Tsunami warning signs

Tsunamis – are we at risk?

We receive a number of questions on this topic, so I hope this will answer some of these.

According to the Civil Protection National Risk Assessment, mainland Portugal presents a high risk of tsunamis, being considered a HILP (High Impact – Low Probability) phenomenon.

The National Emergency and Civil Protection Authority has, therefore, been mapping the seismic and tsunami risk and working with the municipalities, which also carry out studies and define local actions.

Portugal has been strengthening alert mechanisms and emergency plans for a tsunami scenario and has worked in conjunction at various levels – from national to local – to deal with the eventuality of this phenomenon.

What is a tsunami?

A tsunami is a series of waves of extremely long length and duration generated by sudden deformations of the ocean floor. The most common cause is an underwater, high-magnitude earthquake, although they can also be caused by landslides, volcanic eruptions and meteorite impacts. Although infrequent compared to other natural events, tsunamis have enormous destructive potential.

In deep ocean, a tsunami can go unnoticed due to the reduced slope of the seabed. However, as the tsunami approaches the shore, the waves’ energy is compressed over a much shorter distance, creating highly destructive waves. This is because the wave height, often only tens of centimetres, increases rapidly in shallow water.

The speed at which tsunami waves travel can be over 700 km/h, but as they enter shallow coastal waters, they slow down and reduce in length, causing an increase in wave height. Upon impact on the shore, waves can reach several metres in height.

When a tsunami wave inundates a coastal area, it creates strong onshore currents that exert potentially destructive forces in its path, sweeping away all kinds of objects and even people. Some of the most direct effects of tsunami include: loss of life/injuries; damage or destruction to buildings, vehicles, vessels and coastal infrastructure; retreat of the coastline; large amounts of debris washed ashore.

However, other effects with long-term consequences can also occur: contamination of water and soils; failure in the supply of drinking water due to contamination of surface water and aquifers (by salt water and other pollutants, some of them toxic); outbreaks of disease; interruption of economic activities; disruption of services.

Portugal’s experience

Of the tsunamis occurring on the Portuguese coast that include reliable events since 60 BC, 14 of them were generated by earthquakes. A group of five events can be considered regional or pan-Atlantic in scope, having generated enormous damage: namely in 60 BC, 382, 1755, 1761 and 1929.

The most destructive one to hit the coast was in 1755, originating from an earthquake with epicentre southwest of Cape St. Vincent, and which struck a large part of the Portuguese coast with great intensity. This tsunami blasted through some estuaries, such as the Tagus estuary, where there was critical damage (human and material).

In mainland Portugal, the regions most susceptible to tsunamis are distributed along the entire southern and western coast between Cape St. Vincent and Peniche. The estuarine and lagoon areas along these coastlines are also classified as areas of high susceptibility.

Among the most exposed zones, located in areas of high susceptibility to tsunamis, are the main urban settlements of the Algarve and Alentejo coasts and the Sado and Tagus estuaries, as well as the ports and marinas located south of Peniche.

Identification and anticipation

Unlike many other events, a tsunami can be anticipated. We may only have a few minutes to act, but if we all know what to do, many lives can be saved. In most situations, it is possible to forecast a tsunami on the basis of information on the origin of the earthquakes that cause them. Each situation is evaluated, taking into account seismic information and sea level data, which are obtained through seismic and sea level monitoring networks and tide gauges along the coast.

The enormous devastation caused by the Indian Ocean tsunami on December 26, 2004, alerted global authorities, and in particular UNESCO’s Intergovernmental Oceanographic Commission (IOC), to the need to implement a global tsunami warning system, as well as to raise the awareness and preparedness of populations in risk zones. Portugal is part of the Tsunami Early Warning and Mitigation System in the North-eastern Atlantic, Mediterranean and Related Seas (NEAMTWS).

Alert and communication

Among the various means that can be used to disseminate tsunami warnings, sirens are one of the most widely used at international level, as well as the possibility of voice messages for the purpose of informing the population at risk about the self-protection measures to be adopted.

In planning, emphasis should also be placed on the importance of communication during the emergency, so it is essential to have teams/people to provide up-to-date information on the situation and particularly whether the population should remain at the meeting point and until when, or when it is safe to return, thereby avoiding situations of panic and precipitous dispersal.

All of the area potentially threatened by tsunamis should be marked, making the existing danger known to all those who find themselves there, together with evacuation routes and assembly points.

For the Algarve region – one of the main risk areas in our country – a Special Emergency Plan for Earthquake and Tsunami Risk was drawn up, which brings together more than 100 entities. These contingency plans are regularly tested in regional and national exercises.


One of the ways of recognising an impending tsunami is if the sea suddenly recedes outwards from the coastline. This is the time to react, not remain and take photos!

The success of any alert and warning system in saving lives and reducing material damage depends on the rapid dissemination of information to the potentially affected population and on how prepared they are to respond to the emergency. It is important to encourage people to react and recognise that the main warning of a possible arrival of a tsunami is the earthquake itself.

In this context, it is already a practice in some municipalities to consider the evacuation drill as a testing exercise within the Municipal Civil Protection Emergency Plan.

Sourced from ANEPC

By David Thomas
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David Thomas is a former Assistant Commissioner of the Hong Kong Police, consultant to INTERPOL and the United Nations Office on Drugs and Crime.
In 2011, he founded Safe Communities Algarve to help the authorities and the community prevent crime. It is now registered as Associação SCP Safe Communities Portugal, the first national association of its type in Portugal.
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