Prehistoric Mega-quake Identified Off Chilean Coast 3,800 Years Ago

The quake and ensuing tsunami destroyed prehistoric communities at the edge of the Atacama desert – and they wouldn’t come back for 1,000 years

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Vegetation in Pan de Azúcar National Park on the coast of the Atacama Desert.
Vegetation in Pan de Azúcar National Park on the coast of the Atacama Desert.Credit: Aaron Bornstein - Flickr

Prehistoric hunter-gatherers who had been surviving on the edge of Chile’s hyper-arid Atacama Desert for 12,000 years may have been overcome by a particularly intense offshore earthquake followed by a tsunami almost 4,000 years ago, an international team of researchers reported on Wednesday.

The survivors, it seems, then moved to higher ground and didn’t return to their coastal settlements for about a thousand years, the geologists and archaeologists suggest.

The paper is based on analyses of finds at five prehistoric settlements on a narrow stretch of land between the Atacama and the Pacific Ocean. Their source of water was springs, and the people subsisted mainly on marine life, from mollusks to large marine mammals.

Theoretically, they would have known about earthquakes and tsunamis. Chile is situated on the Pacific “ring of fire,” which is characterized by relatively intense volcanism and seismicity. About 3,800 years ago, the team reconstructs, there was a huge seismic event: a rupture over a vast 1,000 kilometers or so along the subduction contact zone of the Nazca and South American tectonic plates. Along that zone, a section of oceanic Nazca tectonic plate slid beneath the South American plate offshore, causing a tsunami that rose 25 to 35 meters (82 to 115 feet) in height when it hit the coast.

Paleo-quakes and tsunamis in prehistory have been detected in other areas in separate work, for instance off the coast of Israel. But this discovery was a first for this region, says the team, which is led by Diego Salazar and Gabriel Easton from the University of Chile.

The Atacama coast, in Chile.Credit: Nasa

How bad was it? Possibly as strong as the worst one in the historic record. That, too, was in Chile and struck in 1960, measuring about 9.5 on the Richter scale (estimates range from 9.4 to 9.6). For comparison, the megathrust earthquake that caused the deadly Indonesian tsunami in 2004 is estimated to have been in the range of 9.1 to 9.3 on the Richter scale.

The approximate date of the newly reported cataclysm was based on organic material found in the tsunami’s deposits in the coastal sites, which had been located on the steeply slanted shoreline of the Taltal region in northern Chile. The structures there show hallmarks of severe shaking: cracked hearths, toppled stone structures and fractured foundations. The team also identified tsunami deposits: sand and shells where they shouldn’t be.

On top of the layer of destruction and debris, the researchers found a sterile layer, indicating that the site had been abandoned for about a thousand years, after which some coastal habitation resumed, they estimate.

It is impossible to know how many people died in the catastrophe. Plausibly, the quake and tsunami devastated the coastal population, who may or may not have learned from their own oral history that sudden movements by the ground may be succeeded by giant waves – but this may have been an unusually big event.

It seems the survivors formed small, highly mobile groups that moved to higher ground in the foothills, where the archaeologists found smaller occupation sites, less densely occupied than the original five settlements. The archaeologists did not find any cemeteries or permanent structures in these smaller, more dispersed settlements, which were built about 30 to 35 meters (about 100 to 115 feet) above sea level – just a few meters beyond the level of the tsunami wave, they estimate.

Atacama Desert in Chile, between Antofagasta and Taltal.Credit: Valerio Pillar

Throughout this region, human occupation generally seems to have diminished after the quake and tsunami, being reduced to a smaller number of less populated sites. The team suggests these slightly higher sites were founded and inhabited by tsunami survivors, among other reasons, because the material culture of shell fish hooks, harpoons and net weights was virtually indistinguishable from that found in the former coastal sites.

Then about 1,000 years later, settlements arose anew on the low coastal ground, on the edge of the Atacama. These were plausibly established by the descendants of the original occupants, Salazar says.

“It’s the same people, there’s no doubt it’s the same people, the same knowledge on how to use the environment, the same technology, but a different settlement system,” he explains. For example, a unique blade style associated with the Taltal region’s hunter-gatherers was found in these coastal sites before and after the tsunami struck, Salazar says. They are thought in the archaeological community to be a ceremonial weapon.

One question that remains is why the people did not return to the coast sooner. Salazar suspects the answer may lie in a change in their subsistence strategy during the interim period, which took them farther afield. Or perhaps they retained an orally conveyed memory of the tsunami, which faded over time. It is impossible to know for sure.

Also, during that period, there would likely have been more major temblors: Chile is one of the most seismologically active regions in the world, and is estimated to experience major quakes every few centuries, as well as the occasional megaquake.

It is likely, Salazar adds, that the people didn’t think of the trembling earth as a natural phenomenon but as an act of supernatural powers. Like other prehistoric peoples, they may have employed shamans to negotiate with, or at least appeal to, these powers, he speculates. But we cannot know; they left no records. And, as he points out, the language of these communities is lost, their mythology is lost and their lore is lost.

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