16:02, July 04, 2008
A group of scientists from Massachusetts Institute of Technology (MIT) said in a report that the magnitude-8.0 earthquake that plagued China's Southwest Sichuan province was rare in its nature and hard to predict.
"The earthquake is rare because it has a long recurrence interval, it is unusual to see strike-slip and thrust motions combined in the same break like this, and also unusual because probably several faults ruptured along the northwestern margin of the Sichuan basin," Leigh Royden, professor of geology and geophysics from the MIT research group told Xinhua in an interview Thursday.
According to the report appearing in the July issue of GSA Today, a publication of the Geological Society of America, the quake resulted from faults with little seismic activity, and similar events in that area occur only once in every 2,000 to 10,000 years, on average.
The team has been doing extensive research in that region of China and the Tibetan plateau for more than two decades, but had found no hints that suggested such a large earthquake might strike the area.
They 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, Royden said. "This earthquake was quite unusual," and may have involved a simultaneous rupture of two or more separate but contiguous faults.
"Our study results show that this earthquake happened in a place, probably THE place, where the relative motions across faults are among the slowest in the eastern Tibetan plateau," said Royden.
"The motions at the earthquake site are very slow, probably around 1 mm per year. But the very large difference in elevation between the plateau and the Sichuan basin means that the stresses could be very high, so could explain the large magnitude earthquakes there," Royden continued.
The region is extremely unusual geologically, according to Royden, because of the very steep slopes at the boundary between the Sichuan Basin to the east and the Tibetan plateau to the west. The elevation rises sharply by about 3,500 meters over a span of only about 50 kilometers.
The motions are related to the distant India-Eurasia collision and the fact that the high part of the Tibetan plateau has a weak crust underneath that is moving east to get out of the direct collision zone, said Royden.
The area where the quake occurred is part of the boundary between two of the Earth's tectonic plates, where the Indian and Asian plates converge in an ongoing collision that has created the Himalayan mountains and the Tibetan plateau.
But in central and eastern Tibet, unlike most other areas of continental collision, much of the movement of crust is hidden from view. Instead of thickening the entire crust by folding and faulting, the surface of the eastern Tibetan plateau is undeformed and is being lifted upward by thickening a weak crustal layer more than 15 km below the surface.
The crust in this deep weak layer is flowing eastward. But, in the area where the earthquake occurred, this flowing material is obstructed by a major obstacle, the Sichuan Basin.
"The crust and mantle beneath the basin appears to form a hard, cold knot" that extends to 250 km depth, Royden said, that forces the flow to "wrap around the knot." The huge elevation difference between the surface of the plateau and the Sichuan Basin provides the underlying stress that led to the quake.
It is a rare event. "While statistically it looks like there should not be a repeat of the Wenchuan earthquake soon, earthquakes can come in clusters," Royden warned. And unfortunately, it is at least now impossible to say that there will not be an earthquake, "this is true anywhere in the world (due to the difficulty of prediction)."
"The Chinese people have to be prepared and build high quality buildings on solid bedrock to minimize the loss of life from earthquakes," Royden said.
The magnitude-8.0 quake struck Sichuan province at 2:28 p.m. (0628 GMT) of May 12, with the Wenchuan county lying at the epicenter. More than 69,000 people have been confirmed dead so far, and more than 374,000 injured.