The seismic swarm that shook the island of La Palma in mid-September was the prelude to the eruption in the volcanic building of Cumbre Vieja. The earthquakes had begun to register four years ago, in 2017, but a week before the first lava hole opened in Cabeza de Vaca, the volume and intensity of the tremors, each time closer to the surface, made herald the imminence of the phenomenon. The succession of earthquakes concentrated in the same place during a period of time usually acts as a precursor, an alert for the scientific community.
In the bowels of the volcano that buried more than 300 properties on La Palma
However, the volcanic reactivation process had already begun to emit the first signals eleven years before. A study published in January this year in a journal of the group Nature by a research team from the Institute of Geosciences, dependent on the Higher Council for Scientific Research (CSIC) and the Complutense University of Madrid (UCM), places the initial phase, the incipient manifestation of the underground movement, between 2009 and 2010.
Through an innovative geodetic observation and interpretation tool (a science that analyzes the shape and dimensions of the Earth), a team led by José Fernández confirmed the existence since that time of small deformations, less than five centimeters, in the terrain of the steep palm tree orography, which already warned that the magma from the depths had begun to move in the bowels of Cumbre Vieja.
“It is the first time that the geodetic observation of small deformations has made it possible to find the beginning of a period of volcanic reactivation”, explains Fernández. Using images from three satellites (ENVISAT, Sentinel-1, and RADARSAT-2) and a high-quality digital terrain model derived from the National Geographic Institute (IGN), his group developed a highly accurate technique to detect if there were small deformations on the surface in three time periods (2006-2010, 2010-2017 and 2017-2020).
“You have to make a precision observation, correcting, for example, the atmospheric effects and comparing the techniques to make sure that what you see are deformations of the terrain,” he says.
Studies carried out between 1992 and 2008 had not detected any anomaly in the area. “As of 2009 we see small deformations,” says the CSIC researcher, who suggests that the causal source of the eruption that has buried homes and crops on the southwest face of the island during this last week has its origin on that date, almost 40 years after the youngest volcano on the island, Teneguía, emerged in 1971.
The deformations observed on the ground (uplifts and subsidence) were compared with other data sources. “In the publications of Involcan (Canary Islands Volcanological Institute) and ITER (Technological and Renewable Energy Institute) gas anomalies appeared that could be associated with an activity in 2010 due to a passage of magma under Cumbre Vieja”. Specifically, a significant increase in helium in the cold source of Dos Aguas. Measurements of the soil of one of the volcanoes in the south of La Palma also recorded two significant increases in carbon dioxide between 2011 and 2013, which could be translated as a “delayed response” to a movement of magma that could have occurred “approximately one year before “, highlights the study. “All the measurements fit you, also other types of observations”, redounds the geodesy specialist.
On the other hand, there were no associated earthquakes, a circumstance that, according to the researcher, does not destroy the hypothesis. “Volcanic reactivation processes can have a weak or undetectable seismic activity as a consequence of a vent that remains open or mechanically weak after an initial stage of activity, avoiding the accumulation of stresses that cause earthquakes”, collects the article, adding that magma can be displaced taking advantage of fractures from ancient eruptions.
Fernández puts the antecedent of 1949. The precursor seismic activity of the San Juan volcano eruption began 13 years earlier, in 1936, in the Aridane Valley, at a distance of between “five and ten kilometers” from the vents. “The seismicity was not felt at that time (there is no record like the one now carried out by the IGN) in the area where the volcanic cones later emerged, but it was at a distance similar to that of now, about 10 kilometers, with which the behavior (described in the study) is consistent with a part of the volcanism existing in the area. The seismicity is sometimes where the magma is rising, which seems to have been the last stage before the recent eruption (week before the volcano erupted), but not at other times. ”
According to the geodetic model prepared by this research group, between 2009 and 2010 a quantity of magmatic material that was found at a depth of between 25 and 30 kilometers rose to 8-10 kilometers, taking advantage of a “weak zone, where a recent eruption occurred. “. Two years later, another upward magmatic movement was observed “along a similar path”, using existing fractures under Cumbre Vieja, in the area where significant deformations were detected prior to the last eruption. This fact would explain the absence of tremors during these periods of volcanic reactivation. Already in 2017 and 2018, two seismic swarms were recorded that the researchers associate “with new paths that favored the rise of the magma.”
The CSIC researcher points out that the area where the ascent began “was possibly closely related” to the 1949 eruption, since there was “a lot of fractured material and many areas that were not consolidated.” “It started to rise in small amounts, varying the deformation that was there before and that allowed us to detect it,” he says.
Fernández maintains that this hypothesis is consistent with the behavior observed in other volcanic processes in the Canary Islands. In El Hierro, in 2011, “there was no significant deformation until shortly before” the eruption of the underwater Tagoro volcano. In Tenerife there was also a crisis between 2004 and 2005 with a “deep intrusion”. The deformations had to do “with the aquifers, with the hydrothermal system” and were also small and variable in time. “If we want to detect – and we can – deformations in the initial stages of the reactivation process, we have to look for these types of small deformations. And we are already seeing that in other areas such as Italy. That allows you to detect magma entry at a greater depth and , therefore, detect earlier. In the case of La Palma, eleven years in advance “, says the CSIC geodesy specialist.
Although the study was stopped in April 2020, the research group is updating the data to find out how the magma movement subsequently evolved from satellite images and data obtained through surveillance groups. “We are very interested in seeing what the rise has been after that period and see if, for example, it took advantage of the fractures of the 1949 eruption,” says Fernández.
The first author of the article published in ScientificReports, group magazine Nature, highlights the importance of the new interpretation tool to anticipate a volcanic reactivation process in its initial phase. “Many times what happens is that you do a geodetic observation and you have 15-20 stations and you already have enough, because it is expensive. You need a team of people and you have a series of data, but you are limited by the number of stations from which When you make a precise observation with satellite and with ascending and descending orbit, that displacement information is thousands of points on the surface. Then you need more powerful interpretation tools than those traditionally used because you have much more information. And if you don’t you do, you are wasting or losing valuable information, “he concludes.