Volcanological surveillance is in full swing. The eruption of a decade ago in El Hierro and the most recent, in The Palm, have shown the need to know much more in depth what moves under the Archipelago. There are many mysteries that still sleep under the Islands, but there is one that does not even have an explanation yet: the seismicity between Gran Canaria and Tenerife.
Every year, in the area there are between 200 and 400 earthquakes of different magnitudes between both islands. “Between Tenerife and Gran Canaria there is a perennial seismicity that we have registered since we have instrumentation and that, in the past, generated some felt earthquakes”, explains Itahiza Domínguez, seismologist at IGN and one of the main researchers of this study. One of the examples that shows the strength of these earthquakes is in the seismic movement of 1989, which was felt and woke up many people in Santa Cruz de Tenerife and also in some parts of Gran Canaria.
It happened on May 9 at 2:30 a.m.. The earthquake had a magnitude of 5.3 and, although it did not cause casualties or collapse of buildings, it did break windows, it moved the furniture of the houses, and caused powerful underground noises, producing generalized panic in the population. What is not yet known “is what exactly is there.”
As the Canary Islands are in the center of the African tectonic plate, for decades it was ruled out that the movement could be caused by the collision of the edges of a plate. The theory has been, since then, that what exists is a volcano; the so-called middle volcano.
But the truth is that researchers are not so sure about it. And it is that, although it is true that in the Canary Islands «almost all the important seismicity occurs before or during volcanic eruptions; It is not like that in that area ”, as Domínguez explains. It is not because the activity between the capital islands occurs every year and without an apparent volcanic activity.
Hence, the National Geographic Institute (IGN), together with the Catalan delegation of the CSIC in the Institute of Marine Sciences, have begun a research project to discover if what sleeps under the islands is the famous middle volcano or seismicity it is caused by some other type of earth movement.
The theories are varied. As there is no tectonics in the area, it is valued that “the very existence of the islands may be causing the movement of the plate there to be different,” says Domínguez. One of the possibilities is that a fault has formed in the area that contributes to generating this seismicity. This was suggested two years ago by a group of geologists from the University of Burgos and the Complutense University of Madrid, after a review of the magnetic data collected by the IGN.
At that time, the researchers explained that they had found evidence of the existence, in the submerged part of the northwest of Gran Canaria, of a large rocky body with inverse polarity to the current magnetic orientation of the Earth and a size that represents practically one fifth of the entire building on the island. According to this study, this building was created based on magmas that emerged in the growth phase of Gran Canaria under the influence of a great fault, and at a time in the Earth’s past in which the magnetic poles were inverted with respect to the present. (hence its reverse polarization).
Seismicity could also be caused “by the own weight” of the buildings that make up the capital islands, as well as their evolution over time. “The pressure changes generated because Gran Canaria is in the process of erosion and Tenerife in formation could also be behind this seismicity”, remarks the IGN seismologist. So, in short, “there are many hypotheses” that have tried to explain what is influencing the occurrence of these earthquakes of such magnitude, but the reality is that “you do not know what is down there.”
An expansion of the network
The next four years – the duration of the project – will finally be able to solve this mystery. The researchers are going to expand, like never before, the network of seismographs between the two islands. They will do so after having obtained 300,000 euros of financing with a project in a public call of the Ministry of Science that promulgates high-quality science. They will install them by land, in the ocean and will also make use of the optical fiber that connects both islands to monitor it. In total, between the two islands, 47 new seismic monitoring devices will be installed, of which 20 will be placed on land (4 in Tenerife and 16 in Gran Canaria), 18 submarines between the two islands (of which 13 will be installed for a limited time). ) and another 9, also submarines, but in the surroundings of both islands.
With all this instrumentation, which will collect data over two years, scientists will be able to “map the area and carry out a structural study of these islands,” Domínguez explained. With these new facilities, the Gran Canaria network will expand like never before. And so far, the island only has three stations; a situation that differs greatly from that of Tenerife, which has more than twenty seismic stations (due to the exhaustive surveillance derived from the activity of the Teide building).
“To locate earthquakes in this specific area, a wide network of stations on both islands is necessary,” says the IGN seismologist. As he remarks, due to volcanology issues, Tenerife has always had a larger network. “The volcanic risk in Gran Canaria is much lower, although not zero,” warns the researcher, who points out that with this expansion of the network, not only will the seismicity between the two islands be characterized, but the information collected will also help to understand the island structure. In addition, as he insists, it is necessary to expand the network on the ground because it is the only one that can get data in real time. And it is that, although the network of sensors that will be installed on the seabed will house data of great interest, they will have to be collected from time to time (between 4 and 6 months) in order to download their data.
The same occurs with the DAS sensor that is to be installed in the submarine optical fiber that connects Gran Canaria with Tenerife. This type of sensor converts the large cable into a wide network of seismographs since it is capable of providing researchers with a virtual seismograph for every 10 meters of cable. This technology is already being used to contribute to the La Palma eruption. For this, 4 kilometers of underground optical fiber from the Instituto de Astrofísica de Canarias (IAC) have been used to measure earthquakes. This technology also makes it possible to see the direction from which the seismic movements are coming.
“We are going to be able to have a combination of surveillance systems that will allow us to better measure what happens under the islands”, remarks the researcher, who hopes that this work “will also contribute to improving the Gran Canaria network.” And is that, for the current project, the seismographs that are going to be used are “for rent”, therefore, once it is finished, they will also be withdrawn. However, he hopes that at least “two or three more points” can be left on the Gran Canaria network, since such an amount will be enough to make the network “much more efficient”. Although the mysterious volcano in the middle is what has moved these researchers to work, for the first time, on this area, the truth is that this deployment of means will also allow us to know what the islands are like inside and thus improve volcanic surveillance in the islands. Islands and even anticipating the eruptive phenomena of the future.
Trip to the interior of La Palma
Although the middle volcano is what drives the research project that the National Geographic Institute will develop in the next four years, the study also contemplates an improvement in seismic monitoring of the youngest and volcanically active islands: La Palma and El Hierro. With this expansion of the seismic network in the underwater slope that surrounds both islands, scientists will be able to achieve something impossible to date, travel to its interior to get to know its volcanic structures in depth.
Until now, as explained by the seismologist of the National Geographic Institute and main researcher of the project, Itahiza Domínguez, “there have been various studies that show structures under the islands that can condition what volcanology is like.” However, the models that have been made in this regard are very rudimentary. “We have a model of parallel layers that is very simple and for volcanic islands they are not a good approximation,” insists Domínguez.
Volcanic islands require a more complex model that researchers believe they can achieve thanks to 3D tomography. And it is that, like the entire Archipelago, La Palma and El Hierro have been formed based on an accumulation of eruptions that have generated an overlap “of layers” that, after all, cannot be studied in depth with a “Homogeneous model” like the one they currently have.
This heterogeneity of the terrain, in turn, is what causes the ground to have “different accelerations” with regard to the magnification of earthquakes. Proof of this is how, during this volcanic crisis, earthquakes have affected different parts of La Palma. Thus, when they occurred on the surface, the oldest area of La Palma (located to the north), felt with enough force to move the objects, in the places that were next to the epicenter they barely managed to wake up the population.
Furthermore, the island building on El Hierro is not the same as the one on La Palma. “On El Hierro it seems that there is a structure in the center of the island with a high density, while on La Palma it is completely the opposite”, remarks Domínguez, who insists that, for this reason, the pre-eruptive processes and eruptions are different on both islands.
With this deployment of means, it will be possible to accurately measure the seismicity that occurs before the eruption, and it will also be possible to determine “what awaits us in the future.” That is, how are the eruptions on each island to be able to forecast them with greater precision. In this sense, Domínguez remarks that, in this case, only data from earthquakes that occur in the area will be used, that is, natural sources. Thus, the use of external signals to carry out this study is ruled out. “Sometimes explosions or boats that generate pulses – such as those in oil exploration – are used to carry out these studies, but they are very invasive techniques,” says the researcher. In fact, these artificial waves, which are like “noise shocks in the sea”, can cause damage to marine ecosystems, since they alter the life that inhabits them.