• Canary Islands What is a seismic swarm?

October will mark the 50th anniversary of the last Cumbre Vieja eruption on La Palma.

Half a century later, the most active volcano in the Canary Islands - with the permission of El Teide - has kept the inhabitants of four municipalities on this small island of 85,000 in suspense since a new wave of earthquakes began last Saturday.

Most are being of low magnitude and are caused by the accumulation of 11 million cubic meters of magma trying to rise to the surface.

There are already more than 4,500 tremors, a thousand of them located -those that have a very low magnitude are registered but not studied in detail-, as detailed in a telephone conversation from Santa Cruz de Tenerife the volcanologist David Calvo, spokesman for the

Canary Islands Volcano Institute (Involcan).

Precisely as we speak, on Thursday at noon, a new 3.1 earthquake occurs, the largest in recent hours.

On Tuesday, the largest earthquake of this wave was registered, 3.5.

As the Scientific Committee of the Special Plan for Civil Protection and Attention to Emergencies due to Volcanic Risk pointed out yesterday,

"it is expected that there will be more earthquakes felt of greater intensity."

As Calvo reviews, "we are facing

the tenth seismic swarm that La Palma has suffered since 2017 and the most dynamic and the one that has been closest to ending an eruption, although we do not know if there will be," he

admits.

Geologists refer to a large number of earthquakes that occur at the same point during a short period

with the term

seismic swarm

.

"This seismic swarm has been produced because a volume of magma is rising from the depth, probably from the upper mantle, to the surface. In its attempt to break the earth's crust, it deforms the surface and generates many earthquakes", summarizes this expert.

Ground deformation on the rise

Some earthquakes in the last few hours are already quite superficial.

According to the National Geographic Institute, since Wednesday there have been fifty earthquakes at depths between 1-5 km.

In addition, the deformation of the terrain has doubled in just one day, going from the six centimeters accumulated until Wednesday to the 10 centimeters measured yesterday Thursday, when the General Directorate of Security and Emergencies of the Government of the Canary Islands

decided to keep the volcanic traffic light yellow

four-color by which you set alert levels (green, yellow, orange, and red).

According to this organism, "it cannot be ruled out that the activity will intensify in the short term."

Los Llanos de Aridane, Fuencaliente, El Paso and Mazo are the towns affected

by the yellow traffic light, a state of alert in which information to the population, surveillance measures and monitoring of volcanic and seismic activity are intensified.

"The deformation means that the ground is bulging in the superficial part of the earth's crust as a result of the intrusion of the volcanic magma. It is as if you bend a loaf of bread or a grain comes out", compares Pablo Gabriel Silva, geologist of the University of Salamanca, to explain what scientists call

volcanic tumescence.

"It is one of the precursor phenomena of the eruption, it also occurred in the submarine crisis of El Hierro in 2011, although there the deformation of the ground reached 40 centimeters", he reviews.

"The ground is rising, as if the island were growing,"

says Calvo, who considers that a deformation of the ground of 10 centimeters "is significant."

According to the experts consulted by EL MUNDO, a deformation like the one that is being reached in Cumbre Vieja and that relatively shallow earthquakes are recorded is indicative that the magma has reached very close to the surface, although it is impossible to know if this material will reach to surface or not, as there have been episodes of great seismicity and even deformation and they stop without reaching the eruption.

"You have to keep monitoring and observing, and wait," they

say. "Here it does not happen like with storms, you see them coming. We handle only two scenarios, whether there is an eruption or not. We can only work in real time," admits Calvo.

"The normal thing is that before an eruption the rise of magma breaks up areas of the crust and generates earthquakes as is happening now. For the magmatic box to break and rise to the surface there would have to be an earthquake of greater magnitude, of 4.5 or 5 approximately, or perhaps it will not break. We have to wait, "agrees Pablo Gabriel Silva.

Here it does not happen like with the storms, that you see them coming.

We handle only two scenarios, whether there is an eruption or not.

We can only work in real time

David Calvo, volcanologist and spokesperson for Involcan

And if any volcano is unpredictable, the characteristics of this monster from La Palma make its evolution even more uncertain.

"

Cumbre Vieja is not a typical volcano, it is decomposable

and unlike Teide (in Tenerife) or Etna (in Sicily), it does not have a single crater. It is a succession of small volcanoes and if there are an eruption, the lava could come out of any of its craters ", explains David Calvo.

The Cumbre Vieja Natural Park occupies some 7,500 hectares and covers six municipalities whose fate depends on volcanic activity.

It was created in 1987 precisely to preserve

the cones and volcanic flows of the different eruptions that have occurred in the area since prehistoric times, as well as its Canarian pine and laurel forests.

Both La Palma and El Hierro, which are the youngest Canary Islands, are in a growth phase, have volcanoes and there must be eruptions

Pablo Gabriel Silva, geologist at the University of Salamanca

In the days prior to that last eruption in 1971, several earthquakes also shook the island of La Palma until on October 26 Cumbre Vieja roared again.

The fire show in Teneguía - as the inhabitants call that part of the volcano, which is always the same - was recorded by the cameras and although it was not destructive, it did cause a death by smoke inhalation.

It was the most intense eruption since 1677, although there was another in 1949 that devastated fields and houses after the passage of volcanic lava.

In 2017, a new period of seismic activity began which, as occurs in volcanic areas, is intermittent and unpredictable.

But as Calvo points out, they are sure that just as there have been other eruptions in the past, there will be new eruptions on La Palma, although they cannot find out when.

"Both La Palma and El Hierro, which are the youngest Canary Islands, are in a growth phase, have volcanoes and there must be eruptions," says Silva, who recalls that the only Canary Island in which there has been no recent volcanism is La Gomera.

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