The Mediterranean is a region where we know destructive earthquakes have occurred for several thousand years. Greece, Turkey, Italy, Egypt, Algeria, and even southern Spain, among other countries, have experienced large earthquakes with hundreds or thousands of victims at some point.
It is a complex territory from a tectonic point of view, due to the fact that different tectonic plates converge in it. Fundamentally, the Eurasian plate limits and collides with the African plate (today it is preferred to call the Nubia plate) at speeds that oscillate between 4 mm per year in the western sector – around the Alboran Sea – and 10 mm per year. in the eastern sector, in the eastern Mediterranean. It is a process that began about 50 million years ago.
These velocities, which may seem extremely small to us, are not at all so from a geological point of view. Moreover, they are the origin of the deformations that gradually occur in the outermost part of the Earth, the lithosphere, and that cause the accumulation of elastic energy that, when the time comes, is released by an earthquake.
When one approaches this region, one observes that this limit is complex. It presents areas where one of the plates enters under another, in what we call a subduction zone -in Italy, Greece and Cyprus- and others where it slides horizontally relative to another in what is called a fault of 4 tear. Examples of the latter case are the Dead Sea fault, which runs through the Sinai peninsula, Israel and Lebanon; the North Anatolian Fault, which runs through the whole of northern Turkey; and the East Anatolian Fault, which runs along the southeast of Turkey and possibly generated the February 6 earthquake. These two faults, which run through this country in its northern and southeastern parts, push Turkey towards the Mediterranean, towards Greece.
Earthquakes in the Mediterranean environment
Faults tend to accumulate elastic energy, just as a stick of wood does when we grasp its ends with both hands and bend it, in some cases for decades and in others for hundreds of years. When the time comes, due to the rupture that is generated, they are capable of causing large earthquakes that often lead to a significant number of victims and destruction.
The Ibero-Mogreb region, in which Spain is located, has suffered major earthquakes in historical times and also in the recent past.
We can cite the earthquakes in Oran (Algeria) in 1790, where some historical accounts speak of total destruction in what is now the historic part of the city, with nearly 3,000 deaths. In 2003, in a commuter town near the Algerian capital, a magnitude 6.8 earthquake caused more than 2,000 deaths and 10,000 injuries.
In Morocco, they have suffered destructive earthquakes such as those of Fez in 1624 and 1755, or the recent ones that occurred in the Al Hoceima region in 1994, 2004 and 2016, with magnitudes 6.0, 6.4 and 6.3, respectively.
In Spain we can cite the earthquake in Arenas del Rey, in Granada, which occurred on Christmas Day 1884, which caused between 1,000 and 1,200 victims. The most recent is that of Lorca (Murcia) in 2011. Although it only had magnitude 5.1, it left nine dead, more than 300 injured, and more than 1,100 homes had to be demolished.
Earthquakes kill, buildings too
The effects produced by an earthquake are mainly due to two factors: the characteristics of the earthquake and the quality and design of the buildings.
Regarding the first, the magnitude, duration, distance and depth of the earthquake fundamentally influence. Greater magnitude implies greater energy released or, what is the same, greater amplitude of the waves generated by the earthquake. The duration of the rupture of the fault implies a greater or lesser time acting on the buildings or infrastructures.
Distance and depth are also important. The closer and shallower an earthquake, the greater its effects. The same thing happens with an explosion: the further away it occurs, the less it will affect us.
But there is also a very important factor, in some cases the most, that does not depend on the earthquake: the quality and design of the buildings and infrastructures.
In seismic engineering, we work with the maxim that earthquakes are not the ones that kill people, but buildings. In areas that are seismically active, and where we know and expect that there will be major earthquakes in the future, it is imperative to design buildings to withstand the ground motions that will be generated by those earthquakes. Fundamentally, even if they fail to withstand the earthquake and experience severe damage, we must at least design them so that they do not collapse.
What we know about earthquakes
Today we still do not know how to predict when the next earthquake will occur and what magnitude it will have. Leading seismologists are of the opinion that this will never happen. What we do know in many cases is where they will occur.
We know which are the most important faults, which have normally generated serious earthquakes in the past, and in certain cases we also know something about their behaviour. It is what we could call a long-term prediction.
With this information, the researchers who work on seismic hazard can provide in each area the values of ground movement that we expect to occur in the future, thus improving the seismic-resistant construction standards in each country.
Urban planners must use this information to be truly effective, for example, when designing buildings, deciding on areas for expansion in a city, or when tightening building regulations for essential infrastructure.