Three earthquakes with magnitudes between 3.3 and 3.9 on the Richter scale shook Lisbon and Setúbal within six minutes of each other, and no damage to any persons or property was reported.
According to Observador, the Portuguese Sea and Atmosphere Institute (IPMA) has not yet determined from which faults the earthquakes originated.
The three earthquakes, with magnitudes between 3.3 and 3.9 on the Richter scale, were recorded off Sines in six minutes this morning by the IPMA.
The first was recorded at six o’clock, 52 minutes and one second, 55 kilometers to the southwest of Sines and originated 17 kilometers deep.
The second, of magnitude 3.3 on the Richter scale, happened at six o’clock, 58 minutes and 11 seconds in the same epicenter but 15 kilometers from the surface.
A second later, another earthquake was recorded 65 kilometers from Sines, which originated 31 kilometers deep.
The quakes were felt in various areas by the population – the IPMA says they were mainly noticed in Odemira and Odeceixe – and the first occurrences began to be shared in social networks.
At the Euro-Mediterranean Seismological Center, a person in Lisbon (111 kilometers north of the epicenter) says he heard “a noise and then a slight shaking that lasted five to 10 seconds.”
In Odemira, 80 kilometers from the origin of the earthquake, another person felt “a short shake like a train”. In Sintra, 122 kilometers from the epicenter, a witness saw “the windows shaking” and compares the sound of the phenomenon to the passing of a train.
And in Cascais, 104 kilometers from the source of the earthquake, another person said that their bed and the bedroom door “moved slightly.”
Despite the reports, all the shakes were evaluated with intensity between levels III and IV on the Mercalli scale because “they were felt by some people inside dwellings” and “the objects hung inside them waved.”
However, none of them “caused personal or material damages.”
In seismic terms, Portugal is conditioned by the interaction between three tectonic plates, parts that make up the Earth’s lithosphere and that move between them: the Eurasian, the North American and the African.
The Eurasian plate and the North American plate are moving away from each other in a movement of divergence that is responsible for the intense seismic activity recorded in the Azores archipelago. On account of this, just before these three earthquakes occurred, a shock of magnitude 2.2 on the Richter scale was recorded at the Hirondelle Flock at six o’clock, 36 minutes and 54 seconds from five kilometers deep.
But earthquakes in mainland Portugal have to do with the clash between a specific part of the Eurasian plate and the African one.
Earthquakes originating from Infante Dom Henrique Hill, the region where the three earthquakes happened, happened because the country is in a very particular region of the Eurasian plate: it is the Iberian microplate, which moves eastwards and welds to the Asian plate.
This microplate, however, is influenced by the African plate, which is moving northwest. As the African plate moves, it compresses the Iberian microplate and creates so-called intraplate failures.
“This is the engine of earthquakes of this nature in Portugal,” geologist Fernando Carrilho told Observador. However, the IPMA has not yet been able to determine which tectonic fault in specific is that the quakes are related.
“They are not very big earthquakes and that makes this discovery difficult. In addition they were recorded to some depth. Most people only realized them because they heard sounds or realized that the objects were shaking,” added Carrilho.
Asked if the occurrence of three earthquakes so close to each other, both in geological and temporal proximity, is normal, Carrilho explained to Observador that the strongest earthquake was the first of all. The other two were replicas, that is, secondary tremors that happen because the rocks that make up the earth’s crust around the fault that originated the main earthquake is being readjusted to those movements.
Earthquakes are sudden releases of energy that occur when materials in the Earth’s crust yield to the pressures to which they are subjected in depth. Sometimes this energy is released gradually, causing several earthquakes in a short period of time and associated with the same geological event.