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Posted on 9 December 2021
The new discovery indicates past tsunamis may have hit the Chilean coast more frequently than previously believed
Researchers from the universities of York and Northumbria identified a large earthquake in 1737 that may have caused a substantial tsunami.
The large earthquake, which occurred in south-central Chile, was previously absent from historical records, which are used to predict how often tsunamis are likely to occur in a region in the future.
Until now, it was believed that tsunami-causing earthquakes had occurred in this area of Chile three times since the 1570s.
The discovery of the 1737 tsunami means the average time between historical tsunami occurrences could be significantly reduced, to an average of 130 years.
The researchers worked to investigate sediments within tidal marshes at Chaihuín, near Valdivia, close to the area where the 1737 earthquake struck.
Their analysis of 130 sediment cores revealed evidence of widespread sandy layers, dating to the same time as the earthquake, that closely resemble deposits made by tsunami waves in other areas.
The researchers also found a mix of marine and freshwater algae species as well as evidence of land subsidence, meaning they were able to rule out storms, river flooding or a distantly generated tsunami as the cause of the sand deposits.
Their findings show that the 1737 earthquake ruptured mainly offshore, at fault depths much shallower than previously indicated in historical records.
The researchers, therefore, suggest that, when predicting the potential risk of future tsunamis, geological and historical records should be considered in tandem, since historical records alone may not provide a complete picture of their occurrence and characteristics.
Ed Garrett, leader of the Creating a Climate Resilient World Research Group in the Department of Environment and Geography at the University of York said: "Extensive fieldwork over several seasons allowed us to map out the tsunami deposit in great detail.
"We think the combination of this comprehensive field data with numerical models of the earthquake and tsunami makes this study really notable."
Lead researcher, Dr Emma Hocking, from Northumbria University’s Department of Geography and Environmental Sciences said: "Tsunami hazard assessment is often based on historic records of flooding along particular coastlines, with the frequency of past tsunami occurrence used to predict the potential future risk.
"However, such records are sometimes incomplete because reporting of tsunamis can be greatly affected by societal unrest or other crises. In this case, it is believed that the lack of chronicles of a tsunami could be attributed to uprisings that had driven settlers from most of the colonial outposts in the area.
"There are records of an earthquake in the area in 1737, but there is nothing in these records to indicate it generated a tsunami. However, we have found evidence to suggest that the earthquake did, in fact, cause a tsunami.
"The implication of this is that the average recurrence interval we previously believed has reduced, from approximately every 270 years to 130 years.
"We therefore suggest that the use of historic records alone may give miscalculations. Geological evidence is essential for verifying and supplementing historical records to obtain robust long-term patterns to inform seismic and tsunami hazard assessment."
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In the study, researchers investigated sediments within tidal marshes close to the area in Chile where the 1737 earthquake struck. The researchers now say geological records, and not just historical records, should be considered together when predicting the risks of future tsunamis.
The paper 'Geological evidence of an unreported historical Chilean tsunami reveals more frequent inundation' is now published in Communications Earth & Environment.
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