What are ‘slow earthquakes’ (and how they could help science to anticipate large earthquakes) | Security | News

It is something impossible with current knowledge and technology, but research in recent years has led to the experts to be one step closer to identify when the conditions are being met on Earth for a great earthquake to occur.

Experts in geophysics have focused, among other fields, on the so-called “Slow earthquakes”.

It is about “landslides that take place in a geological fault, in general, and in particular in subduction zones between two plates that are in contact ”, he explains to BBC World the seismologist Víctor Cruz-Atienza, researcher at the Institute of Geophysics of the National Autonomous University of Mexico.

The expert and his colleagues recently published a study on these types of earthquakes that develop in certain seismic regions, such as those of the Mexican southeast, where there are two interacting plates.

His research determined that there were slow earthquakes -also called silent– behind the last four major earthquakes in that country.

Unlike the tremors that shake the surface, slow earthquakes release energy little by little over weeks or months, which makes them imperceptible and not destructive at all.

But experts say studying them is very important to better understand how earthquakes are generated, since not always a slow one anticipates a “normal” one, but it is a factor to take into account.

What Happens in the Deep

Earthquakes generally occur when the interaction of tectonic plates releases surface energy, which causes the ground to shake sharply.

However, there are other types of interactions in layers lower or higher than the layers in which earthquakes occur that are felt on the earth’s surface.

One such event is slow earthquakes which, because they do not release energy abruptly, are not noticeable.

Ruiz points out that some have reached magnitude 7 on the Richter scale, which would be a considerable danger if they were earthquakes with consequences on the surface, but the fact that they occur over weeks or months eliminates the risk.

It is as if there were plates, cups and cutlery on a table, explains the Chilean geophysicist. If that table were moved quickly, what was on top would shake. But if the cabinet were moved very slowly, things would practically remain still.

“A slow earthquake can be the same size as a large one, as a ‘normal’ one, but since it moved very slowly it is not perceived,” says Ruiz.

Cruz-Atienza explains that they can be measured with differential GPS devices, “very high precision”, which measure the deformation of the continents with an accuracy of approximately two millimeters.

“With that we can measure to what extent the continent is deformed and how there is an elastic rebound, the return of the slow sliding or the slow earthquake, with the contact of the plates under the continent,” says the expert.

Behind great earthquakes

From the study of slow earthquakes, scientists have determined that several large earthquakes that have shaken different regions of the world were preceded by these “silent” events.

Among them is the magnitude 9.1 earthquake of 2011 in Japan, which caused a tsunami and the failure of the nuclear power plant in Fukushima. Also the 7.8 magnitude in New Zealand in 2016 or the of Chile of 2014 of magnitude 8.2.

In the case of Mexico, Cruz-Atienza has determined that slow earthquakes preceded four major earthquakes in the country, including the one in September 2017 that caused building collapses in Mexico City, as well as the one in February 2018 near the city. of Pinotepa Nacional.

“We demonstrate the stresses or deformations that this slow and deep earthquake induced in the most superficial area of ​​plate contact and that it was the one that triggered the rupture of this 7.2 magnitude earthquake that caused a lot of damage in Pinotepa”, explains the expert.

His research in southern Mexico found that slow earthquakes occur every 3.5 years in the state of Guerrero and every 1.5 years in Oaxaca, product of the sliding of the Cocos plate (oceanic) and that of North America (continental).

Each region of the world has its own periodicity.

However, both Cruz-Atienza and Ruiz warn that, with current evidence, It cannot be said that slow earthquakes are a phenomenon that will always produce earthquakes on the surface.

“There are many slow earthquakes that have not produced earthquakes. Slow earthquakes, at least with the observational capacity we have today, seem to be a necessary but not sufficient condition to produce an earthquake. There must be other conditions to produce them ”, explains Cruz-Atienza.

What does it teach scientists then?

The study of slow earthquakes is an important advance for researchers to have evidence that activity in the earth’s crust is advancing towards an event with destructive potential.

“They have allowed the scientific community understand much better the behavior of geological faults where dangerous earthquakes occur. These slow earthquakes that we do not perceive modify the state of stresses, tensions, and deformations of the continental crust that can eventually cause large earthquakes ”, says the Mexican researcher.

The observation of slow earthquakes has only occurred in the last 20 years, but for Ruiz “it opens a window to understand the physics that controls earthquakes.”

“It is still very difficult to conclude whether slow earthquakes are a general observation. Since we don’t have a good number of slow earthquakes on record, the question remains. These observations must be tried to continue to maintain them over time to be able to make more accurate conclusions”Says Ruiz.

What does science take to anticipate the possibility of a dangerous earthquake?

Both researchers agree that it requires more observation and research of how these events are generated.

In addition, it is necessary the installation of more measuring instruments along seismic regions, such as those of the entire Pacific coast of Latin America.

“For now, what is most missing is to increase the instrumentation to be able to measure earthquakes in a terrestrial way. Having a lot of data and if all comply with being preceded by a slow earthquake, ”explains Ruiz.

And although there are numerous geophysical investigations in Latin America, the region is still behind in instrumentation compared to other parts of the world. (I)