungay.