What are “slow earthquakes” (and how science can help predict large earthquakes) | protection

The ability to predict when a major earthquake will occur is a longing that seismologists share.

It’s impossible with current knowledge and technology, but research in recent years has led experts to get one step closer to determining when conditions are met on the ground of a major earthquake.

Geophysicists have focused, among other fields, on so-called “slow earthquakes”.

It’s about “landslides that occur in a geological fault in general and especially in subduction zones between two plates in contact,” explains BBC seismologist Mundo Victor Cruz-Atenza, a 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.

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

About 300 people were killed in the September 19, 2017 earthquake in Mexico City. Photo: Environmental Protection Agency

Unlike earthquakes that shake the surface, slow earthquakes release energy little by little over the course of weeks or months, making them imperceptible and completely non-destructive.

But experts say their studies are crucial to better understanding how earthquakes arise, since it’s not always slow to anticipate for a “normal” person, but it is a factor to take into account.

What happens in the depths?

Earthquakes generally occur when tectonic plates interact, releasing surface energy, causing the Earth to shake sharply.

However, there are other types of interactions in layers lower or higher than the layers in which earthquakes occur and which are felt by the Earth’s surface.

One such event is slow earthquakes that go unnoticed because they do not suddenly release energy.

Some have reached 7 on the Richter scale, Ruiz notes, which would be a huge risk if the earthquakes had consequences at the surface, but the fact that they occur over the course of 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 moved too slowly, things would stay more or less the same.

“A slow earthquake can be about the same size as a large earthquake, as a ‘normal’ earthquake, but because it was moving very slowly, it’s not perceptible,” Ruiz says.

Slow earthquakes are observed in subduction zones where tectonic plates interact. Photo: Getty Images

Cruz-Atienza explains that it can be measured using differential, “ultra-high resolution” GPS devices, which measure the deformation of continents with an accuracy of about 2 mm.

“By doing this, we can measure how much the continent is deformed and how there is elastic recovery, slow slip or slow earthquake, with plate contact under the continent,” the expert says.

Behind the great earthquakes

By studying slow earthquakes, scientists have concluded 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 Japan, which caused a tsunami and the failure of a nuclear power plant in Fukushima. Also, a score of 7.8 in New Zealand in 2016 or from Chile in 2014 a score 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 the plate contact, and that it was this rupture from this 7.2 magnitude earthquake that did 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) segments.

Each region of the world has its own frequency.

However, Cruz-Atenza and Royce caution that with current evidence, it cannot be said that slow earthquakes are a phenomenon that will always result in surface earthquakes.

The 8.2-magnitude earthquake in Iquique, Chile, was preceded by a slow quake. Photo: Getty Images

There are many slow earthquakes that do not produce earthquakes. Slow earthquakes, at least with the monitoring capacity we have today, appear to be a necessary but not sufficient condition to cause an earthquake. There must be other conditions for its production,” Cruz-Atenza explains.

What do scientists know next?

Studying 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 allowed the scientific community to understand much better the behavior of geological faults where dangerous earthquakes occur. These slow, unnoticed earthquakes modify the state of stresses, tensions and deformations in the continental crust that can eventually cause large earthquakes,” says the Mexican researcher.

Seismic alerts are the product of advances in the understanding of earthquakes. But scientists are looking to create more prediction tools.

The observation of slow earthquakes has only occurred in the past 20 years, but for Ruiz it “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 must be attempted and maintained over time to be able to provide more accurate conclusions,” says Ruiz.

What does science take to predict the likelihood of a serious earthquake?

The researchers agree that it requires more observation and research on how these events are generated.

Experts say installing more measurement tools is necessary in Latin America to boost research. Photo: Getty Images

In addition, it is necessary to install more measuring instruments along seismic zones, such as those on the entire Pacific coast of Latin America.

“At the moment what we lack the most is to increase the hardware so that we can measure earthquakes in a ground way. There is a lot of data and if everything should be preceded by a slow earthquake,” Ruiz explains.

Although there have been several geophysical investigations in Latin America, the region still lags behind in instrumentation compared to other parts of the world. (I)

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