A massive earthquake that killed tens of thousands in the Chinese province of Sichuan last year was a “once in 4,000 years” event, according to a paper published in Nature Geoscience.

The Wenchuan quake in May 2008 “took the local population as well as scientists by surprise”, write Zheng-Kang Shen, of the State Key Laboratory of Earthquake Dynamics, and colleagues. “Although the Longmen Shan fault zone—which includes the fault segments along which this earthquake nucleated—was well known, geologic and geodetic data indicate relatively low deformation rates.”

However, by analysing GPS and radar data the researchers found that three different rock structures between different segments of the fault all failed one after another. “These connecting structures may represent barriers that rarely fail, and would fail only when high stress has accumulated after multiple rounds of smaller events broke the adjoining individual segments,” they write.

These three barrier regions corresponded to the areas of maximum damage at the towns of Yingxiu, Beichuan and Nanba. Such failures should only occur every 4,000 years, the team estimates.

“You really have to accumulate enough elastic energy to have them rupture through – but once rupture starts, it would rupture a series of barriers to get a cascade style,” lead author Shen told AP.

For more on the Sichuan quake, see Nature’s May 2009 news feature: The sleeping dragon.

Image: Alex Witze