A gigantic tsunami-unleashing earthquake that struck northern Chile 3,800 years ago wreaked such devastation on coastal populations, it took 1,000 years for humans to return to the shore, scientists say.
The ancient super-quake would have had a magnitude of around 9.5, and was so powerful it generated a tsunami that hurled boulders hundreds of meters inland in New Zealand, which is thousands of miles – and an entire ocean – away.
The discovery is evidenced by uplifted land structures (aka littoral deposits) and samples of marine rocks, shells, and sea life washed far ashore by tsunami waves into the higher stretches of Chile's Atacama Desert. It serves as a grim warning of the destructive potential of major tsunamigenic earthquakes that may have previously escaped our notice.
"We found evidence of marine sediments and a lot of beasties that would have been living quietly in the sea before being thrown inland," says geologist and tsunami specialist James Goff from the University of New South Wales, Australia.
"And we found all these very high up and a long way inland so it could not have been a storm that put them there."
Tsunami deposits visible in a trench. (University of Southampton)
The research team, led by anthropologist Diego Salazar from the University of Chile, conducted several years of research in the Atacama Desert region, which is particularly vulnerable to megathrust earthquakes due to its proximity to the convergence of the Nazca and South American tectonic plates, with the former being subducted under the latter.
This phenomenon and its seismic backlash is what led to the most powerful earthquake on record, the 1960 Valdivia earthquake in southern Chile; thousands of years prior, it seems the same tectonic tensions led to an equally diabolical yet undocumented precursor in the north of the country.
"It had been thought that there could not be an event of that size in the north of the country simply because you could not get a long enough rupture," says Goff.
"But we have now found evidence of a rupture that's about one thousand kilometers long just off the Atacama Desert coast, and that is massive."
In their investigations, the researchers used radiocarbon dating to get a sense of the age of the littoral deposits, which stretch over some 600 kilometers (about 370 miles) of Chile's coastline.
Readings from several of the deposit sites suggest the existence of a "tectonic event that would have uplifted littoral deposits all along the study region, generated a paleotsunami, and triggered social disruption at a regional scale," the researchers write in their paper.
Collapsed stone structure. (Gabriel Easton)
At the time of the event, the people living in this part of the world were hunter-gatherer communities. Archaeological evidence suggests the tsunami wave generated by the quake toppled their stone structures – and not just once, but twice, with a strong current of tsunami backwash wreaking havoc as it flowed back out to the sea.
The effects on any people lucky enough to have survived the immediate disaster were long-lasting, with evidence suggesting the area remained uninhabited by human populations for as long as 1,000 years, despite people living on this stretch of coastline for nearly 10 millennia before the crisis.
"The local population there were left with nothing," says Goff. "Our archaeological work found that a huge social upheaval followed as communities moved inland beyond the reach of tsunamis."
With time and the passing of dozens of generations, the local people's boldness (or perhaps forgetfulness) grew, and people eventually made their way back to the ocean about 1,000 years later.
"The abandonment of previously occupied areas and changes in the mobility patterns and spatial arrangements of settlements and cemeteries were probably resilience strategies developed by hunter-gatherer societies," the researchers write.
"However, knowledge of these giant events and their consequences seems to wane over the passage of time."
Aside from filling the gaps in our historical understanding of this gigantic event – an earthquake about as powerful as anything known to humanity – the research is a cautionary note about the risks similarly powerful megathrust quakes might pose in the future, the researchers say.
"While this had a major impact on people in Chile, the South Pacific islands were uninhabited when they took a pummeling from the tsunami 3,800 years ago," Goff says.
"But they are all well-populated now, and many are popular tourist destinations, so when such an event occurs next time the consequences could be catastrophic unless we learn from these findings."
The findings are reported in Science Advances.