Evidence of an earthquake off the coast of Chile as powerful as any recorded has been found by archaeologists. The quake generated an enormous tsunami, devastating for people living on the coastal plain. For a thousand years after the megaquake, inhabitants of the area moved their dwellings and burial sites to higher ground – despite the inconvenience when the sea was their main food source.
The most energetic earthquake since the invention of seismometers took place in 1960 in southern Chile, registering 9.5 on the moment magnitude (Mw) scale. Even thousands of kilometers away in Hawaii, 10 meter (35 foot) waves wreaked havoc on shorelines facing the wrong way. Estimates of deaths in Chile range as high as 6,000, despite an epicenter beneath a relatively unpopulated area.
Chile has experienced many other large earthquakes before and since as a result of the South American Plate riding over the Nazca Plate, the same process forcing up the Andes. So it is unsurprising that Professor James Goff of the University of Southampton and co-authors claim in Science Advances to have found deposits left behind by tsunamis further north.
The evidence is seen in the form of marine sediments and fossils far above the current sea level, let alone that in earlier times. Some have likely dates coinciding with smaller recorded tsunamis from Japan.
Although such past events were to be expected, the height and length of one dated around 3,800 years ago was not. For a thousand kilometers, sediments are deposited so high above what was then the shoreline that the quake responsible must have measured around 9.5 (Mw).
“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,” Goff said in a statement.
If the size of this quake, stretching for around a thousand kilometers, surprised the researchers, the response of Atacama’s Indigenous people was more amazing still.
The Atacama is one of the least habitable places for humans on Earth, with parts thought to have experienced no rain for up to a million years. However, the waters that border it are among the most abundant on Earth thanks to nutrients brought up by the Humbolt current. For at least 12,000 years, humans have survived there, with a diet rich marine foods.
For a thousand years of that time, however, behavior changed. Stone structures and graveyards disappeared from the Atacama’s coast. Instead, equivalent traces of human presence were found 40 kilometers (25 miles) inland. Yet the diet of the people of the area barely varied, suggesting they preferred to make long treks to access the sea’s bounty, before returning inland to avoid its wrath.
“The local population there were left with nothing,” Goff said. “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.”
There is evidence Indigenous Australians have preserved tales of natural disasters for many thousands of years, so the fact the tsunami survived in the collective memory of the Atacameño for a thousand is not a total shock. More extraordinary is that the warnings were so powerful people avoided living near their primary food source.
This was no mere local disaster. Goff was alerted to the possibility of an event this large when studying enormous boulders on New Zealand’s Chatham Island placed by a tsunami. Judging from their locations, Goff concluded the waves that dragged them there must have originated in northern Chile.
What has happened once can happen again, and Goff warns the tsunami would have devastated many Pacific islands that were uninhabited at the time, but are now heavily populated and unprepared for a repetition.