Italy is, seismologically speaking, one of the most active countries in Europe. The fourteen volcanoes, of which four are still active, and many earthquakes do proof that. The 6.2 magnitude earthquake of the 24th of August, still lingers in everybody’s head. But why is it that the earth shakes this often in Italy? And if it happens that much, couldn’t we have known an earthquake would reoccur?
In order to understand why there are so many earthquakes in Italy, we have to take a closer look at Italy’s geological situation. There are three main causes for the tensions prevailing in the area. First of all, the country’s geographical position is right on top of the border between the Nubia plate (Africa) and the Eurasian plate. These are tectonic plates: giant slabs of rock, making up the earth’s upper crust or lithosphere, which can move on a viscous layer, called the asthenosphere. At the boarder of these two tectonic plates, a process called subduction has been taking place for approximately 50 million years. In other words, the African plate slowly dives underneath the Eurasian plate in the asthenosphere. This process is responsible for the building of the alpine mountain belt, the formation of the Apennines (i.e. the accretionary wedge) and the volcanism in the country. To complicate things even further, there is a second cause which needs to be taken into account: the subduction of another plate, the Adria micro-plate underneath Eurasia and the Apennines. Finally, we also have to consider the opening of the Tyrrhenian basin to the West, which initiates stress on the area. The expression of all these different tectonic styles, present at the same time in a broad region, leads to the formation of some complex fault systems in the Apennines (figure 2).
In order to understand why earthquakes do occur, we first have to clarify what an earthquake is. An earthquake is the shaking of the surface of the earth, resulting from the sudden release of energy in the earth’s crust. At a fault two blocks of rock move along each other. However, the movement is not constant, because the surfaces of these rocks are not perfectly smooth. If there is a bump on one of the surfaces, the movement can stop, but the tension will keep building up. At a certain moment, the tension reaches a maximum, the bump breaks, the block can slip through and the energy that triggers the earthquake is released (figure 3). This is what happened in Italy at the 24th of August at 3:36 am.
Could it have been predicted?
Italy is known for its many earthquakes. The previous severe earthquake happened 2009 in l’Aquila, only 50 km away from the epicentre of the earthquake at the 24th of August. These two events can be linked to each other, because they were triggered in the same fault system. You can split up one fault system into different segments. Each segment is activated separately, and under normal conditions they get activated chronologically. Segment 1 releases its energy and initiates stress on segment 2. When segment 2 releases its energy, it will initiate stress on segment 3 and so on. Therefore, it is very likely that the segment which was activated during the 2009 earthquake, put stress on the segment activated in August 2016. This tension can be measured. Hence, in a way, scientists already knew that an earthquake was about to happen. However, it is impossible to exactly calculate when and where the earthquake will occur. And that is what people want to know. So yes, we knew an earthquake would happen. But no, we could not pinpoint the exact date and place.
Text: Flore Van Maldeghem