Archaeological and geological evidence point to 3,800-year-old magnitude-9.5 earthquake and tsunami on the Chilean coast.
By Rebecca Owen, Science Writer (@beccapox)
Citation: Owen, R., 2022, Ancient Chile quake upended coastal Atacama communities, Temblor, http://doi.org/10.32858/temblor.256
Sumatra, 2004. Chile, 2010. Japan, 2011. These earthquakes are some of the largest and most destructive in modern memory. Now, a team of interdisciplinary scientists have pieced together evidence for an even larger earthquake that struck off the coast of Chile approximately 3,800 years ago. The recent study bridges a gap between the archaeological record of human settlements within Chile’s Atacama Desert and the geological evidence of a magnitude 9.5 earthquake that disrupted life there for thousands of years. The event challenges the record for most powerful earthquake ever recorded and also provides an opportunity to address how researchers imagine and prepare for future seismic disasters.
Large earthquakes occur infrequently because the stress required for a big rupture builds slowly over geologic time. “If two plates are [pushing towards one another] at six centimeters a year, and the fault between them slips 10 meters in an earthquake, that process can happen at most every 150 years on average,” says geologist Judith Hubbard from Nangyang Technological University in Singapore. “The result is that the largest earthquakes are experienced least frequently. This is good! It also means that we’re less likely to know about these events, because they may only occur every 500, 1000 or even 5000 years.”
Chile’s Atacama Desert is a 600-mile patch of arid moonscape, bordered by a coastal mountain range to the east and the Pacific Ocean to the west. Communities contending with inhospitable weather and unpredictable terrain have inhabited the region for nearly 12,000 years.
The ancient residents of the Atacama coastline had to be resilient to live in such an extreme setting. Life was made even more difficult when a monstrous earthquake and tsunami struck around 3,800 years ago. Records of the event, etched in the marred landscape, slowly eroded over time. For the last seven years, researchers sifted through archeological sites and layers of earth to uncover evidence of this extreme seismic upheaval.
In the Atacama, the researchers discovered the sandy remnants of fossilized marine life far inland and well above sea level — evidence of past coastal uplift. These deposits coincide with damage to several nearby archaeological sites, including a vertical crack in an ancient iron oxide mine and erosion evident on a shell midden. Radiocarbon dating from unearthed shells and charcoal suggested this incident occurred approximately 3,800 years ago.
During a tsunami, the sea would have risen 20-40 meters over these ancient communities, toppling stone structures and washing algae and sea creatures ashore as water flowed in and then receded.
Across the Pacific Ocean, in New Zealand, boulders were pushed inland by an orphan tsunami from the same era. Now, that tsunami is linked to this same ancient megaquake that devastated the Atacama, demonstrating the global impact of a prehistoric, supersized earthquake.
“Uplifted coastlines, impacts on past civilizations, archaeological sites and tsunamis on the other side of the Pacific: these are all natural and expected outcomes of a mega-earthquake on this stretch of the South American subduction zone,” says Hubbard, who was not involved in the study. “The authors demonstrate they all can match to the same earthquake scenario.”
The researchers found evidence that communities along the Atacama coastline abandoned their structures after the earthquake and tsunami. “One of the fascinating things about this research is not only that the archeological sites document the occurrence of this event, but they document the social effects of this event too,” says Diego Salazar, an archeologist at the University of Chile and lead author of the study. Many residents would have died. Many fled for higher, more predictable ground, and the coastline was depopulated for several thousand years. As a result, this disaster disappeared from oral history, long forgotten by the Atacama’s eventual inhabitants.
Connecting the past to the future
The subduction of the Nazca tectonic plate beneath the South American plate, 100 miles (160 kilometers) off the Peruvian and Chilean coasts, creates a volatile subduction zone where large quakes are possible. At least 15 magnitude 8.0 or larger events have struck this boundary in the last 120 years, sending tsunami waves roiling through coastal communities. The largest quake ever recorded on Earth — the magnitude 9.5 Valdivia earthquake — ruptured a 620-mile-long (1,000 kilometer) section.
Researchers simulated a tsunami modeling to determine the extent waves would have inundated the coast during the ancient Atacama disaster. They identified four simulations that most closely matched their field observations to suggest a similarly powerful rupture unfolded 3,800 years ago — highlighting how strong earthquakes have plagued this boundary for much longer than modern records can attest.
Tsunami modeling for a Mw 9.5 mega-earthquake affecting northern Chile. More information in our new paper: "Did a 3800-year-old Mw ~9.5 earthquake trigger major social disruption in the Atacama Desert?"https://t.co/g5M95qTA1h pic.twitter.com/dxJKg2XFja
— José González-Alfaro (@Josgonal) April 6, 2022
Co-author Gabriel Easton, a geologist at the University of Chile, explains that it is dangerous to underestimate or limit the potential power of earthquakes in seismically active regions based on relatively recent history only. “This occurred before in northern Chile and certainly will occur in the future, so why won’t it occur in other subduction margins, too?” asks Easton.
Though residents and officials today are aware of the seismic hazards this region presents, their awareness is incomplete. Salazar, Easton, and their colleagues say they want to return the ancient disaster to the collective understanding of what it means to be resilient and prepared. “No one remembers this event, obviously,” says Salazar. “We want to put it in our social memory so that it helps articulate our responses to future events.”
Salazar, D., et al. (2022). Did a 3800-year-old M w~ 9.5 earthquake trigger major social disruption in the Atacama Desert?. Science advances, 8(14).