Previously unrecorded Chilean tsunami identified

Date:
December 9, 2021
Source:
Northumbria University
Summary:
A large earthquake off the coast of south-central Chile in 1737 may
have caused a substantial tsunami that was absent from historical
records.

Historical records are used to predict how often tsunamis are likely
to occur in a region in the future. Until now, it was previously
believed that tsunami-causing earthquakes had occurred in this area
of Chile three times since the 1570s, including after the magnitude
9.5 earthquake of 1960. However, this discovery of an unrecorded
tsunami means that tsunamis may have struck the Chilean coast
more frequently than previously believed. This means the average
time between historical tsunami occurrences could be significantly
reduced, to an average of 130 years.



FULL STORY ==========================================================================
A large earthquake off the coast of south-central Chile in 1737 may have
caused a substantial tsunami that was absent from historical records,
according to new research published in the Nature journal, Communications
Earth & Environment today (Thursday 9 December).


========================================================================== Historical records are used to predict how often tsunamis are likely to
occur in a region in the future. Until now, it was previously believed
that tsunami- causing earthquakes had occurred in this area of Chile
three times since the 1570s, including after the magnitude 9.5 earthquake
of 1960.

However, this discovery of an unrecorded tsunami means that tsunamis may
have struck the Chilean coast more frequently than previously believed.

This means the average time between historical tsunami occurrences could
be significantly reduced, to an average of 130 years.

Researchers from the universities of Northumbria and York in the UK
worked to investigate sediments within tidal marshes at Chaihui'n,
near Valdivia, close to the area where the 1737 earthquake struck.

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 mixture of marine and freshwater algae
species and 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 geological and historical records
should be considered in tandem when predicting future tsunami risk, adding
that historical records alone may not provide a complete documentation
of their occurrence and characteristics.

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 tsunamis have occurred more often than
we previously believed, and 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."
Ed Garrett, leader of the Creating a Climate Resilient World Research
Group in the Department of Environment and Geography at the University
of York, added: "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." ========================================================================== Story Source: Materials provided by Northumbria_University. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Emma P. Hocking, Ed Garrett, Diego Aedo, Mati'as Carvajal, Daniel
Melnick. Geological evidence of an unreported historical Chilean
tsunami reveals more frequent inundation. Communications Earth &
Environment, 2021; 2 (1) DOI: 10.1038/s43247-021-00319-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211209124241.htm

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