University of Southampton: Scientists discover ancient earthquake, as powerful as the biggest ever recorded

A new study has discovered that an ancient super-earthquake took place in Northern Chile, on the same scale as the largest recorded quake in history. The earthquake, 3800 years ago, had a magnitude of around 9.5 and the resulting tsunami struck countries as far away as New Zealand where boulders the size of cars were carried almost a kilometre inland by the waves.

Earthquakes happen when two tectonic plates rub together and rupture – the longer the rupture, the bigger the earthquake. Previously, the largest known event in the world happened in 1960 in Southern Chile.

“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.

The study was led by Professor Diego Salazar at the University of Chile and has been published in Science Advances.

The research team found that the enormous rupture of the plates caused the coastline of northern Chile to lift up and a massive tsunami was also generated. “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.”

3800 years ago the coastline of the Atacama Desert was home to hunter-gatherer communities. Excavations of archaeological sites found stone buildings which had been destroyed by the waves, lying beneath the tsunami’s deposits. Some of walls had toppled towards the sea, most likely from the strong currents of the tsunami’s backwash.

“The local population there were left with nothing,” said Professor 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 behaviour, 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.”

Before this study began, Prof Goff had been investigating a site in New Zealand on Chatham Island which included a vast number of boulders, some the size of cars, that had been thrown hundreds of metres inland. These boulders date back to around the same time period as the earthquake in Northern Chile.

By pure coincidence, a week later, he was asked to join Prof Salazar’s team to study sites in northern Chile which have now provided the answer.

“In New Zealand we said that those boulders could only have been moved by a tsunami from northern Chile and it would need to be something like a 9.5 magnitude earthquake to generate it. And now we have found it,” he said.

The researchers believe that their findings provide a very important deeper time context for understanding earthquake and tsunami hazards in the Pacific region, and how severe the effects will be next time such a super-earthquake happens.

“While this had a major impact on people in Chile, the South Pacific islands were uninhabited when they took a pummelling from the tsunami 3800 years ago. 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.” Prof Goff concluded.

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