Scientists are close to solving the mystery of an earthquake that struck Lisbon, Portugal, in 1755. The magnitude 8.7 temblor on All Saints’ Day killed tens of thousands of people. Scientists were only able to figure out how destructive it was years later. Surveys showed that the earthquake flattened 85 per cent of the buildings in the region. Using this and other data, researchers determined the scale of the disaster and concluded that it was a significant one. However, what perplexed them the most was that Europe’s most destructive earthquake should have never happened. There were no known subduction zones in the region, the place where one tectonic plate sits under another. When they shift, the currents trigger earthquakes and are also responsible for volcanic eruptions. This confused scientists, who were unable to understand how such a massive quake happened. Now, a study published this week in Nature Geoscience seems to have an answer. The findings suggest that a new “Ring of Fire” might be forming in the Atlantic.
The study showed that a process known as delamination is occurring in the Earth’s mantle beneath offshore Portugal. The hot, dense layer is peeling off, something which was believed to be impossible in old, strong oceanic crust, the researchers wrote. University of Lisbon geologist and lead author João Duarte said, “Delamination has never been identified in a pristine oceanic basin.” The researchers wrote that this process has the ability to trigger enough stress, resulting in earthquakes.
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Ring of Fire triggers 450,000 earthquakes
The more important finding is that they believe a new subduction zone might be forming under the Atlantic. They believe the process can lead to a chain similar to the Ring of Fire forming in the region millions of years from now. Notably, the Ring of Fire in the Pacific triggers 450,000 earthquakes every year, besides causing volcanoes to erupt. It runs from Russia, Japan, the western coast of the US and moves south towards Antarctica as well.
1969 earthquake in Portugal showed what could be going on
Atlantic is tectonically quiet, while Portugal’s ocean basin has no deep ocean trenches and volcanic arcs. But then in 1969, another earthquake of magnitude 7.9 struck the region. Scientists started researching what had caused it and found a blob plunging 250 kilometres into the mantle. They also noted that quakes originated 20 kilometres below the surface. This was strange since most earthquake epicentres lie within 10 kilometres of the surface. This meant that something was going deep inside Earth. Duarte and the team built simulations, and their “best fit” model showed the mantle delaminating from the crust and dropping into Earth. The researchers believe that this process likely reveals how subduction begins, and the 1755 earthquake was the first “fingerprint”.
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