4.8-magnitude earthquake rattles Lisbon
The incident has raised concerns regarding the country's readiness for a stronger earthquake, particularly in Lisbon, a region known for its seismic risk.
A 4.8-magnitude earthquake was felt in the Lisbon and Setúbal regions on Monday at 1:24 PM local time.
According to the Portuguese Institute for Sea and Atmosphere (IPMA), the tremor was widely felt but caused no injuries or structural damage.
The earthquake’s epicenter was located just off the coast, near Praia da Adiça, on the Costa de Caparica.
Experts note that its shallow depth of seven kilometers intensified the effects at the surface. The tremor was felt strongly in Sintra, Almada, and other parts of Greater Lisbon.
The tremor was also perceived, albeit with less intensity, in several municipalities from the central region of Portugal down to the Algarve, including Odemira, Coimbra, Albufeira, Portimão, Alcobaça, Leiria, Cascais, Lisbon, Loures, Mafra, Oeiras, and Vila Franca de Xira, and even into parts of Spain.
Three small-magnitude earthquakes, one of which was felt by the population, were also recorded within just over an hour on the morning of Tuesday, February 18, in the Seixal region.
Website Crash and Response
Following the quake, IPMA’s website experienced a surge in traffic, leading to a temporary crash.
Between 1:25 PM and 1:51 PM, the site faced intermittent outages, with full access restored after a restart at 1:51 PM.
The primary cause of the disruption was the overwhelming number of visitors filling out seismic questionnaires, which help refine intensity and magnitude assessments.
During the outage, IPMA received over 420,000 active connections, 8.5 times the average daily traffic.
In response, IPMA activated a simplified webpage to provide critical information and mitigate service disruption. This incident highlighted the need for infrastructure upgrades, and a €4.5 million investment plan is now in place to enhance Portugal's seismic monitoring and public alert systems.
The upgrades, expected by June 2026, include additional seismic sensors, tsunami alert systems, and reinforced cybersecurity measures.
What caused the earthquake?
Unlike major quakes along the Azores-Gibraltar fault, which marks the boundary between the Eurasian and African plates, this earthquake was an “intraplate” event within the Eurasian plate.
Historical data indicates that the last significant earthquake in this region occurred in 1858, with an estimated magnitude between six and seven.
The “complex geology and geological structures of the Greater Lisbon region,” including the ocean, formed millions of years ago under the influence of a volcano.
“In broad terms, it is an area with sedimentary rock formations from the Jurassic and Cretaceous periods (145 to 92 million years ago), over which the volcanic event that gave rise to the Sintra Magmatic Complex and the Lisbon Volcanic Complex occurred, these being around 90 million years old,” explained the IPMA in a statement.
“During the Cenozoic era, the continental sedimentary complex of the Paleogene (50 to 30 million years ago) and the essentially marine sediments of the Miocene (25 to 10 million years ago) were deposited. The sedimentary complex from the Pliocene and Quaternary periods (5 to 0 million years ago) is primarily continental, sandy-gravelly, with shallow marine, fluvial, and dune units."
It is within this geological context that, as a result of compressive tectonics, the Sintra and Arrábida mountain ranges were formed, as well as other smaller ones in the Lisbon region.
This tectonics affected the entire Iberian Peninsula, giving rise to mountain ranges such as the Pyrenees, the Central System, and the Baetic System.
Experts classify the earthquake as “moderate,” though its shallow depth made it more perceptible.
More recently, a 5.3-magnitude earthquake struck offshore in August 2024, but there is apparently no link between these events.
Preparedness
Portuguese President Marcelo Rebelo de Sousa, currently visiting Brazil, stated that initial reports from Prime Minister Luís Montenegro confirmed no significant consequences.
Lisbon Mayor Carlos Moedas reassured residents about the city’s preparedness, noting that 86 emergency gathering points are available in case of a larger quake.
He emphasized the importance of public awareness and the need for continued investments in seismic resilience.
Authorities are urging residents to stay informed through the city’s emergency alert system, Aviso LX, and to familiarize themselves with seismic safety protocols.
The recent website crash has underscored the urgency of upgrading public access to real-time seismic data, with planned improvements aimed at ensuring more reliable communication in future events.