In March 2022, three months after the volcano on the island of La Palma stopped spitting lava, geologist Miguel Ángel Rodríguez-Pascua received a call that at first he did not know how to fit.
There was a neighbor from El Paso who said that his house had become full of cracks.
The surprising thing was that this man had been repairing his walls for years.
He covered a crack and over time, it came back out.
And so on since the 1980s. How could this be due to the eruption?
And why didn't the house across the street not have a single chip?
Since then, Rodríguez, from the Geological and Mining Institute of Spain (IGME-CSIC), and the rest of his team have analyzed dozens of similar cases.
There are water tanks cracked from end to end and significant fractures in walls, fences, sidewalks, walls, garage ramps, even bus stations.
In none of the identified cases is the damage related to the lava flows or to the swarm of thousands of earthquakes that presaged the eruption.
The reason, they have just concluded, is that these buildings are right above two active faults, the first to have been discovered in the Canary Islands.
So far, at least ten houses located on these fracture zones have been confirmed and at least two of them have been left uninhabitable.
“Until now it was thought that there were no active faults in the Canary Islands,” explains Rodríguez.
“These are the first that have been identified in the archipelago.
Without them, there would not have been an eruption like the one on La Palma,” adds the scientist.
Faults are zones of breakage in the Earth's crust.
These fractures range from the mantle, about 10 kilometers underground, where there is magma, and at a depth similar to that at which the hypocenters of the earthquakes were detected before the eruption, to the exterior.
The opening of these cracks is ultimately responsible for the molten rock crawling to the surface and causing a lava eruption like the one that occurred on La Palma between September and December 2021.
The two new faults of La Palma, named Tazacorte and Mazo, can be considered the fastest in the entire Spanish territory.
They move about three millimeters a year, triple that of the Carboneras fault, one of the fastest on the Peninsula, explains Rodríguez, who has directed the work to identify these two fractures by collecting more than 400 data on the ground.
Researchers believe they have been active for tens of thousands of years.
It is “probable” that they are also responsible for the eruption of the San Juan volcano in 1949, adventure.
The IGME study shows how the Tazacorte fault was active before, during and after the eruption.
All of the mouths of the volcano were aligned along this fracture, which follows a northwest-southeast direction.
Starting on October 2, the eruption changed phase and the hypocenters of the earthquakes and the mouths changed orientation to west southwest-east northeast following the Mazo fault.
More than two years after the volcano went out, the cracks in the crust continue to move, as confirmed by neighbors who have been repairing the walls of their houses for years.
Two plaster witnesses show the dates on which the cracks in a house reopened.
Some neighbors have been repairing their homes since the 1980s.IGME
“Everyone wonders when the activity is going to stop, but we can't know,” acknowledges Raúl Pérez, another IGME geologist who co-authored the study of the faults, who carried out intense work during and after the eruption, sometimes entering into the most dangerous areas together with units of the Military Emergency Unit.
“These faults have been accumulating energy for thousands of years and now it is being released little by little with that slow and constant movement that we detect and which is what causes houses to crack.
There was a lot of skepticism regarding this issue, which is why we needed to study it in detail and take three years of data to confirm it. How dangerous is it?
That must be determined by an architectural analysis that is outside our powers, but if there is follow-up there should be no danger,” he adds.
The two cracks cross La Palma from side to side.
That of Tazacorte will be about 8.5 kilometers from the coast to the volcanic cone and another 20 kilometers towards the opposite side of the island;
and Mazo's about six kilometers to the crater and another 16 to the opposite flank.
It is likely that they extend through the oceanic crust, under the sea, although for now it has not been possible to verify this.
Wall of a house built on a fault in the Corazoncillo area, in La Palma.IGME
Miguel Ángel Rodríguez-Pascua and a colleague analyze the cracks in a house in La Palma.Arturo Rodríguez (Arturo Rodriguez)
Measurement of cracks in an affected home.Arturo Rodríguez (Arturo Rodriguez)
An IGME geologist analyzes one of the two active faults of La Palma near the Las Manchas Cemetery.
Water tank affected by faults in the Cogote mountain area, in La Palma.IGME
A team from IGME-CSIC analyzes the cracks in a house affected by active faults in La Palma.Miguel Angel Rodriguez Pascua
Analysis of cracks in the Puerto Naos bus station, in La Palma.Arturo Rodríguez (Arturo Rodriguez)
The faults of La Palma are slow moving, so the risk of earthquakes or affected houses collapsing is lower, scientists consider.
Each side of the fault moves slowly without causing sudden movements, but cracking the buildings in its path.
Starting in the 1980s, houses on La Palma began to be built on concrete footings, a material that resists deformation well.
On the other hand, the surrounding structures, such as patios, ramps, swimming pools, and water tanks, do not have any structure underneath and crack.
Many of the most damaged buildings are in the Corazoncillo area.
One of the most affected points is the famous Las Manchas cemetery, located just above the Mazo fault.
The IGME team collaborates with the affected town councils and with the Cabildo of La Palma.
They go house by house and determine if the cracks are due to the activity of these faults.
Their location has forced us to change the location of the future interpretation center of the new La Palma volcano, which was planned to be built just above a fault.
Scientists have installed almost 60 crackmeters in homes, witnesses that monitor the progressive opening of cracks inside houses.
The ideal, they explain, would also be to install a more precise system with GPS stations.
The La Palma eruption has been unique in the history of Spain.
For the first time, an army of scientists could follow it on land, sea, air and space.
It was like watching geological time, which acts on scales of millions of years, passing before your eyes in fast motion.
After the eruption, when people were able to return to the areas that had been cordoned off, cases of cracks and damage began to appear in apparently intact areas, some at a great distance from the flows.
“In some houses the cracks were brutal;
There were even fumaroles of water vapor,” recalls Nieves Sánchez, IGME researcher.
“I remember one that was completely destroyed, it had been left uninhabitable, while the one next door was perfect.
Why is mine yes and the other no?, the owner asked herself.
It has been terrible in some cases, but there has been great collaboration.
We are trying to turn some of these uninhabitable houses into museums, but it all depends on what the owners want to do,” she details.
Rodríguez-Pascua highlights that the work in La Palma has been unique due to this social aspect.
“I work on active faults, earthquake geology and archaeoseismology, and I see that sometimes you sound like a guru saying that there have been earthquakes in the past and that at some point there will be another one.
Many people call you an alarmist, they try to deny it;
They don't accept it.
In La Palma it has not been necessary to convince anyone.
It has been the neighbors themselves who warn you that their house is breaking down.
What we have done is explain to them why, and generally the majority has understood it,” he highlights.
The new faults of La Palma also explain, for example, why during the eruption lava came out from points that were very far from the cone, explains David Sanz Mangas, co-author of the study, which has just been published in the specialized journal
Applied Sciences
.
José Fernández-Turiel, CSIC volcanologist, who has not participated in the study, explains: “The work is an interesting contribution in that it highlights the surface tectonic manifestations of a volcanic island;
an under-researched field of work.”
The scientist delves into the implications of the new work.
“Oceanic volcanic islands grow very quickly in geological time.
La Palma has been formed in about two million years, forming a building more than 5,000 meters high, more than 3,000 under the sea and 2,400 on the surface.
This rapid growth means that they tend to collapse through giant landslides.
These landslides produce faults that can reach up to three kilometers deep.
These are usually recognized on the ground by the plans they leave.
They are also recognized in the subsoil in the galleries drilled for water.
There are good examples in Tenerife and El Hierro.
Other fractures that may have that dimension in depth are those that take advantage of the magma to ascend and it is the magma itself that opens them in its path.
These are recognized on the surface or in the subsoil by volcanic rock dikes.
When the magma cools, it solidifies and forms these dams, which are the fossilized magma progression conduits.
In this new study they assume that the Tazacorte fault plane reaches up to 20 kilometers deep based on data from September 12 to 19.
Another recent work, with data from seismic sources from the same dates, does not draw a simple plane like that of the Tazacorte fault.
This model is more plausible, since in the context of an oceanic island the magma trace follows a more complex path, because there are no simple planes to travel through.
The magma breaks the socket and moves where it is easiest for it to make its way," he details.
Alessandro Maria Michetti, volcanologist at the University of Insubria and expert in the faults produced by the Etna volcano, which can cause very dangerous earthquakes, says: “It is a very well carried out study and shows the success of predicting the area in which the volcano would erupt.”
The geologist says that in Italy this type of work helps to divide the land into microzones and prevent houses from being built above dangerous areas, something that he also recommends in La Palma.
Other types of constructions that are more flexible are possible, he details, such as highways or gas pipelines that pass just above one of the most dangerous faults in the country, the Pernicana.
On La Palma, concludes the Italian researcher, this type of study “will help understand where the next eruption may occur.”
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