China Earthquake Was Rare Type

The major earthquake that devastated China last month was something of a seismological oddity, seismologists report in a new analysis. The faults that caused the temblor rarely rumble.

More than 69,000 people are confirmed to have died from the magnitude 7.9 quake that struck China's Sichuan province around noon on May 12, leveling school buildings and other poorly-constructed structures.

The earthquake was also a complete surprise to scientists. MIT seismologists who had been operating an array of 25 seismograph stations in the region for more than a year had found no hints that a large temblor might hit.

"Nobody was thinking there would be a major seismological event," in that area, said MIT's Leigh Royden. "This earthquake was quite unusual."

The region is extremely unusual geographically, Royden said, 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 by more than two miles (about 3.5 kilometers) in a span of just 30 miles (50 kilometers).

The area is at the boundary between the Indian and Asian tectonic plates, which are engaged in an ongoing collision that has created the Himalayas and the Tibetan plateau.

In central and eastern Tibet, unlike most other areas where two continental plates are colliding, the movement of the crust is hidden. Instead of folding and faulting, the surface of the eastern Tibetan plateau is undeformed and being lifted upward by the thickening of a weak crustal layer more than 9 miles (15 km) below the surface.

The crust in this layer is flowing rapidly eastward away from central Tibet. But in the area where the earthquake occurred, it is obstructed by a major obstacle, the Sichuan Basin.

"The crust and mantle beneath the basin appears to form a hard, cold knot," Royden said, which forces the flow to "wrap around the knot." The huge elevation differences between the surface of the plateau and the Sichuan Basin provided the underlying stress that led to the quake, she added.

Similar events in the area occur only once in every 2,000 to 10,000 years on average, the researchers say, though they caution that because earthquakes can sometimes occur in clusters, residents and officials should still be wary of another possible large-scale earthquake.

The new analysis is detailed in the July issue of GSA Today, a publication of the Geological Society of America. The research was funded by the National Science Foundation.

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