Ancient Chilean tsunami scared local people away for 1000 years

A tsunami 3800 years ago devastated the coastline of Chile and encouraged hunter-gatherers to move inland, where they stayed for the next 1000 years

By Michael Marshall

An earthquake as large as any in recorded history struck the coast of Chile about 3800 years ago, triggering a tsunami that caused devastation along 1000 kilometres of coastline. In the wake of the tsunami, local hunter-gatherers began spending less time near the coast and moved cemeteries further inland, staying there for 1000 years or more, despite not having a system of writing to convey information about the disaster. 

It is a remarkable example of a society transforming itself to handle natural threats, say the researchers who studied the event.

The team, led by Gabriel Easton at the University of Chile in Santiago, spent years in the Atacama desert on the west coast of South America, gathering evidence of an ancient tsunami.

At multiple sites, they found a layer of distinctive sediment dumped by a tsunami. Radiocarbon dates from charcoal and shells in archaeological deposits directly overlying the tsunami sediment suggest it happened about 3800 years ago.

It is impressive that the team has found evidence over such a wide area, says Eugenia Gayo, director of Millennium Nucleus Upwell in Concepción, Chile. “It’s robust.”

The coast of Chile lies on a subduction zone, where one of the tectonic plates that make up Earth’s surface is being forced under another. As a result, the region is prone to large earthquakes. However, the written record in this region is quite short, so it is unclear how big the quakes can be and how often the biggest ones occur.

“We propose that this earthquake was similar to the Valdivia earthquake that occurred in 1960 in southern Chile,” says Easton. “This is the largest earthquake ever recorded in history.” The Valdivia quake had a magnitude of about 9.5, and Easton’s team says the tremor 3800 years ago was similar.

In theory, the Valdivia quake could have been a one-off caused by a very rare combination of circumstances, says Easton. But if a similar quake happened within the past 5000 years, that can’t be true. “This is our proposal, that this area in northern Chile is capable to produce earthquakes of this size,” he says.

Other subduction zones may also have been underestimated, says Easton. He points to the 2011 Tōhoku earthquake, which caused devastation in Japan. Many seismologists thought the region could only produce earthquakes of about magnitude 8.3, but the Tōhoku quake was 9.0 or 9.1.

People have lived in the Atacama for more than 12,000 years. Although the desert gets little rainfall, the marine ecosystems along the coast are rich so hunter-gatherer societies have thrived.

However, Easton and his team documented major shifts that occurred around 3800 years ago. Archaeological sites near the coast show less evidence of habitation, suggesting people stopped going there or at least spent less time there.

Furthermore, cemeteries were moved inland and uphill. The local people mummified their relatives’ bodies and placed great value on having their dead ancestors nearby – a practice that continues to this day in communities in the Andes. “The most important thing that the families and the communities had at that time were their parents,” says Easton, and they took great care to protect them.

This new pattern of behaviour lasted a long time, with many sites only being reoccupied between 1500 and 1000 years ago. “This is kind of surprising, because people usually have a short memory for this kind of event,” says Gayo. Even maintaining the behaviour for 1000 years would have meant sustaining it for 40 generations. “That is a lot.”

It isn’t clear how the memory was preserved. Easton says the message may have been passed on orally, and perhaps through pictures on stone.

For Gayo, the lesson is that sometimes it is necessary to make big changes to adapt to natural hazards. That includes modern societies, which are threatened by growing climate extremes and rising seas. “You need to transform radically,” she says.

Journal reference: Science Advances, DOI: 10.1126/sciadv.abm2996

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