What happens under the earth’s surface when the strongest earthquakes occur | Horizon: The European Union’s Journal of Research and Innovation

Like nearly all stronger earthquakes, this was a massive earthquake. This occurs in subduction zones, which are places where one tectonic plate is pushed beneath another. If the plates slip suddenly, you get a massive earthquake. The magnitude of the 2010 Chile earthquake was 8.8: strong enough to knock buildings off their foundations.

We understand subduction zones poorly, which is why the geophysicist Professor Anne Souquet, based at the University of Grenoble Alps in France, planned to visit Chile. She wanted to install seismographs to collect data. By chance, I arrived a week after the earthquake. “It was terrifying,” she said. “The apartment we rented has cracks in the walls so you can put your fist inside.”

Prof.Suckett says most people who study massive earthquakes focus on frontal tremors just before the main earthquake. But the unusual characteristic of massive earthquakes is that they are often followed by a series of other very powerful earthquakes many years later and hundreds of kilometers away. For example, the 2010 Chilean earthquake was followed by other events in 2014, 2015 and 2016 that focused on regions above and below the coast of Chile. Professor Souquet wanted to look at the sequence of tremendous earthquakes and investigate possible links between those great earthquakes. This requires close examination of seismic and geodetic data on a larger scale than previously performed.

“Nobody knows what the initial impulse was, the thing that led to the first slip.”

Professor Matteo Alvaro, University of Pavia, Italy

Giant confidence

We know that massive earthquakes are the result of a tectonic plate plunging beneath another. But beyond that, we have very little understanding of subduction dynamics and how it can lead to instability that leads to another major event a few years later. There is some evidence that this may have something to do with the release and migration of fluids at great depth. Professor Souquet’s DEEP-Trig project is about bridging this gap. “This is a virgin territory in terms of notes,” she said.

The first step of the six-month-old project was to add to a network of around 250 GPS instruments that it has contributed to in Chile since 2007 and to build a new network of devices in Peru. Currently unable to travel to South America due to the Covid-19 pandemic, she was working with local contacts to initiate the installation. She is also working on mathematical tools to start analyzing historical data from the area.

The important thing will be to obtain systematic observations of the relationship between slow slip and seismic fractures over a wide range of time and space. This would be a very large entry to science.

At the University of Pavia in Italy, mineralogist Professor Matteo Alvaro is also interested in giant earthquakes – albeit much older.

It turns out we can have as unique a window into subduction zones as they were millions of years ago. There are certain places, few and far between, where rocks that have passed through the subduction zones are forced up to the surface. By analyzing these rocks, we can deduce the depths and pressures at which subduction occurred and build a picture of how subduction works – and possibly how massive earthquakes occurred.

Crystal

Usually it works like this. Geologists find a rock made of a mineral with a so-called crystal embedded within it. This inclusion was held within the mineral as two submerged plates pressed against each other at a great depth, perhaps 100 km or more below the surface. It will have a specific crystal structure – a specific and repetitive spatial arrangement of atoms – which depends on the pressure it experienced while it was forming. The crystal can reveal the pressure to which the inclusion was subjected and thus the depth in which it was formed.

The problem is that this is an oversimplification. It is only preserved if the inclusion is in the shape of a cube – and never will be. This whole notion of compression is equal to depth – we all know that might be wrong, says Professor Alvaro. “The natural questions are, okay, but how wrong are we?” This is what he decided to discover in his TRUE DEPTHS project.

The plan was simple in principle. Professor Alvaro wanted to measure the pressure the crystal was subjected to while being held within the mineral. If he could understand the small displacement of the atoms from their usual positions in a typical uncompressed crystal structure, this would provide a better measure of the pressure exerted by the surrounding rocks during the crystal formation and thus a more accurate measure of the depth in which they were formed. To study atomic structure, he uses a combination of X-ray crystallography and a technique called Raman spectroscopy.

Professor Alvaro has just demonstrated the first successful application of his technology. He looked at a sample of rock from a site known as the Mir tube in Siberia. This is a column of molten kimberlite that rose very quickly from huge depths. (We get most diamonds from kimberlite tubes like this one, and indeed, Mir has been mined on a large scale.) Professor Alvaro looked at garnet rocks with tiny quartz inclusions inside them. “Kimberlite is the lift that brings it to the roof,” he said.

Effects

By measuring the pressure on the impurities, he can confirm that they formed at a pressure of 1.5 gigapascals (about 15,000 times the pressure found at the Earth’s surface) and a temperature of 850 ° C. This isn’t entirely surprising, but it’s the first evidence that Professor Alvaro’s method really works. He is now looking to do more measurements and build a library of examples.

He also wonders, more speculatively, if impurity formation and deformation could have been the first catalyst for massive earthquakes. The idea is that these small changes cause cracks in the larger rocks that eventually slide out of place. Professor Alvaro plans to explore this further.

Professor Alvaro said: “Nobody knows what the initial impulse was, the thing that led to the first slip.” We started to think – and it’s probably quite a crazy idea – maybe this bug. A group of them, may undergo an immediate change in phase and thus a change in size. This could possibly be the first trigger.

The research in this article was funded by the European Union. If you liked this article, please consider sharing it on social media.

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