Portuguese geologists explain 1755 earthquake

Scientists 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.