A tsunami wiped out ancient communities in the Atacama Desert 3,800 years ago

A recent study of geological deposits and archaeological remains has identified a massive earthquake and tsunami that wiped out communities along the coastline of Chile's Atacama Desert around 3,800 years ago. Studying the ancient disaster—and people's responses to it—could help with modern hazard planning along the seismically active coast.

A long-forgotten disaster

Broken walls and toppled stones reveal the calamity that struck Zapatero, an ancient community in what's now northern Chile, about 4,000 years ago.

The people who lived along the coast of the Atacama Desert 5,700 to 4,000 years ago built villages of small stone houses atop massive piles of shells (Zapatero's shell-filled midden is two meters deep and spans six square kilometers). Usually, these houses stood adjacent to each other, opening onto inner patios. People buried their dead beneath the houses' floors. The cement floors were made from algae ash, seawater, and shells—the same material that held the stone walls together.

But stones and mortar failed in the face of the ocean's power. One house at Zapatero stands in ruins, with the stones from its walls toppled inland as if struck by a giant wave. Another lies with its stones scattered back toward the sea, in exactly the pattern you'd expect from "strong currents associated with tsunami backwash," University of Chile archaeologist Diego Salazar and his colleagues say. In a third house, the floors are covered in a layer of a washed-in sand laden with the remains of marine algae and echinoderm spines, mingled with chunks of rock, shells, and sediment ripped up from the ground.

Elsewhere on the Zapatero midden, Salazar and his colleagues found similar layers of sand and ripped-up ground left behind by an ancient tsunami, along with channels gouged out by the tsunami's strong, sudden current. When the archaeologists radiocarbon-dated shells from these layers, they found that many of the shells were actually older than the ones in undisturbed layers underneath—evidence that something had churned up the ground and ripped these older shells from their resting places to deposit them on the surface.

The same story is written in ruins and sediment at other archaeological sites along a several-hundred-kilometer stretch of the Atacama coastline. In recent surveys, Salazar and his colleagues also found geological evidence of an earthquake and tsunami that struck the region: layers of sandy, shell-laden seafloor sediment lifted several meters above sea level by seismic upheaval. The researchers radiocarbon-dated shells in these uplifted chunks of ancient coastline, along with shells and charcoal in the layers just above and below the tsunami deposits, and narrowed the date of the ancient disaster to around 3,800 years ago, give or take a century or two.

Combined, the geological and archaeological evidence points to a natural disaster of epic proportions: a rupture along a 1,000-kilometer stretch of the fault system where the Nazca Plate is slowly sliding under the South American Plate. The estimated magnitude 9.5 megathrust earthquake would have shoved parts of the coastline upward and triggered a tsunami 19 to 20 meters high along a huge stretch of the Chilean coast (and all the way across the Pacific in New Zealand, where geologists have also found deposits from a tsunami of about the same age).

The combined earthquake and tsunami struck a devastating blow for ancient people who lived close to the Pacific Ocean with a hyperarid desert at their backs. Archaeological evidence reveals that people abandoned the coast for centuries after the disaster.

Abandoned villages and scattered camps

The Atacama Desert is a hard place to live. It's the driest desert in the world outside Antarctica, with less than 1 millimeter of rain a year. But people have lived—and thrived—here for at least 12,000 years. In part, they've pulled it off by turning to the sea.

Just offshore, the Humboldt Current wells up with nutrient-rich water, fueling a rich, teeming coastal ecosystem that's still one of the world's most productive fisheries. Thanks to the long, slow tectonic collision between the Nazca Plate and the South American Plate, the region is also fraught with seismic hazard. But for millennia, people traded that sporadic, long-term risk for the riches of the ocean. They left behind archaeological evidence of their presence and their adaptations to life in this unique environment.

But in the wake of the earthquake and tsunami 3,800 years ago, people deserted the settlements of shell middens and stone houses that dotted the Atacama coast. The sea has always been vital to life in the Atacama, but it's clear that, for centuries, no one wanted to live too close.

Above the layers of sand and debris from the waves, mixed with toppled walls, there's little or no trace of human activity at sites like Zapatero. The only evidence speaks of very short visits: small hearths and a sparse scattering of artifacts lying atop flood debris and broken stone walls. When people had to return to the ruins of their ancestors, they clearly didn't want to stay long.

Archaeologists can see that wariness in the abandoned buildings and short-lived camps at places like Zapatero, but they can also read it in larger-scale changes that span the whole north Chilean coast. In one 100-kilometer stretch near Taltal, an area of northern Chile rich in archaeological sites, a survey revealed a 65 percent decrease in the number of settlements after around 3,800 years ago.

That date marks not only the estimated arrival of the tsunami, but the boundary between two archaeologically distinct cultures, Archaic IV (5,700 to 4,000 years ago) and Archaic V. After that boundary, settlements are scarcer, and both homes and cemeteries tend to be farther inland and on higher ground. Close to shore, what settlements there are get smaller, with fewer artifacts left buried and scattered.

Ancient mine gets the shaft

Even very important resources, like the iron oxide mine at San Ramón, were abandoned.

"Iron oxide was used as a pigment for several reasons, including the realization of pictures on stones that can be found in several sites along this region of the coastal Atacama Desert," University of Chile geologist Gabriel Easton, a co-author of the recent study, tells Ars. These pigments appear to have been important for local communities and were involved in their rites and ceremonies.

A 3 centimeter-wide vertical crack in the wall of the mine probably dates to the earthquake 3,800 years ago, and after that, work here seems to have stopped. "The San Ramón 15 archaeological site constitutes one of the most ancient [pieces of] evidence of mining activity in the Americas, exploited since 12,000 years ago, and abandoned after around 4,000 years ago, most possibly because of the effects caused by the earthquake in the region," Easton tells Ars.