Recent Earthquakes Shake Seixal: What You Need to Know

Two Earthquakes Recorded in Seixal

This Tuesday morning, February 18, the region of Seixal experienced two earthquakes, measuring 2.4 and 1.8 on the Richter scale. These earthquakes mark the second and third tremors in less than 24 hours in mainland Portugal, following a 4.7 magnitude earthquake that occurred off the coast of the Setúbal Peninsula on Monday afternoon.

The 2.4 magnitude earthquake struck at 10:36 AM, located 12 kilometers southwest of Seixal. It was felt most strongly in the municipality of Sesimbra, in the Setúbal district, but tremors were also reported in Lisbon, Almada, and Palmela, according to the Portuguese Institute of the Sea and Atmosphere (IPMA). Fortunately, there have been no reports of casualties or property damage. A less intense aftershock occurred about an hour later at 11:34 AM.

The Euro-Mediterranean Seismological Center initially calculated the magnitude of the first earthquake at 2.6, with its origin (the hypocenter) found at a depth of 11 kilometers on land, near the Setúbal Peninsula. The earthquake on Monday was more shallow, occurring at a depth of 7 kilometers in the ocean, but all three earthquakes in the past 24 hours have originated in the same region.

Understanding the Earthquake Magnitudes

The variation in magnitude calculations can be attributed to the complex physical and mathematical processes involved in determining an earthquake's epicenter, which relies on data sets, algorithms, and seismic wave propagation models, as explained by the IPMA.

The 4.7 Magnitude Earthquake

According to João C. Duarte, a researcher and professor at the European Geosciences Union, the Monday earthquake occurred along a tectonic fault parallel to the Arrábida Fault, which forms the region's topography. The 4.7 magnitude earthquake was caused by a reverse faulting movement, where one rock block moves upward relative to another, specifically in an East-West direction.

Duarte noted that this fault system is linked to the Lower Tagus Valley fault system, which poses significant risks, as it has previously generated earthquakes of magnitude 6 or higher, such as the 1531 Lisbon earthquake and the 1909 Benavente earthquake. While there is no immediate cause for alarm, he emphasized the importance of being aware that we live in a seismic zone that has a history of generating significant earthquakes.

The Seismicity of Portugal

The seismic activity in Portugal is influenced by the interaction of three tectonic plates. The Eurasian Plate and the North American Plate diverge, allowing magma to rise through the Mid-Atlantic Ridge. However, the Eurasian Plate and the African Plate are converging, leading to faults in the Iberian Peninsula. The accumulated energy from this convergence is released once the elastic deformation limit is reached, resulting in ruptures.

Portugal and Spain are particularly vulnerable to these movements due to their position on the Iberian microplate, a region of the Eurasian Plate that gradually moves eastward while being compressed by the African Plate. This compression increases the likelihood of new tectonic faults forming within the plate itself.