Deep Earthquakes Could Reveal Secrets of the Earth’s Mantle

On 6 February, 2023, a powerful 7.8 magnitude earthquake and a series of strong tremors and aftershocks devastated southeast Türkiye and northwest Syria. Sadly, the death toll has passed 41,000 and continues to rise.

It turns out that most of the earthquakes we hear about on the news are relatively shallow, originating in the top crust of the Earth. But occasionally, there are earthquakes that originate deep within the Earth -- down to 724 kilometres below the surface. These earthquakes are not as well-studied as shallower ones, because they’re not as destructive to human settlements. But because they reach down into the mantle, Sunyoung Park, a geophysicist with the University of Chicago, thought they might offer a way to understand the behaviour of the mantle.

Park and her colleagues looked at one such earthquake, which occurred off the coast of Fiji in 2018. The quake was magnitude 8.2, but it was so deep – 563 kilometres down -- that it did not cause any major damage or deaths.

However, when the scientists carefully analysed the data from GPS sensors on several nearby islands, they found that in the months following the quake, the Earth was still moving, settling in the wake of the disturbance.

“You can think of it like a jar of honey that slowly comes back to level after you dip a spoon in it, except this takes years instead of minutes,” said Park.

By examining how the Earth deformed over time, Park and her team found evidence of a layer about 50-miles thick that is less viscous (that is, “runnier”) than the rest of the mantle, sitting at the bottom of the upper mantle layer. They think this layer may extend around the entire globe.

This low-viscosity layer could explain some other observations by seismologists that suggested there are stagnant slabs of rock that don’t move very much, located around the same depth at the bottom of the upper mantle.

“It has been hard to reproduce those features with models, but the weak layer found in this study makes it easier to do so,” Park said. The study has implications for how Earth transports heat, cycles and mixes materials between the crust, core, and mantle over time.

“We’re really excited,” Park said, “There’s a lot more to find out with this technique.”

From Science Daily, 22 February 2023.

• Topics:

• Natural Disasters
• Nature