The ability to predict when a major earthquake will occur is a longing that seismologists share.
It is impossible with current knowledge and technology, but research in recent years has led to experts To be one step closer To determine When the conditions are met On the ground of a great earthquake.
Geophysics experts have focused, among other fields, on the so-called “slow earthquakes”.
It’s about “landslides that happen in a geological faultIn general and in particular In subduction zones Between two panels in contact,” he explains to BBC Mondo Seismologist Victor Cruz-Atenza, researcher at the Geophysics Institute of the National Autonomous University of Mexico.
The expert and colleagues recently published a study of these types of earthquakes that occur in certain seismic zones, such as those in southeastern Mexico, where there are two interacting plates.
Select his search 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 monthsWhich makes it imperceptible and not destructive at all.
But experts say Their studies are very important To better understand how earthquakes are created, since Not always slow to expect a “normal” personBut it is a factor that must be taken into account.
What happens in the depths
Earthquakes generally happen When tectonic plates interact, surface energy is released, causing the ground to shake sharply.
However, there are other types of interactions in layers lower or higher than the layers in which earthquakes that are felt by the Earth’s surface occur.
One such event is slow earthquakes that go unnoticed because they don’t suddenly release energy.
Ruiz refers to it Some have reached 7 on the Richter scale, which would be a huge risk if they were earthquakes with surface consequences, but the fact that they happen over weeks or months eliminates the risk.
It is as if there are plates, cups, and cutlery on a table, explains the Chilean geophysicist. If this table was moved quickly, what was above it would shake. But if the government moves too slowly, things will remain practically the same.
“A slow earthquake can be about the same size as a large earthquake, as a ‘normal’ earthquake, but because it’s moving so slowly, it’s not perceptible,” Ruiz says.
Cruz Atenza shows that it can be measured Using Differential GPS Devices, “Very High Accuracy”, which measures the deformation of the continents with an accuracy of nearly 2 millimeters.
“By doing this we can measure how much the continent is deformed and how there is elastic recovery, slow slip back or slow earthquake, with plate contact under the continent,” says the expert.
Behind the great earthquakes
By studying slow earthquakes, scientists have determined that many of the large earthquakes that shook different regions of the world were preceded by these “silent” events.
Among them is the 9.1-magnitude earthquake in 2011 in JapanCausing a tsunami and nuclear power plant failure in Fukushima. Also 7.8 degrees in New Zealand in 2016 or the From Chile in 2014 with a strength of 8.2.
In the case of Mexico, Cruz-Atenza identified that slow earthquakes preceded four major earthquakes in the country, including the September 2017 earthquakes that caused buildings to collapse in Mexico City, as well as the February 2018 earthquake near the city of Pinoteba. National.
“We show the stresses or deformations that this slow and deep earthquake created in the most superficial region of plate contact and that it was this rupture of this magnitude 7.2 earthquake that caused a lot of damage in Pinoteba,” the expert explains.
His research in southern Mexico found slow earthquakes Every 3.5 years in Guerrero and every 1.5 years in Oaxaca, a product of the Cocos (oceanic) and North American (continental) plate slides.
Each region of the world has its own frequency.
However, Cruz-Atenza and Royce both caution that with the current evidence, It cannot be said that slow earthquakes are a phenomenon that will always lead to earthquakes on the surface.
There are many slow earthquakes that do not produce earthquakes. Slow earthquakes, at least with the monitoring capacity we have today, appears to be a necessary but not sufficient condition to cause an earthquake. There must be other conditions for its production,” explains Cruz-Atenza.
What do scientists know then?
The study of slow earthquakes is an important advance for researchers to obtain evidence that activity in the Earth’s crust is progressing toward an event with destructive potential.
“They let the scientific community Much better understanding of the behavior of geological faults Where dangerous earthquakes occur. The Mexican researcher says that these slow, unaware earthquakes modify the state of stresses, tensions and distortions in the continental crust that can eventually cause major earthquakes.
The observation of slow earthquakes has only occurred in the past 20 years, but for Ruiz “opens a window to understanding the physics that control earthquakes.”
“It is still very difficult to conclude whether slow earthquakes are a general observation. Since there are not a large number of slow earthquakes recorded, the question remains. These observations should be attempted Continue to maintain it over time to be able to provide more accurate conclusionsRuiz says.
What does science take to predict the likelihood of a dangerous earthquake?
The researchers agree that it requires More monitoring and research on how to create these events.
In addition, it is necessary Install more measuring tools Along seismic zones, such as those on the entire Pacific coast of Latin America.
“At the moment, what we are most lacking is an increase in hardware to be able to measure earthquakes in a ground way. There is a lot of data and if it all corresponds to being preceded by a slow earthquake,” Ruiz explains.
Although there have been many geophysical investigations in Latin America, the region still lags behind in instrumentation compared to other parts of the world. (I)