Chile earthquake and aftershocks: What is causing these powerful quakes?

The powerful magnitude 8.2 earthquake that struck off the coast of Chile on Tuesday night was a particular shock to the region, but it was only one quake among around 150 significant quakes that have shook the area since mid-March. Nearly half of those are considered aftershocks of this latest big quake, but what's been causing all of these temblors?

The west coast of South America, especially along the coasts of Peru and Chile, is one of the most active earthquake zones in the world. Some of the strongest quakes we've ever recorded have happened along this zone. One in particular currently tops the charts as the most powerful ever recorded — the 1960 Great Chilean Earthquake. It struck with a magnitude of 9.5 on the Richter scale just off the coast of southern Chile, near Valdivia, and it set off a devastating tsunami that hit the Chilean coastline with 25 metre waves, and reached all the way across the Pacific Ocean with 10 metre waves striking New Zealand, Hawaii, the Philippines and Japan. More recently, in 2010, a magnitude 8.8 quake shook the region just to the south of Santiago, causing a similar tsunami to the one in 1960. This latest earthquake ranks further down the list of most powerful quakes along the Chile coastline, but it still reveals what the region is capable of producing. So, why is it so active there?

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Just off the coast of Peru and Chile is the aptly-named Peru-Chile trench, a roughly 7,000 kilometre-long region where two tectonic plates — the Nazca plate pushing east and the South America plate moving west — collide. This collision forces the denser Nazca plate downward, underneath the lighter South America plate (called 'subduction'), and the friction, crumpling of the bedrock and buildup of material along the boundary between the two plates has sculpted the coastline and lifted the immense Andes Mountains towards the sky. Along with this sculpting and building, though, come plenty of shaking and rattling as these two plates constantly rub against one another.

You can see this in action in the short video from the UK Geological Society:

Terms like 'strike-slip', 'dip-slip', 'oblique-slip' and 'thrust' tend to fly around when talking about earthquake faults. For the Nazca plate and South America plate, they have a dip-slip relationship, as the Nazca plate dips underneath and the two plates slip against each other. This results in thrusts, where the South America plate, which is above the fault, gets suddenly pushed upward. Particularly large thrusts — the like the one that caused Tuesday night's quake — are typically called megathrusts. These displace a lot of earth in a very short time, which is what causes the devastating tsunamis that typically accompany them.

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Unfortunately, even though this region of the world is so prone to experiencing earthquakes, there's still no way to tell exactly when they'll happen. The only thing that governments and residents can do is prepare themselves for when they'll happen. They know from experience that these quakes often cause tsunamis, and buildings built there these days follow strict guidelines to make them more earthquake resistant (swaying with them, rather than trying to resist the motion). This means that, although it's not perfect, the region is also one of the best protected from these disasters.

(Photo courtesy: Getty Images)

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