Almost 300 years ago, a tsunami hit the coast of Chile, but no one has found it yet

The south-central coast of Chile may be more susceptible to tsunamis than the historical record suggests.

Geological research in the tidal swamps of Chihuen revealed the fallout of a long and high wall of water that hit the ground in 1737. However, written documents from that time did not describe such a wave.

“There are records of an earthquake in the area in 1737, but there is nothing in these records to indicate that it caused a tsunami,” says Emma Hawking of Northumbria University in the UK.

This is a problem, because it suggests that our predictions about a future tsunami are based on miscalculation. Rather than repeating once every 280 years, earthquakes that have the potential to produce tsunamis may arrive as frequently as once every 130 years.

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

This ground-breaking event eventually caused a deadly tsunami that hit the Chilean coast at an altitude of about 25 metres, while also hitting the coasts of Japan, the Philippines, New Zealand and Hawaii.

Written records indicate that earthquakes near Valdivia were followed by tsunamis in 1837 and 1575, but for some reason, the 1737 earthquake lacked a corresponding wave.

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

But when the researchers analyzed the sediments and unicellular algae found in Chihwen, they found evidence of a tsunami inundating the land.

Aerial view of tidal swamps in Chihwen. (Northumbria University)

“By combining deformation with tsunami modeling, we show that our evidence of coastal subsidence and tsunami inundation in Chihwyn can be better explained by shallow sloping rather than deep underground sloping as suggested previously,” the authors wrote.

The earthquake that caused this tsunami was likely to have a depth of about 20 kilometers (12 miles) or less. After all, a shallow earthquake hitting the sea is more likely to cause a tsunami in its wake.

In Chihwen, the authors found three distinct sand layers, deposited by locally sourced tsunamis.

Layer A deposits coincided with the 1960 earthquake and tsunami, while the B and C sands represent tidal waves from the 1737 and 1575 earthquake, respectively.

Although other earthquakes occurred during the time that layer B was deposited, the 1737 earthquake was the closest to this salt marsh. Other geological research elsewhere along the coast has not revealed similar deposits, indicating that the tsunami from the 1737 earthquake struck a smaller area than the 1960 tsunami.

Therefore, the same fault lines seem capable of causing slightly different natural disasters over time – something we need to be very aware of in the future.

Hawking explains that “tsunami risk assessment often relies on historical records of floods along certain coasts, with past tsunami frequency being used to predict potential future risks.”

“However, such records are sometimes incomplete because tsunami reporting can be significantly affected by societal disturbances or other crises. In this case, it is believed that the lack of historical records of tsunamis can be attributed to the uprisings that drove settlers out of the Most of the colonial outposts in the region.”

As a result, researchers are calling for caution when it comes to using historical records to predict future earthquakes and tsunamis.

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

The new findings only come from one area of ​​the Chilean coast, about 20 kilometers south of Valdivia, so more research will be needed in other nearby areas to understand the scope and timing of the 1737 tsunami.

The study was published in the journal Communications Earth & Environment.

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