India, once a giant island before crashing into the underside of Asia about 40 million to 50 million years ago, continues to slide north at a geologically quick pace of two inches a year. The tectonic stresses push up the Himalaya Mountains and generate scores of earthquakes from Afghanistan to China.
In the eastern part of the region where Asia and India collide, the Tibetan plateau is pushing southeast against the flat Sichuan basin. “It’s definitely a seismic area,” said Julie Martinez, a geophysicist with the National Earthquake Information Center of the United States Geological Survey in Boulder, Colo.
On Monday afternoon, an upward thrust fault broke, generating an earthquake with a preliminary magnitude of 7.9. An aftershock with a preliminary magnitude of 6.0 followed 15 minutes later, and smaller aftershocks continue in the area.
The epicenter lay in the Longmen Shan, mountains that rise steeply to the west of Chengdu, the capital of Sichuan. B. Clark Burchfiel, a professor of geology at the Massachusetts Institute of Technology, who performed research in the area, said that section of the Longmen Shan fault would not have been expected to generate an earthquake of that magnitude.
“It’s active, but I wouldn’t have put an earthquake that big on it,” he said.
However, much of the visible tectonic history along the fault has been obliterated by landslides, and the earthquake on Monday may have occurred along a previously unknown fault, he said.
Kenneth Chang, The New York Times, May 13, 2008