Seismic waves are like brushstrokes on an invisible canvas. By understanding how they move through various materials, we can visualize what hides beneath our feet, from magma being generated in the crust to upwelling superheated material in the solid mantle.
Using this wizardry, a pair of geophysicists from the University of Illinois at Urbana-Champaign have found that there are “tears” in the mantle beneath the elevated, massive Tibetan Plateau. Considering that this region is one of the most seismically complex and frequently active parts of the world, this is no small fry discovery, but a looking glass into an enigmatic part of the planet’s innards.
The team explains that seismic wave data suggests that the part of the more rigid Indian upper mantle appears to have been torn into four main pieces. Writing in the Proceedings of the National Academy of Sciences, they suggest this explains not only several geologically young rifting (tectonic spreading) events, but also the genesis of several fairly deep quakes beneath southern and central Tibet.
So – what caused the tears, and what do they reveal about the past, and future, of the region?
Largely thanks to the collision of India with Eurasia around 50 million years ago – which created the Himalayas and the Tibetan Plateau – there are vast fault networks crisscrossing throughout.
It’s these faults, and their staccato movements, that have led to some truly devastating tremors. These include the 2015 disaster at Mount Everest’s Base Camp and in Nepal, as well as the 1950 Assam-Tibet quake. Both killed thousands of people.
Earthquakes can occur in plenty of ways whenever you have a fault. Perhaps one side is sliding beneath the other, or maybe they’re grinding alongside each other; either way, this movement isn’t smooth.
There’s always friction, and the constant push/pull of the region’s tectonic plates means you build up stress. Release that through sudden movement, and you’ve got yourself an earthquake.
It’s far more complicated than that in reality though, and Tibet, for one, doesn't always play by those rules.
The origins of plenty of quakes can be pinpointed by tracing the seismic waves back to their source, but they’re not always where we expect them to be. Sometimes they’re at unusual depths, far away from where we’d expect friction to occur.
The ultradeep 2015 Ogasawara quake in Japan is a great example of this. Sourced from 680 kilometers (423 miles) down, it took place far from the descending Pacific Plate, and its origins are still debated by seismologists today.
The same type of enigma applies to the Tibetan Plateau. Several earthquakes in the region were traced to depths up to 160 kilometers (99 miles), far deeper than most, but the team’s data links them with these four tears in the upper Indian mantle.
These tears are less dense than the surrounding mantle, which means that they have unique mechanical properties. This not only explains several surface features in the region, but it also helps to explain how earthquakes are generated and what damage they can do to the surface world.
So where did these tears come from? Xiaodong Song, a geology professor at the University of Illinois, told IFLScience that it’s all do with a problematic tectonic rendezvous.
The Indian Plate, moving north, encounters resistance in the form of the stronger Lhasa block. This means it advances more in the east and west, but gets somewhat stuck in the center, which creates rips in the upper mantle. The tears may also be alongside “pre-existing weakness zones, such as the Indian basement ridges.”
Far more work is required to corroborate these findings, but the discovery of the tears is good news: Plugging this new data into computer models may improve how we understand, and possibly predict the nature of, future earthquakes.
It’s important to stress the things this study does not reveal, the most important of which being that it gives us no concrete information as to when and where any future quakes will occur, nor how powerful they will specifically be. As ever, preparation is the best form of mitigation – and this latest study certainly boosts our ability to do just that.