© Reuters Rodrigo Garrido
The quake hit at just before 11 pm at a depth of 25 kilometres, off the coast of Coquimbo, 230 kilometres north of Santiago in Chile.
Early reports suggest at least five people have been killed and millions have been evacuated from the nearby Chilean coast, as Tsunami warnings spread around the Pacific. Waves 4.6 metres high were seen hitting in Chile and tsunami warnings are active for California and New Zealand.
Seismologists say the event is linked to one from 2010. In February that year, a quake of 8.8 magnitude killed more than 700 people and shifted the Earth's axis.
Earthquakes happen along the coast of Chile because the Nazca plate under the Pacific Ocean is moving towards, and slipping under, the South American plate. It's moving at a rate of about eight centimetres every year. The friction between the plates creates tension that builds up, and is then released when the plates slip, creating earthquakes.
The massive 2010 earthquake released tension in a very large part of the fault, but the slipping stopped at a pile of underwater mountains called the Juan Fernández Ridge. "This one that happened this evening started just north of where the last one finished," says Kevin Furlong from Penn State University.
That meant that the section which slipped today was ready to slip, says Furlong. But it also put extra stress on the remaining fault. "Because if the one next to it moves, and it didn't, it sort of gets twisted and torqued a bit," he says.
It's unclear yet if the earthquake has relieved the stress in that region, lowering earthquake risk in the coming years. "That's probably information that will come out in the coming days," says Gavin Hayes from the US Geological Survey.
"But looking at the history of large earthquakes in the region, there was a significant earthquake on this same section in 1943 and just to the north in 1922. So it hasn't had a big event in 70 years. So it has been accumulating strain energy for a long time."
But just as this quake was waiting to happen since 2010, other parts of the Chilean fault are expected to go soon too. Just last year a powerful quake struck further north near the border with Peru, killing six people.
As Hayes, Furlong and others showed recently that quake failed to release stress building up for over a century, and added additional stress to the region just south of it. A devastating quake relieving the remaining stress could be as big as 8.9, Hayes says.
Comment: Volcanic eruptions and earthquakes increasing across the planet