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Rock crystals from depth give microscopic clues to Earth’s seismic movements

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The stresses from these faults—small enough to disrupt the atomic building blocks of the crystal—could change how hot rocks move under the Earth’s crust, thus transferring pressure back to Earth’s surface, starting with the countdown to the next earthquake.

The new study, published in Nature Communications, is the first to map crystal defects and surrounding force fields in detail. “They are so small that we have only been able to observe them with the latest microscopy techniques,” said lead author Dr David Wallis from Cambridge’s Department of Earth Sciences, “but it is clear that they can greatly influence how deep rocks move, and even control when Where will the next earthquake happen?

By understanding how these crystal faults affect rocks in Earth’s upper mantle, scientists can better interpret measurements of ground motions after earthquakes, which provide vital information about where stress builds up — and thus where future earthquakes will occur.

Earthquakes occur when parts of the Earth’s crust suddenly slide into each other along fault lines, releasing stored energy that spreads through the Earth and causes it to vibrate. This movement is generally a response to the buildup of tectonic forces in the Earth’s crust, causing the surface to bend and eventually rupture in the form of an earthquake.

Their work reveals that the way the Earth’s surface stabilizes after an earthquake, and stores stress before the event repeats, can ultimately be attributed to small defects in rock crystals from the depths.

“If you can understand how fast these deep rocks can flow, and how long it will take to transfer pressure between different regions through a fault zone, we may be able to get better predictions about when and where the next earthquake will strike,” Wallis said.

The team subjected olivine crystals – the most common component of the upper mantle – to a range of pressures and temperatures in order to replicate conditions up to 100 kilometers below Earth’s surface, where the super-hot rocks (about 1,250 degrees Celsius) move like syrup. .

Wallis likens their experiments to a blacksmith working with hot metal—at the highest temperatures, their specimens glowed white hot and pliable.

They observed distorted crystal structures using a high-resolution form of electron microscopy, called electron backscattering diffraction, which Wallis pioneered in geological materials.

Their results shed light on how hot rocks in the upper mantle can mysteriously turn from flowing like syrup immediately after an earthquake to becoming dense and sluggish over time.

This change in thickness – or viscosity – transfers stress back to the cold, brittle rocks in the crust above, where it builds up – until the next earthquake strikes.

The reason for this shift in behavior remained an open question, “We’ve known that micro-processes are a key factor in controlling earthquakes for a while, but it was difficult to observe these subtle features in sufficient detail,” Wallis said. “Thanks to modern microscopy technology, we were able to look at the crystal framework of hot and deep rocks and see how significant these tiny flaws really are.”

Wallis and co-authors show that irregularities in the crystals become increasingly intertwined over time; They battle for space because of their competing force fields – and it’s this process that makes rocks more viscous.

Until now, it was thought that this increase in viscosity was due to competition pushing and pulling the crystals against each other, and not due to microscopic defects and their stress fields within the crystals themselves.

The team hopes to apply their work to improving seismic hazard maps, which are often used in tectonically active regions like Southern California to estimate where the next earthquake will occur. Current models, usually based on where earthquakes have occurred in the past, and where stress should build up, take into account only the most urgent changes across the fault zone and do not take into account gradual stress changes in rocks flowing deep into the Earth.

Working with colleagues at Utrecht University, Wallis also plans to apply the lab’s new constraints to models of ground motion after the serious 2004 earthquake in Indonesia, and the 2011 Japan earthquake—both of which triggered tsunamis and resulted in tens of thousands missing. of spirits.

Reference:

David Wallis et al. “Dislocation reactions in seismic creep to control olivine in the upper mantle.” Nature Communications (2021). DOI: 10.1038 / s41467-021-23633-8

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