What happens under the Earth's surface when the strongest earthquakes occur

At 3.34 local time on February 27, 2010, Chile was shaken by one of the most powerful earthquakes in a century.

The upheaval caused a tsunami that devastated coastal populations.

The two events combined killed more than 500 people.

The earthquake was so powerful that, according to NASA calculations, it displaced the Earth's axis by no less than three inches.

Like almost all maximum power earthquakes, the one in Chile was a mega-earthquake.

These phenomena occur in subduction zones, as the places where one tectonic plate is pushed under another are called.

If the plates collide and slide suddenly, a massive earthquake occurs.

The 2010 Chile shake was of magnitude 8.8, strong enough to displace buildings from their foundations.

We know little about subduction zones.

That is why geophysicist Anne Socquet, from the Grenoble Alpes University in France, planned a trip to Chile.

He wanted to install seismic monitoring instruments to collect data.

Coincidentally, it arrived just a week before the earthquake.

"It was terrifying," he recalls.

"In the walls of the apartment we rented there were cracks that could fit a fist."

Most researchers studying mega-earthquakes are primarily concerned with the shaking that precedes the main earthquake, Socquet explains.

However, an unusual feature of these colossal earthquakes is that they are often followed by a series of other powerful mega-earthquakes that occur several years later, with the epicenter hundreds of kilometers away.

For example, the earthquake that shook Chile in 2010 was followed by other seismic episodes in 2014, 2015 and 2016, centered at different points along the country's coast.

Socquet wanted to study these cataclysm sequences and investigate possible relationships between mega-earthquakes.

This required a detailed examination of the seismological and geodetic data on a larger scale than had previously been done.

The mega-earthquake

We know that mega-earthquakes are the result of the subduction of one tectonic plate with another. But, apart from this, we do not know almost everything about subduction dynamics and how they can cause instability that leads to another seismic event of extraordinary magnitude a few years later. There are indications that it could have to do with the release and migration of fluids to great depth. The DEEP-trigger project, led by Socquet, tries to fill this gap. "In terms of observations, it is virgin territory," says the researcher.

The first step in the six-month life of the project was expected to consist of expanding the network of around 250 GPS instruments installed in Chile and to which Socquet had contributed since 2007, and setting up another new network in Peru.

Since it is currently unable to travel to South America due to the covid-19 pandemic, the geophysicist has worked with local contacts to begin the installation.

It is also developing IT tools to begin analyzing the area data previously collected.

“The key issue will be to have systematic large-scale temporal and spatial observations of the relationship between slow slip and seismic fractures.

This will constitute a great contribution to science ”.

"Nobody knows what the initial trigger is, what causes the first slip"

At the University of Pavia, the mineralogist Matteo Alvaro is also interested in mega-earthquakes, although much older ones.

It is the case that we have a unique window to the subduction zones as they were millions of years ago.

There are certain places, few and far between, where rocks that have passed through subduction zones are pushed to the surface.

Through their analysis, it is possible to deduce the depths and pressures at which the landslide took place, and get an idea of ​​how subduction works, and perhaps how mega-earthquakes are triggered.

The glass

Normally, it works like this: Geologists find a rock made of a mineral with what is called an inclusion crystal inside. This inclusion was trapped in the mineral when two subducting plates pressed against each other at great depth, perhaps 100 kilometers or more below the surface. Depending on the pressure it experiences when it is formed, the inclusion will have a particular crystalline structure - a specific and repetitive spatial arrangement of the atoms -. The crystal can reveal to what pressure the inclusion was subjected, and consequently, to what depth it originated.

The problem is that this principle is an oversimplification. It is only true if the crystal is cubic, something that almost never happens. "We all know that the idea that pressure equals depth can be wrong," warns Alvaro. "The natural question is to what extent are we wrong." That was what he decided to find out with his TRUE DEPTHS project.

In principle, the plan was simple. The researcher wanted to measure the stress experienced by the crystal while it was trapped in the mineral. If you could know the infinitesimal displacement of atoms from their normal positions in a typical unstressed crystal structure, this would give you a more accurate measure of the pressure exerted by the surrounding rock when the crystal formed, and thus a measure more exact depth at which this occurred. To study atomic structure, the mineralogist uses a combination of X-ray crystallography and a technique called Raman spectroscopy.

Researcher Matteo Alvaro successfully demonstrated the first application of a combination of X-ray crystallography and a technique called Raman spectroscopy to a rock sample from a place known as Mir's Chimney, in Siberia. Vladimir

Recently, Alvaro demonstrated the first application of his technique with good results.

He examined a rock sample from a place known as Mir's Chimney, in Siberia.

It is a column of molten kimberlite that emerged very quickly from enormous depths (Most of the diamonds are mined from kimberlite vents like Mir's which was, in fact, intensively mined).

Alvaro examined the garnets with tiny quartz inclusions that sprouted.

"Kimberlite is the elevator that takes you to the surface," he explains.

The trigger

By measuring the pressure exerted on the inclusions, he was able to confirm that they had formed at a pressure of 1.5 gigapascals (about 15,000 times higher than that recorded on the Earth's surface) and at a temperature of 850 degrees.

Although this fact is not entirely surprising, it is the first proof that the technique proposed by the scientist works.

Now, Alvaro wants to take more measurements and create a collection of examples.

He also wonders, at a more speculative level, if it is possible that the formation and deformation of inclusions could act as the first trigger for mega-earthquakes.

The idea would be that these tiny changes cause cracks in larger rocks, which would end up causing a fault to slide.

Alvaro intends to delve into this idea.

"Nobody knows what the initial trigger is, what causes the first landslide," says the mineralogist.

“We have begun to think - and at best it is a totally crazy idea - that these inclusions could be.

A cluster of inclusions, perhaps subject to an instantaneous phase change, and therefore to a volume change.

That could be the original trigger. "

This article

was originally published in English in

'Horizon', the EU research and innovation magazine.

The research for this article was funded by the EU.

Translation of NewsClips.

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