Friday, July 1

It took 300 years for scientists to “hunt down” this tsunami and now they are re-calculating their predictions: the value of a good historical record.

“Whoever forgets his story is condemned to repeat it,” he said. the poet and philosopher Jorge Ruiz de Santayana. Although surely when thinking about it I had in mind the great revolutions and wars that hit half the world between the 18th and 20th centuries, the maxim, which underlines how much we play in keeping the past in mind, is applicable to all fields. It happens in politics, it happens in the most everyday day to day and it happens, equally, in science.

In fact, seismologists have just verified it in a rather curious way in Chile: overnight they have identified a tsunami that shook the country’s coast in 1737 and of which they had no prior evidence, which has forced them to redo their prediction calculations. Beyond its local impact, the finding is a barbaric example of how hard the experts are risking to dispose of some good and complete historical records and check them with external sources.

A secret hidden among the sediments

The case was published a few days ago the Nature magazine. Scientists already knew that in 1737 Chile had suffered an earthquake, but until now it was believed that its impact had not gone beyond, unlike, for example, another of 9.5 magnitude registered in 1960 and that unleashed a terrible tsunami that hit the coasts of Chile, Japan, the Philippines, New Zealand and Hawaii.

Something similar – earthquakes followed by gigantic waves – would have occurred, according to written records, in the surroundings of Valdivia in 1575 and 1837. Not so during the earthquake recorded in the 18th century (1737). The reason? The researchers believed that the tremors on that occasion had been generated by a deep rupture between tectonic plates below the earth, not under the sea.

In light of the new findings, they were dead wrong. To the analyze sediments and single-celled algae located in Chaihuín, a coastal town located at the mouth of the river that bears the same name, unicellular algae were found that reveal that the area was flooded by a tsunami in the 18th century. In total they identified three layers: the first and most recent coincides with the 1960 earthquake and the oldest is associated with the 1575 earthquake; but in the middle they ran into another one that researchers directly relate to the 1737 earthquake.

Although there were other earthworks that hit the area during the period in which this intermediate layer was formed, the one in 1737 was the closest to the marsh in which the samples were taken. During the analysis, other options, such as storms or floods, were also ruled out. Given that no similar deposits have been found elsewhere on the coast, the researchers believe, however, that its impact was less devastating than that of the 1960 tsunami.

Beyond the historical interest of the discovery or its usefulness in writing a new chapter in Chilean history books, what does it imply that seismologists have unmasked the eighteenth century tsunami? Scientists are clear: data is key to refining predictions. “The tsunami hazard assessment is based on historical flood records along particular coastlines and the frequency of past tsunamis is used to predict potential future risk,” notes Emma Hocking, University of Northumbria, in Science Alert.

New data, new calculations

With the new information on the table, scientists have indeed been forced to revise their estimates. Until now the calculations took into account events such as those of 1575, 1837 and 1960; not the one from 1737. Conclusion: its frequency may be higher than they believed and be reduced, on average, to 130 years. “The implication of this is that tsunamis occurred more frequently than previously thought,” says Hocking. statements collected by EFE.

“With the addition of the 1737 tsunami along with the previously known events of 1960, 1837, and 1575, the historical interval of recurrence of tsunamis generated anywhere in the Valdivia segment of the Chilean subduction zone it is reduced to 130 years. This is valid even if the inferred flooding from the tsunami is not associated with the 1737 earthquake, but with another earthquake of similar age that is not listed in the historical catalog ”, details the study published in Nature, in which it is recalled that the interval that had been initially proposed for breaks such as 1960 with the information available until now was around 270 and 280 years.

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Hocking stresses the importance of managing good chronicles – the acquaintances spoke of an earthquake in the 18th century, but did not mention any tsunami – but also the need to complete them with other data, such as those obtained with field work. “Geological evidence is essential to verify and supplement historical records in order to obtain robust long-term patterns that inform seismic and tsunami assessments ”, the expert abounds.

The disaster that killed a hundred thousand people and created modern seismology

What went wrong in the archives of the 18th? “Tsunami risk assessment is often based on historical flood records along certain shores, and the frequency of past tsunamis is used to predict potential risk in the future. However, these records are sometimes incomplete because tsunami reports can be badly 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 revolts that had expelled the colonists from most of the colonial positions”, Ditch. There remains, yes, the lesson for future studies on tsunamis.

Images: Yisris (Flickr)

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