“In that aspect, it is better to have a house in Albaicín than in Zaidín”
While Tenerife pay attention to the small earthquakes that shake its volcanic territory, another Spanish province that is also becoming an expert in seismic swarms has had its own succession of tremors. And this Saturday, July 26, an earthquake of magnitude 3.6 occurred in Granada that was preceded and followed by a few other much smaller earthquakes. Logically, just as happened in 2021, fear arises among Granadans due to the possibility of a larger earthquake. But could there really be large earthquakes in Granada? To answer this question, I have spoken with Ana Crespo Blancprofessor of geology at the University of Granada and expert in earthquakes. As he tells us, with science in hand, the answer depends on the flaws we look at. With those currently active in this seismic swarm, the most abundant in the province, a high intensity earthquake cannot occur. With some other, responsible for large earthquakes in the past, it could occur, but it is much less likely. In addition, nuances must be taken into account that make the risk currently even lower. Let’s talk about failures To talk about earthquakes, we must be clear about what faults are. And no, I’m not talking about the Valencia festivities. The faults are fractures in the earth’s crust throughout which there has been a displacement measurable of rock blocks. Due to tectonic forces, on the sides of these fractures Elastic tension accumulates until a limit is reached, in which the fault slides and energy is released in the form of seismic waves. This is important, because the larger the fault, the more energy will be accumulated and released. Shooting an object with a small slingshot is not the same as throwing an object with a large catapult. How do faults influence the earthquakes in Granada? The failures that caused the seismic swarm of 2021, as well as those that have been causing the most earthquakes in Granada these days, are too small to cause a large earthquake. “All the active faults we have are relatively short, on the order of less than a kilometer, so not much energy can be accumulated,” explains Crespo Blanc, clarifying that he refers to this being the answer based on the science we know today. As a result, not enough energy can be built up to generate a large earthquake. Now, in the history of Granada, two large earthquakes are known, which were very intense and destructive. One took place in 1884, in King’s Sandsand the other occurred in Albolote in 1956. We cannot know the exact magnitude of the first one, since there were no measurement systems like the current ones, but it is estimated that it had a magnitude of more or less 6.2 to 6.5. The second reached magnitude 5. We are talking, now, about quite large earthquakes. What happened then? A larger fault and poorly constructed buildings Normally, in seismic swarms, such as those that gave rise to the 2021 Grenada earthquakes, or even those that have been detected recently, the energy accumulated in the faults is released little by little. That results in tremors that are not too large. However, it could happen that it is all released at once. In that case, the magnitude increases. This is what happened in Albolote. However, today a magnitude 5 earthquake should not be extremely destructive. It is true that it is much more intense than those that normally take place, but we must add that in 1956, the buildings in Granada were not prepared for earthquakes. “In that year, the constructions were generally of poor quality,” recalls the geology professor. “Then the earthquake triggered landslides, there were people who lived in caves and there were also deaths in relation to those landslides, not so much with the house falling on top of the person.” In fact, Crespo Blanc points out that “that earthquake was the trigger for the anti-seismic standard which has been applied in Granada since the 70s.” With this rule, applies to the entire Spanish territory with risk of earthquakesa series of limitations and obligations are taken into account when constructing buildings, according to the different levels of risk. For example, if the seismic acceleration is 0.08 g or 0.12 g, the maximum height of a wall structure will be 4 stories, each of them no higher than 20 times the thickness of the wall. Thanks to that, earthquakes today cause much less damage in Spain than in the past. Although in the past there were some of very considerable magnitude, such as Arenas del Rey, it did take place on a much longer fault. It is something much more exceptional, which is not abundant in Granada. The ones that are normally active and cause seismic swarms are the small ones, which cannot cause earthquakes of great magnitude. Could one like the one from the 19th century be produced? Maybe one day, if all the energy from that larger fault is released at once, but it’s much less likely. The third factor: the composition of the terrain We have already seen that the most abundant faults in Granada are too small to generate large earthquakes. There is no need to fear a large earthquake related to the most recent swarms. We have also remembered that the buildings in Granada are built to withstand these types of events. However, there is a third factor that we must also take into account: the composition of the terrain. In this sense, Crespo Blanc explains that “if the soil is soft, for example sandy, an amplification of the waves occurs, which does not occur if the soil is rocky.” For this reason, he points out that, in that aspect, “it is better to have a house in the Albaicin than in Zaidín.” In any case, if you have a house in Zaidín you should not fear this swarm of earthquakes either. The flaws are too small so that the earthquake becomes extreme, no matter how soft the ground is. Image | … Read more
