The unusually high magnitude of the 2008 Wenchuan earthquake was caused by the unexpected failure of solid rock separating different fault segments, suggests a study published online in Nature Geoscience. The magnitude 7.9 quake struck along a number of smaller faults that are separated by solid rock; these solid-rock barriers generally prevent multiple faults from rupturing.
Zheng-Kang Shen and colleagues used Global Positioning System and satellite radar data to show that the quake motion was focused at the barriers. Three prominent barriers failed one after the other during this quake, which allowed widespread motion throughout the fault zone. The researchers point out that the highest number of fatalities occurred in towns near the barriers, which they suggest is due to particularly strong quake motion in these regions.
The team estimates that similar magnitude earthquakes are uncommon along this fault zone, occurring once in about 4,000 years.