Seismic waves from the devastating earthquake in the Atlas Mountains also arrived in Italy on Friday night – “very slow movements, detected only by physical measuring stations,” explains Dr. Carlo Meletti, a seismologist at the National Institute of Geophysics and Volcanology (INGV).
What kind of event was this?
The earthquake was in all likelihood one of the strongest ever recorded in Morocco, because if we look at the historical data available, we can see that there have been very few events that reached such a high magnitude as on Friday. In addition, there was probably no preparedness for such an event in the region of the Atlas Mountains. The North African country has two areas of high seismic activity, with frequent earthquakes, but these are located in the Mediterranean part, in the north of the country, where there is very frequent seismic activity, like in Algeria and Tunisia. However, in this case, the earthquake hit a mountainous area, with peaks reaching several thousand meters, which starts in Algeria and cuts across Morocco, passing near the city of Marrakesh and continuing towards the Atlantic Ocean. It is a mountain range that was formed by the thrust of the African plate: the African continent is pushing northward into the Eurasian plate, and mountain ranges arise when one plate is colliding against another.
With this geological history, isn’t the Atlas region an earthquake zone?
Earthquakes are not very frequent in this area, although there was one case, that of the Agadir earthquake in 1960, which also took place along the Atlas Mountains but more toward the Atlantic Ocean, and which completely razed the city of Agadir to the ground, killing 15,000 people. The earthquake on the night of September 8-9 involved the same axis.
Is there any possible adaptation mechanism that could be used to manage the recurrence time for such an event?
Past data might allow us to make an estimate, but it is very scarce. One would have to have historical archives covering long periods. What is certain is that the Atlas region doesn’t have the same level of seismic risk as the country’s north. Although every earthquake is an instantaneous event, when we look at earthquakes, the timescales are very long: these are processes that have been going on for tens of thousands of years, unrelated to other external factors. There are always probabilistic estimates being made, which in the case of Morocco tell us that the north is at a higher risk. But if you compress a certain area, sometimes it ruptures on one side, sometimes on another, so an earthquake can happen along the Atlas Mountains, even though it will more often hit the Mediterranean coast.
Something that we do know, for Italy, where sociological studies have also been done, is that when an area is not affected by earthquakes for a period of time, it loses the memory of earthquakes after 50 years. This also happened in L’Aquila: in 2009, many people said they didn’t know they were living in an earthquake zone, and the last previous earthquake in a long series had taken place in 1950. It’s likely that in an area where the last earthquake, the Agadir earthquake, happened in 1960, there was also no awareness. In 2012, in Emilia, nobody expected such an event: the last earthquake had been 500 years before.
What might happen now?
Earthquakes can in turn trigger other nearby earthquakes, a few tens of kilometers away. In Emilia, for example, there was one on May 20 and another on May 29, 9 days later, about the same magnitude. Another adjacent part of the fault line ruptured. It is a domino effect, something we call “fault interaction.”
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