Why Was South Asia Hard Hit by Major Quake?

Why Was South Asia Hard Hit by Major Quake?

John Roachfor, National Geographic
Photo Gallery: Earthquake Devastation in Kashmir >>

The magnitude 7.6 earthquake that shook a broad swath of South Asia on October 8 resulted from the same forces that give rise to the world's tallest mountains, the Himalaya, experts say.
The Earth's crust is broken up into a jigsaw puzzle of plates constantly on the move. Some collide, others drift apart. They all jostle along in fits and starts like uncomfortable strangers in a packed crowd.

In the worst-hit nation, Pakistan, the Indian continental plate to the south is trying to subduct, or dive beneath, the Eurasian plate to the north, said Robert Yeats, a geologist at Oregon State University in Corvallis who studies the region.
This ongoing collision forces the Earth's crust to buckle, producing the Himalaya, Karakoram, Pamirs, and Hindu Kush mountain ranges.

Shallow Faults

The compression of the plates also creates a sinuous array of smaller faults in the upper layers of the Earth's crust, Yeats said. Movements in these shallow faults—known as thrust faults—are responsible for devastating earthquakes.

"The bad thing about crustal earthquakes is [that] when an earthquake [originates] so close to the surface, it just produces very, very strong ground motion," he said.

The October 8 earthquake originated only about 6.2 miles (10 kilometers) deep. Because it was so shallow, the shaking forces were much greater than similar-magnitude earthquakes that occur deeper in the crust, Yeats added.

The shaking triggered massive landslides that buried villages carved into the steep mountainsides. The estimated death toll from the quake currently stands at about 40,000 people.

To build on the rugged Himalayan terrain, villagers dig into a mountain and pile the dug-up dirt and rocks below to create a flat spot.

"What that does is steepen the mountain even more than it originally was. A little shake and the uphill side of the road will likely have rocks that come loose and fall, and the downhill side just slides away completely," said Wayne Pennington, a geologist at Michigan Technological University in Houghton.

Active Region

The Indian subcontinent, which includes India, Pakistan, and Bangladesh, creeps northward at a rate of about 1.6 inches (40 millimeters) per year. It slammed into Eurasia about 50 million years ago, beginning the uplift of Earth's crust that produces the region's lofty peaks.
The collision of continental plates, Pennington said, tends to produce shallower faults than when the crash involves a plate that forms part or all of an ocean basin.
"An oceanic plate consists of more dense material, so it slides beneath the continental plate much more willingly," he said. "When two continental plates collide, neither wants to be the one that goes underneath the other."

As a result, a highly active region of shallow thrust faults arc across the Himalayan foothills in the cross-border Kashmir region, which extends across northern India and northern Pakistan.
Earthquakes occur "fairly routinely" along the entire length of the plate boundary, Pennington said. But at any given location sizeable earthquakes can be several generations apart, "so there's very little oral history of them."

The October 8 earthquake occurred in a region that was long overdue for a major temblor, according to a note posted on the Web site of seismologist Roger Bilham of the University of Colorado at Boulder.

Bilham is currently en route to Pakistan. According to his note, the last major earthquake to hit the region was in 1555, though scientists lack sufficient information to determine its magnitude.
Other major quakes that have struck in the region include a magnitude 7.5 quake in northeast Afghanistan in 1842; a magnitude 7.8 quake in Kangra, India, in 1905; and a magnitude 7.5 earthquake in 1935 in Quetta, Pakistan, which killed at least 50,000 people.

Future Quakes

Seismologists cannot say for certain whether the October 8 earthquake increased stress on nearby faults, making another major earthquake more likely.
Nevertheless, major crustal earthquakes tend to have lots of aftershocks, Yeats said. This case is no exception: More than a hundred have shaken the region since Saturday. Several dozen aftershocks have been greater than magnitude 5.

"A year from now we'll still see clusters of earthquakes around the epicenter of this one," Yeats said. "They will be smaller and will fall off in number and magnitude over the next few months."