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."
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