EARTHQUAKES IN THE ALGARVE

The Algarve is one of the regions in mainland Portugal with the highest seismic risk, along with Lisbon and the Tagus Valley. This vulnerability is due to the region's proximity to the boundary between the African and Eurasian tectonic plates, located approximately 200 km southwest of Cape St. Vincent.

Seismic activity in the Algarve is not a recent phenomenon. Throughout history, the region has been affected by numerous earthquakes, some of great magnitude, which have left deep marks on the landscape and collective memory of the population. The most devastating of all was the 1755 earthquake, which remains one of the largest earthquakes ever recorded worldwide.

Geological Framework

Tectonic Plates

Mainland Portugal is located on the Eurasian Plate, very close to the boundary with the African Plate. This boundary is complex and presents distinct tectonic characteristics, being subdivided into three main segments:

Gorringe Bank: shows a compression movement (thrust) of the Eurasian Plate over the African Plate

GLORIA Fault: where there is a relative displacement velocity between the two plates of about 3.39 cm/year

Terceira Rift: shows a separation velocity between the plates of approximately 0.76 cm/year

Active Faults in the Algarve

Southern Portugal is particularly exposed not only to earthquakes generated at the plate boundary but also to the presence of several active faults that cross the Algarve region:

Portimão Fault / Quarteira Fault

Faro-Loulé Fault (where the 1722 earthquake occurred) / Horseshoe Abyssal Plain (trace over 100 km long)

The Great 1755 Earthquake

Earthquake Characteristics

The earthquake of November 1, 1755, is considered one of the largest earthquakes in world history. With an estimated magnitude between 8.5 and 9 on the Richter scale, this catastrophic event not only destroyed Lisbon but also caused deep devastation throughout the Algarve.

Impact on the Algarve

The Algarve suffered massive destruction. The city of Lagos, then capital of the region and seat of the Portuguese Discoveries, was almost completely destroyed. In Vila do Bispo, only one house remained standing. The city of Silves lost its Cathedral, Tower, Castle, Walls, and countless homes.

The tsunami that followed the earthquake hit the entire Algarve coast with waves reaching 15 meters in height near Cape St. Vincent. Almost all coastal towns and villages were severely damaged, except for Faro, which was partially protected by the sandy banks of the Ria Formosa.

Most Affected Locations

Lagos: Tsunami waves reached the top of the city walls

Silves: Recorded 14 fatalities, with destruction of historic monuments

Portimão and Albufeira: Widespread destruction in coastal areas

Armação de Pêra: Severely affected by the tsunami

Seismic Risk and Prevention

Regional Vulnerability

According to studies by the Portuguese Insurance Association (APS), the Algarve is among the areas with the highest seismic risk in the country. This vulnerability is aggravated by several factors:

Proximity to active faults and the tectonic plate boundary

High population concentration in coastal areas

Old and earthquake-vulnerable housing stock

Low uptake of earthquake insurance (only 19% of homes)

Schools and public infrastructure in need of structural reinforcement

Prevention Measures

The National Authority for Emergency and Civil Protection (ANEPC) has developed several awareness and prevention initiatives:

"A Terra Treme" Exercise: Held annually to teach the 3 life-saving actions: DROP, COVER, and HOLD ON

Special Emergency Plan: The Algarve has a Special Civil Protection Emergency Plan for Seismic and Tsunami Risk

Warning System: Pilot project for population warning sirens at Praia da Rocha

Evacuation Drills: Regular tsunami evacuation tests

What to Do in Case of an Earthquake

During the earthquake:

DROP to the ground to avoid falling

Take COVER under a sturdy table or against an interior wall

HOLD ON until the shaking stops, protecting your head and neck

After the earthquake:

If on the coast, move immediately to higher ground (above 10 meters)

Stay away from damaged buildings

Do not use elevators

Follow instructions from authorities

The Algarve is a region of high seismic risk, where the possibility of a major earthquake is real. Experts warn that an event similar to 1755 could happen at any time, with an estimated return period of about 1,800 years.

Individual and collective preparedness is essential. Public awareness, reinforcement of vulnerable infrastructure (especially schools and hospitals), and uptake of earthquake insurance are essential measures to minimize the impact of a potential major earthquake in the region.

The Portuguese Institute for Sea and Atmosphere (IPMA) continuously monitors seismic activity, providing updated information on its website. The public is encouraged to report felt earthquakes through available questionnaires, thus contributing to a better understanding of seismic risk in the region.

Sources and References

Portuguese Institute for Sea and Atmosphere (IPMA) - www.ipma.pt

National Authority for Emergency and Civil Protection (ANEPC)

Portuguese Society of Earthquake Engineering (SPES)

Dom Luiz Institute (IDL) - University of Lisbon

National Laboratory of Civil Engineering (LNEC)

Jornal do Algarve, Sul Informação, Postal do Algarve