Two “twin earthquakes”: something rare happened in Portugal

Two “twin” earthquakes, both with magnitude 4.1 on the Richter scale, and, subsequently, two aftershocks were recorded in the early afternoon of this Thursday, with the epicenter in Alenquer, in the district of Lisbon, reported the IPMA.

O first The shock, measuring 4.1 on the Richter scale (which measures the energy released), occurred at 12h14 locations, with an epicenter at a depth of 14 kilometers, and four kilometers west-northwest of the village of Alenquer, the seat of the municipality.

One second earthquake, with the same magnitude on the Richter scale, occurred two minutes later, at 12h16also in Alenquer, 12 kilometers deep, indicated the IPMA, highlighting that “many people may not have even realized that there were two different episodes”.

The institute later registered two replicas in the same municipality, at 3:15 p.m. and at 15h162.2 and 2.4, respectively, on the Richter scale.

The earthquakes did not cause personal or material damage and had a maximum intensity of V on the Modified Mercalli scale, felt in Alenquer.

Intensity V means that the earthquake is considered to be moderate to strongimplying that it was felt inside the house, with hanging objects swinging and vibrations in the windows and doors.

The tremors were felt with less intensity in the municipalities of Montemor-o-Novo (district of Évora), Alcobaça, Batalha, Bombarral, Caldas da Rainha, Leiria, Óbidos and Peniche (Leiria), Arruda dos Vinhos, Azambuja, Cadaval, Cascais, Lisbon, Loures, Lourinhã, Mafra, Oeiras, Sintra, Sobral de Monte Agraço, Torres Vedras, Vila Franca of Xira, Amadora and Odivelas (Lisbon), Portalegre, Benavente, Cartaxo, Constância, Rio Maior, Salvaterra de Magos and Tomar (Santarém), Alcochete, Almada, Barreiro, Moita, Palmela, Seixal, Sesimbra and Setúbal (in the district of Setúbal).

In a statement, the National Emergency and Civil Protection Authority (ANEPC) also revealed that, as of 12:30 p.m., it had no record of personal or material damage.

“We appeal to the population to take extra care in areas previously affected by mass movements and structural instability due to recent adverse weather phenomena”, highlighted ANEPC.

The greatest “of the last nine years”

Os two earthquakes were the biggest “in the last nine years”reported the Portuguese Institute of the Sea and Atmosphere.

“With origins close to the Alenquer area, these appear to be the largest earthquakes – in terms of magnitude – in the last nine years”, reads a statement from IPMA.

After the first hour post-earthquake, they had already been received at IPMA, through the macroseismic questionnaire onlinemore than 800 testimonies.

“Twin earthquakes” are very rare

The earthquakes surprised researchers because of almost perfect similarity. This is what the geophysicist explained to Luis Matiasfrom the University of Lisbon.

“What is curious is the fact that there are two earthquakes and, looking at the records, it is as if they were twins. There are two identical earthquakes, probably generated in the same structure, same fault“, he stated.

Although preliminary calculations point to distant epicenters, the researcher admits that they will have occurred very close to each other and associated, he highlighted, with the same fracture zone.

Luís Matias told the morning newspaper that the scientific interest of the episode lies in the rarity of the phenomenon, explaining that “twin” earthquakes like these, with this magnitude are uncommon in Portugal.

“We are not all prepared”

In an interview with , the geologist Filipe Rosasdirector of the Dom Luiz Institute at the University of Lisbon, warned that the risk does not just depend on the magnitude of a simo: “What kills is not earthquakes, it is human constructions.”

The expert thinks that “that we are not at all prepared” for these phenomena, remembering that the seismic risk is particularly sensitive in this region as it is an area with “a very high population density” crossed by active faults.