The turning point occurred with the abrupt halt in volcanic activity a week ago. On Monday, September 27, the volcano woke up without emanating a single hint of toxic smoke, lava or ash. It did not roar either and the lava flows, which were very dense at that time, were completely stopped. It was the calm that preceded the storm, because after that moment, the eruption has gained strength and the lava that emanating from the volcano is so liquid that its destructive power has increased remarkably. In fact, by then the lava flow had engulfed 420 buildings in the area. At the moment, they already exceed 1,000 and 72 hectares of crops. This new stage has also caused a change in the casting, which has gone from measuring just over 500 meters wide, to being 1,250 in some sections, as indicated by the Cabildo de La Palma.
“This new seismicity in the area began when there was a stop in the volcano’s activity, it seems that before we had an activity in which the material came from a closer pocket and now it comes from deeper”, rMark Itahiza Domínguez, seismologist from the National Geographic Institute (IGN), who points out that all this makes sense considering the readjustments that the volcanic system has undergone.
Seismicity plays a leading role in these changes experienced in the Cumbre Vieja volcano. And it is that, once it finished feeding on the small magmatic reservoir that was barely six kilometers deep – which was estimated at about 20 million cubic meters -, it has looked for another way to remain active: devour the magma contained in the depths of Fuencaliente.
The current seismic zone is the same one in which the swarm began on September 11
However, for all the magma found in that pocket – contained deep within the island for decades – to come out of hiding, it must push the earth, which results in seismicity. “It seems to be a readjustment of the system,” indicates Itahiza Domínguez, who highlights that this activity is in the same area where the seismic swarm that gave rise to this eruption began. “This is where the intrusion under the island began,” explains the researcher.
It should be remembered that the first earthquakes that occurred in this swarm, but also in the previous ones – up to nine between 2017, 2018 and 2020 – had a point of confluence: the south of La Palma. In fact, during the first days of this seismic-volcanic crisis that began in mid-September, earthquakes were detected between 10 and 13 kilometers deep. From September 11 to 13, the activity continued in the south of the island, precisely in the Fuencaliente and Mazo area. However, as the magma made its way to the surface, it found a hole to the west through which to flow until it emerged at the current point.
The activity is stronger now than it was then but is at the same distance from the surface. Thus, small swarms are being recorded with fewer earthquakes than then –58 recorded yesterday–, but a higher percentage has a magnitude greater than 3 and, therefore, they are being more widely felt by the population.
This activity has intensified in recent days, since last Friday there were only 19 earthquakes, of which five had a magnitude greater than three. The following day the seismic activity intensified, registering 46 earthquakes, although the strongest were only seven. On Sunday there were fewer earthquakes (35), but the largest ones increased in number, detecting a total of eleven. Yesterday was a particularly active day as the IGN registered more than 70 earthquakes, of which almost 20 had a magnitude greater than 3. This means that, in just four days, the number of earthquakes of greater magnitude has tripled. Likewise, in these two days the strongest earthquakes have been detected so far, in that area, two of 3.7 and one of 4. The first two earthquakes occurred on Sunday and yesterday in Villa de Mazo between 11 and 12 kilometers deep and the strongest of the series, it took place in Fuencaliente.
The 16 historical eruptions show a migration of seismicity during volcanic activity
Of the total earthquakes that occurred yesterday, only five were felt by the population with an intensity of 3 or 4. As explained by the IGN, the depths of the hypocenters are between 10 and 15 kilometers, with the exception of six earthquakes that have occurred at great depth (30 and 36 km).
No sign of another volcano
“What is happening is a response to the system to the eruption and to pressure changes,” Domínguez remarks. This seismic signal together with the stabilization of the deformation of the terrain that, according to the IGN, accumulates around the eruptive centers already open in Cabeza de Vaca, for the moment, ratify the position that a new eruption will not occur in any a place far from the main eruptive cone, such as Fuencaliente. In fact, according to the researchers, although it is impossible to rule out any scenario, for the moment that of a new mouth that arises in that part of the island is the least likely of all.
The volcano, therefore, is not surprising what scientists expect of it. The possibility that it could feed on magmatic pockets located deeper was considered just a few days after the eruption. This was expressed by the geologist of the University of La Laguna (ULL), Ramón Casillas, who at that time insisted that if the volcano only expelled “what is in that first magmatic bag”, the one that was only six kilometers away deep, the eruption might last “just a few weeks.” However, for the researcher it was much more likely that the volcano ended up feeding on magma reservoirs that are found in deeper pockets, between 12 and 20 kilometers deep.
The beginning and the end
This type of seismicity after the volcanic rupture of the ground has happened «in all the historical eruptions [las 16 que se han producido desde el siglo XV]». According to the IGN seismologist, in all these episodes, most of which occurred in Tenerife, La Palma and El Hierro, “there have been earthquakes felt by the population.” You don’t have to go back to the year 1700 to find an example of this.
The Tagoro eruption, in El Hierro, showed that seismicity can be both the manifestation of the birth of a future volcano and a symptom of its death. “We had several years of seismic activity after the Tagoro eruption at a deeper level – as is happening now – but in a different area from where the eruption had occurred,” says Domínguez. In El Hierro, after several years of swarms with random periodicity in the area, the activity ceased with the achievement of several strong earthquakes –with a magnitude greater than 3– between November and December 2013. Since then, the volcano has become to sleep and the seismic activity of that island has been reduced to a couple of low magnitude earthquakes every year.
The Ramón Margalef ship departs from La Palma
The Spanish Institute of Oceanography (IEO-CSIC) has just finished the first multidisciplinary oceanographic campaign in the south-west area of the island of La Palma. The objective has been to respond to the requirements of the scientific committee of Pevolca (Emergency Plan for Volcanic Risk of the Canary Islands) on an emergency basis. During 10 days of uninterrupted work, he has carried out a complete study of the physical-chemical and biological properties of water and of the geomorphology of the seabed before and just after the arrival of the wash in the ocean. In total, about 3,000 seawater samples have been collected, representing more than 500 liters, collected from the surface to 1,200 meters deep and, some of them, a few meters from the casting. Among the analyzes, the temperature of the surface water in the surroundings of the lava strip and biomarkers to monitor the alterations in the ecosystem stands out. The oceanographic vessel Ángeles Alvariño is scheduled to arrive in the area on October 14. | LP