Almost 300 years ago a tsunami hit the Chilean coast, but no one has found it so far

Chile’s south-central coast may be more vulnerable to tsunamis than historical records suggest.

Geologic research among the Chaihuín middle marshes has now revealed fallout from a long, high wall of water that struck the earth in 1737. Written documents from the time, however, do not describe such a wave.

“There are records of an earthquake in the area in 1737, but nothing in these records indicates that it generated a tsunami,” says Emma Hocking of the University of Northumbria in the UK.

This is a problem, as it suggests that our predictions of future tsunamis are based on a miscalculation. Instead of recurring once every 280 years, earthquakes that have the potential to produce tsunamis can happen as frequently as once every 130 years.

The discovery is based on layers of sediment found in an intertidal swamp near Valdivia, a historic town on the southern coast of Chile that was struck by a massive 9.5 magnitude earthquake in 1960.

This groundbreaking event ultimately triggered a deadly tsunami that struck the Chilean coast from a height of around 25 meters, while also hitting the coasts of Japan, the Philippines, New Zealand and Hawaii.

Written records suggest that the earthquakes near Valdivia were followed by tsunamis in 1837 and 1575, but for some reason the 1737 earthquake did not have a corresponding wave.

In the past, researchers have explained this missing tsunami by suggesting that the 1737 earthquake was caused by a deep rupture between two tectonic plates underground, as opposed to under the sea.

But when researchers analyzed the sediment and single-celled algae found in Chaihuín, they found evidence of tsunami-caused flooding on land.

Aerial view of the Chaihuín swamp. (University of Northumbria)

“By combining deformation and tsunami modeling, we show that our evidence of coastal subsidence and tsunami flooding at Chaihuín is best explained by a shallow mega-thrust slide offshore rather than a deeper slide. underground as previously suggested, ”write the authors.

The most likely depth of the earthquake that caused this tsunami would be around 20 kilometers (12 miles) or less. After all, a shallower earthquake that hits the open sea is more likely to produce a tsunami in its wake.

In Chaihuín, the authors found three distinct layers of sand, deposited by tsunamis of local origin.

Layer A deposits coincide with the 1960 earthquake and tsunami, while B and C sands represent tsunamis derived from the earthquakes of 1737 and 1575, respectively.

Although there were other earthquakes during the deposition of layer B, the earthquake of 1737 was the closest to this salt marsh. Other geological research elsewhere along the coast did not find similar deposits, suggesting that the 1737 earthquake tsunami hit an area smaller than the 1960 tsunami.

The same fault lines therefore appear capable of producing slightly different natural disasters over time – something we need to be very aware of in the future.

“Tsunami risk assessment is often based on historical records of flooding along particular coasts, with the frequency of past tsunamis being used to predict potential future risk,” Hocking explains.

“However, these records are sometimes incomplete because tsunami reports can be greatly affected by social unrest or other crises. In this case, it is believed that the lack of chronicles of a tsunami could be attributed to the uprisings. who had driven the settlers from most of the colonial outposts in the region.

As a result, researchers urge caution when using historical data to predict future earthquakes and tsunamis.

To give us a better idea of ​​what might happen in the future and when, we need to compare historical records to direct geological evidence.

The new finds are only coming from an area on the Chilean coast, about 20 kilometers south of Valdivia, so more research will be needed in other nearby areas to truly understand the scope and timing of the 1737 tsunami.

The study was published in Earth and environment communications.

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