Afghan quake: The corner of a continental collision
Initially measured by the US Geological Survey as magnitude 7.7, the quake is now listed by the USGS as magnitude 7.5.
Even this revised assessment makes Monday's event a terribly powerful tremor. Around the world, only about 20 quakes each year, on average, measure greater than magnitude 7.0.
But the origin of the shaking was more than 200km (125 miles) below the surface - much deeper than the magnitude 7.8 quake that brought widespread destruction to eastern Nepal in April. That event was only 8km deep and was followed by many aftershocks, including one in early May of magnitude 7.3.
Strong tremors felt in Ludhiana, earthquake was 7.7 on Richter Scale with epicentre in Hindukush, Afghanistan pic.twitter.com/PItp3TM9kK
— ANI (@ANI_news) October 26, 2015
The much greater depth of Monday's quake appears to have lessened the ground shaking that it produced, although its effects were felt over a wide area.
Early reports of deaths after 7.5-magnitude earthquake strikes Afghanistan https://t.co/noZgpYEdYD pic.twitter.com/OnzoOZ0TUr
— Bloomberg Business (@business) October 26, 2015
"The rupture dimensions will be very similar, but it's very far away from the Earth's surface, so there is strong shaking but it is much less severe than for a shallow earthquake," said Prof Martin Mai, an earthquake physicist from the King Abdullah University of Science and Technology in Saudi Arabia.
READ more: Afghan-Pakistan quake: Death toll rises to 311
Monday's tremor shares an overall cause with the Nepal earthquakes in April and May: the slow collision caused by India pushing north into the Eurasian continent. But they are not directly related.
Here's what we know so far about the earthquake to hit the Hindu Kush region of Afghanistan and Pakistan https://t.co/WBqsz4rv4V
— Sky News (@SkyNews) October 26, 2015
"Those Nepal earthquakes are not directly linked to this; they did not set in train a chain of events that caused this earthquake," said geoscientist Prof David Rothery, from the Open University. "It's not part of an earthquake swarm."
It is in this rugged region that the sideways slip between India and Afghanistan meets the head-on impact of the Himalayan fault line. There are many small, interacting faults and forces pushing in different directions.
"It's a really intricate area," Prof Mai told BBC News. "This is where several plates have met, over several millions of years, and formed this really complex deformation pattern.
Fortunately, the particular area that shook on Monday has tended to see deep earthquakes, rather than shallow ones.
"This is an area in which, predominantly, the earthquakes that occur are at a depth of 100-200km, while the event of 2005 occurred in a region [300km south-east] where we see more shallow earthquakes, historically," Prof Mai said.
A narrow slab of the northward-pushing Indian continent has been forced downward by the collision and now sits almost vertically. It is being further pulled and ruptured by forces in the Earth's mantle - and those ruptures produce deep earthquakes.
"These earthquakes occurring at 200km are indicative of the processes of deformation that are occurring deep beneath the crust, as this piece of slab is being drawn down into the mantle underneath the Hindu Kush," said Simon Redfern, professor of mineral physics at the University of Cambridge.
"The earthquake today fits this pattern."
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