On Monday morning, August 26, many Portuguese were shaken in their beds. A moderate earthquake of magnitudemagnitude 5.4 was indeed recorded at 5:11 a.m. Widely felt in the country, this event did not however cause any damage or casualties. It does however remind the population that the region is subject to a high seismic hazard and that the threat of a powerful earthquakeearthquake still hovers.
L’epicenterepicenter is located off the coast of Lisbon, 84 kilometers to the south. These earthquakes of underwater origin always pose the risk of a tsunami, as during the disaster that hit the capital in 1755, following an earthquake with an estimated magnitude of between 7.7 and 9.0! There will be between 40,000 and 50,000 victims. But why the ground trembletremble-is there so much in this region?
A region at the interface between two tectonic plates
It should be remembered that Portugal is located in a complex area from a tectonic point of view, which is marked mainly by the boundary between the Eurasian plate and the Nubia plate (Africa). This boundary is materialized by the presence of a large faille transformingfaille transforming (Gloria fault), which cuts the crustcrust of the Atlantic Ocean between the Azores and the Moroccan coast. On either side of this fault the oceanic crustoceanic crust is not produced with the same vitessevitesse at the level of the ridge: it is faster in the north than in the south. It is this difference in speed in the movementmovement of the two plates which will sometimes cause earthquakes in this region. Add to this the complexity of the Gibraltar area, where the remains of an ancient ocean are sinking into the coatcoatand we obtain a very unstable region which explains the earthquakes frequently recorded in Portugal, Morocco and southern Spain.
