The analysis, done by a team of geoscientists at MIT (Massachusetts Institute of Technology), shows that the quake resulted from faults with little seismic activity, and that similar events in that area occur only once in every 2,000 to 10,000 years, on average. Hough Clark Burchfiel, Schlumberger Professor of Geology, and Leigh Royden, professor of geology and geophysics in the Department of Earth, Atmospheric and Planetary Sciences at MIT, have been doing extensive research in that region of China and the Tibetan plateau for more than two decades, they had found no hints that suggested such a large earthquake might strike the area.
The team operated an array of 25 broadband seismograph stations in this region of western Sichuan for more than a year.
"Nobody was thinking there would be a major seismological event in that area," said Leigh Royden, professor of geology and geophysics in the Department of Earth, Atmospheric and Planetary Sciences at MIT.
"This earthquake was quite unusual, and may have involved a simultaneous rupture of two separate but contiguous faults," she added.
However, the researchers caution that because earthquakes can sometimes occur in clusters, people should still be wary of another possible large-scale earthquake.
The magnitude 7.9 quake struck Sichuan province on May 12 at around noontime, which may have increased the human death toll because many people were at school, and the school buildings turned out to be especially vulnerable to collapse because of poor construction.
More than 69,000 people have been confirmed dead so far, and more than 374,000 injured, with fears of further casualties because several lakes created by rockfall dams may give way and cause sudden flooding.