Today we interview Ferran Estrada i Llàcer, member of the ICM Continental Margins group. Throughout his career, the researcher has delved into different aspects related to tectonics, basin formation and turbidie systems. He has also been involved in palaeoclimatic projects and in others about convulsive events, such as tsunamis and mega-floods.
1. Is the Iberian Peninsula a high-risk area for earthquakes?
After the Ring of Fire bordering the Pacific Ocean, the Mediterranean is one of the most active seismic zones on the planet due to the northward displacement of the African plate, which produces a compression on the southern edge of the Eurasian plate and significant seismic activity in places such as Turkey, Greece, Italy, the Alboran Sea and a large part of the Maghreb, in the African plate.
In Spain, the most active seismological zone is located in the southwest of the Iberian Peninsula, on the Mediterranean coast, where the system formed by the Alborán-Sea-Betic-Rif, known as the Gibraltar Arc, is located. Likewise, other areas such as the Catalan coast or the Pyrenees are seismologically quite active, although, in general, with much lower magnitudes.
Also, Andalusian cities such as Granada, Malaga or Almeria, located in the Betic mountain range, are frequently affected by earthquakes, although this activity also extends to areas such as Murcia or around Melilla and Alhucema, on the Moroccan coast. To get an idea of the scale of seismic activity in this area, this year, from April to September, 2355 earthquakes have been recorded.
2. Why is an earthquake more likely to occur here?
The Alboran Sea is formed by a microplate trapped between the Eurasian and African plates, which moves at a rate of 4.93 millimetres per year. In the central area of the Alboran Sea, according to our studies, a process of tectonic indentation, i.e. the collision of two blocks of continental crust, is currently taking place. This movement is similar to what happened in the Himalayas, but at an initial stage and on a smaller scale.
For this reason, and because of the complex geodynamics of the area around the Alboran Sea, the area between Morocco and the Andalusian coast is the most active. However, seismic activity is not only confined to the sea, everything is connected (land and sea) in a large system of deformation and seismic activity known as the "Trans Alboran Shear Zone".
3. How do earthquakes lead to tsunamis?
In general, tsunamis are caused by the sudden vertical displacement of the seabed due to a fault associated with an earthquake, although it can also be caused by sediment landslides, volcanic activity (lava flows entering the sea) or by the impact of a meteorite in the sea.
In the case of faults, this vertical movement, in turn, displaces the water mass above the fault blocks and, which generates a wave that moves towards the coast. As the sea surface becomes shallower near the coast, the wave grows in height, resulting in the typical tsunami waves we are all familiar with.
4. Has it happened before in Spain with similar consequences to the tsunamis in, for example, Southeast Asia?
In Spain we have several historical examples, the most famous of which was the tsunami that devastated the coasts of Huelva and Cadiz in 1755, with waves of up to 20 metres. The impact of this tsunami was devastating, although the damage in the case of Southeast Asia, due to the intensity of the earthquakes, the size of the faults and a significant concentration of population, was greater.
In addition, we have historical records of tsunamis on the Andalusian coast and evidence of tsunamites (sediments deposited by the action of a tsunami) in the Almeria area. Based on the still scarce information available from this area of the Mediterranean, we believe that the magnitude of the tsunamis is not comparable to that of the 1755 earthquake or those in Southeast Asia.
5. Which coastal cities are most affected and what are the potential wave heights?
In the case of the Averroes Fault, which is located in the Alboran Sea, due to its orientation and location, the coastal villages most affected would be those located between Motril (Granada) and El Campo de Dalías (Almería). Specifically, the most affected town would be Balerma, where we have calculated that the waves could reach a height of 6 metres. Other less affected areas, with waves ranging between 1 and 2 metres, would be Malaga and the Moroccan coast to the west of Melilla.
6. How can a citizen know when a tsunami is going to occur?
In general, there are early warning systems managed by the National Geographic Institute (IGN), with sensors that give advance warning to the Directorate General for Civil Protection and Emergencies of the arrival of a tsunami. However, these systems are only effective when there is sufficient time between the earthquake and the arrival of the wave. In the case of the Averroes fault, for example, this time span is too short for the early warning systems to be able to warn the population sufficiently in advance.
One possible indicator of an approaching tsunami wave is the retreat of the sea, which exposes areas that are normally underwater. However, this is not always the case, as sometimes the arrival of the wave occurs directly.
This depends on the orientation of the tsunamigenic fault and the displacement of the blocks: if the fault block sinks, this results in a retreat of the sea surface, whereas, if the fault block rises, this results in a rise of the sea surface and a wave at the coast. In our case, both happen at the same time.
7. How much time do we have to run away?
In the case of the Averroes fault, very little time, although it depends on the area of the coast where we are. In general, the further away we are from the source of the earthquake and the associated fault, the longer it will take for the tsunami to arrive. According to our tsunami model, the first wave would reach the town of Balerma in 21 minutes, which is the time between the formation of the earthquake and the impact on the coast. This is very few minutes, so the reaction time will be very short, and we will practically be aware of the tsunami when we see the first signs on the coast.
8. What projects is your group working on to improve these predictions?
We are currently working on the projects AGORA, about the contrast of geological activity and associated risks between the eastern and western sector of the Alboran Sea and the adjacent mountain ranges; and PAPEL, about the tectonic activity and risks associated with the Palomares and Guadiana Menor faults. These are interdisciplinary and multidisciplinary projects that aim to study other structures with seismic potential, submarine landslide and tsunami risk in order to improve prediction methods.