Around 1,800 BC, a devastating tsunami wiped out everything unfortunate enough to be on the coast of Chile. The devastation was so severe that it scared hunter-gatherers inland, where they stayed for a thousand years. Now, researchers have tracked the signs of the earthquake that caused the tsunami, and they believe it may have been on the scale of the largest earthquake in recorded history.

Forensic geology

The Earth may seem like a static place, but that’s only because we’re not looking at the right scale. The Earth’s crust is made of rigid plates (called tectonic plates) that move about, fueled by currents inside the mantle. These plates are moving around each other at a speed of a few centimeters per year — about the same speed your fingernails grow. That may not seem like much, but give it enough time (millions of years), and the changes can be striking.

The movement of these plates also causes earthquakes. Especially when the plates move towards each other and collide, the earthquakes can be particularly devastating. Chile is one of the areas most prone to this type of earthquake, and it’s no coincidence that the largest earthquake in recorded history (the Valdivia Earthquake) took place in Chile.

The Valdivia Earthquake had a whopping 9.5 magnitude, which at the time, was surprising for researchers.

“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,” explained Professor James Goff, Visiting Professor at the University of Southampton who co-authored the study. “But we have now found evidence of a rupture that’s about one thousand kilometres long just off the Atacama Desert coast and that is massive,” he continued.

But as it turns out, not only was such an earthquake possible — but another one just like it may have happened relatively recently in Chile’s geological history.

The main clues for this older earthquake came from tsunami evidence. When a massive earthquake displaces a large volume of water, it can cause a giant wave — a tsunami. Excavations from a 3,800 year-old site on the coastline of Chile showed that hunter-gatherer communities had been destroyed by giant waves. In addition, researchers also found tsunami traces in the Atacama desert.

“The Atacama Desert is one of the driest, most hostile environments in the world and finding evidence of tsunamis there has always been difficult,” explained Prof Goff. “However, we found evidence of marine sediments and a lot of beasties that would have been living quietly in the sea before being thrown inland. 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.”

By analyzing this data, and carrying out out tsunami modeling studies, the team concludes that the event was caused by a huge, 1,000-km-long megathrust rupture along the Nazca and South American tectonic plates, suggesting that the earthquake may have been of magnitude 9.5 and triggered a huge tsunami.

There weren’t too many people living in the area, the researchers explain. But those that were unfortunate enough to be caught by the tsunami would have been completely wiped out.

There’s an important lesson to learn here. We know that earthquakes of this caliber can happen in that area, and they may be happening more often than we’d like. Should such another event take place now, it would be a catastrophe for millions of people living near the Chilean coast.

“The local population there were left with nothing,” said Goff. “Our archaeological work found that a huge social upheaval followed as communities moved inland beyond the reach of tsunamis. It was over 1000 years before people returned to live at the coast again which is an amazing length of time given that they relied on the sea for food. It is likely that traditions handed down from generation to generation bolstered this resilient behavior, although we will never know for sure. This is the oldest example we have found in the Southern Hemisphere where an earthquake and tsunami had such a catastrophic impact on people’s lives, there is much to learn from this.”

The study was published in Science.