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.