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Portuguese geologists explain 1755 earthquake

earthquakeScientists have put forward a hypothetical geological reason to explain Portugal’s two deadly earthquakes that occurred in 1755 and 1969.

Both earthquakes were unexpected as the Iberian peninsula is located far from the effects of shifting tectonic plates, the usual reason for earthquakes.

A group of geologists, led by João Duarte of the University of Lisbon, have raised the hypothesis that the Eurasian plate delaminated near the coast of the Iberian peninsula as a result of the action of oceanic water on the rock layers below the seabed.

Topographic models have indicated the existence of a seismic anomaly at a depth of 250 kilometres, says Duarte in an article he presented at the General Assembly of the European Union of Geosciences.

"We interpret this anomaly as a lithospheric drip caused by the delamination of the oceanic lithosphere. If this is the case, this is the first time the delamination of the oceanic lithosphere has been identified," the research report states.

This phenomenon, called the serpentinization process*, can be explained by the fact that rocky structures absorb oceanic water over time and, in this way, their properties are altered.

This caused the European plate to separate the upper layer of the lithosphere from the lower layer, the researchers said.

If this process is proven, it can lead to the creation of a new seismic zone, where one tectonic plate is being pushed under another. Under hypothetical circumstances, the plate eventually broke, creating strong earthquakes.

Geologists suggest that the emergence of this rift could explain the deadly magnitude of 8.7 from the 1755 earthquake that mainly devastated Lisbon, killing between 10,000 and 100,000 people. The 7.9 magnitude earthquake in 1969 was in the same region.

Both earthquakes remain a mystery because the Iberian peninsula is not situated near the limits of tectonic plates and, therefore, not near seismic zones.

 

* Serpentinization is a processes whereby rock (usually ultramafic) is changed, with the addition of water into the crystal structure of the minerals found within the rock.

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Comments  

0 #5 Sam Arcane 2019-06-05 13:39
Have a wander along the Vale Do Alamo - Just south of Rocha da Pena, then tell me there are no active plates :-?
+1 #4 Daphne 2019-05-08 20:25
Can we assume that GALP / ENI paid for this research as it is nonsensical to be claiming that the Eurasian and African tectonic plates do not mesh near the Portuguese coast! It is odd that Surfers Against Sewage and the anti-oil lobby have not stepped forward to make ADN comments - yet.
+1 #3 elspeth flood 2019-05-08 17:13
I don´t know a thing about geology earthquakes etc, but i know that in the 30 years I have lived here there have been two noticeable earthquakes. If the Algarve is so earthquake -free, why are houses, even swimming pools, built on earthquake rafts?
+4 #2 Peter Booker 2019-05-08 08:35
"…because the Iberian peninsula is not situated near the limits of tectonic plates…"

But the edge of the Eurasian plate passes along the south of the Algarve, and the epicentre of the Great Earthquake (often called the Lisbon Earthquake) of 1755 was in the Marquês de Pombal fault, not too far to the SW of Cape St Vincent.

What on earth does he mean by "near"? Is this a new scientific term?

You write, Ed, that both earthquakes were unexpected. I must have missed something, since it is my belief that current understanding of plate tectonics is in its infancy. And therefore all great earthquakes are unexpected. But, based on experience, we know enough to expect minor earthquakes in the Algarve region.
0 #1 Mutley 2019-05-07 21:23
Just what I always thought.

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