The area stretching across Pakistan (search) into India (search) and Afghanistan (search) is a hotbed for seismic activity that erupts each time the Indian subcontinent slams into Asia. But it's the shallow faults that make these temblors so deadly.
Saturday's magnitude-7.6 quake, centered just outside Pakistan's capital, was about six miles deep, causing buildings to sway in three nations and killing thousands as weak structures tumbled, crushing people under mounds of rubble.
"It's how close you are to where the earthquake initiated, because ground motions fall off very rapidly away from the earthquake," said Harley Benz, a seismologist who runs the U.S. Geological Survey's National Earthquake Information Center (search) in Colorado.
"Shallow earthquakes are very dangerous because they're very close to the built environment, unless they're in remote areas," he said.
In comparison, a 7.5-magnitude earthquake jolted Peru last month. He said it occurred about eight miles beneath the surface in a more remote area. Only a handful of people died and several hundred homes were destroyed.
The December magnitude-9.0 earthquake that occurred off Indonesia's Sumatra (search) island and spawned a tsunami (search) that killed more than 176,000 people in 11 countries occurred about 18.6 miles below the surface.
The crash between the Indian plate and the Eurasian plate in the latest quake triggered the temblor along a range front that extends about 1,250 miles, Benz said.
It's the same type of collision that formed the Himalayas (search), millions of years ago. As the Indian subcontinent continues to creep about 1.6 inches farther north every year, mountains are still being formed in the Himalayan, Karakoram, Pamir and Hindu Kush ranges by the uplift from the collisions, he added.
About 50 million people are at risk of encountering Himalayan quakes in this area, many of them in the densely populated capitals of Bangladesh, Bhutan, India, Nepal and Pakistan, according to research published four years ago by scientists from the University of Colorado who took an in-depth look at the region's seismic activity.
The area along India's northern border in disputed Kashmir (search) is by far the hottest spot in that country, said A.K. Shukla, director of India's Earthquake Risk Evaluation Center. He said the latest quake, unfortunately, will not be the last and may not be the largest to come.
"It's not a question of surprise, because that area is a highly seismic one," he said. "It's not surprising at all."
Saturday's quake was centered about 60 miles northeast of Islamabad in the forested mountains of Pakistani Kashmir. At least 22 aftershocks followed within 24 hours, including a 6.2-magnitude temblor.
In 1935, a magnitude-7.5 earthquake was recorded in Quetta, India, killing 50,000. In 1974, just north of the recent quake's epicenter, a magnitude-6.2 earthquake occurred, generating 5,800 casualties.