[BEIJING] Scientists have mapped the areas most at risk of experiencing aftershocks from the Sichuan earthquake in China.
Aftershocks often occur, sometimes months or years after a large magnitude earthquake. The Sichuan quake on 12 May measured 7.9 on the Richter scale.
Tom Parsons from the US Geological Survey and colleagues from the University of California, Santa Barbara and Penn State University used a model called 'stress-transfer analysis' to map stress changes in the faults of the Sichuan basin.
According to their results, published in Nature on 6 July, Ya'an Thrust, Xiong Po Thrust, and Xianshuihe Fault, all near Longmen Fault where the earthquake occurred, show a notable increase in static stress changes, and therefore face a high risk of aftershocks.
"The 12 May earthquake … caused grievous losses, yet its legacy includes possible large aftershocks in the near future because it increased failure stress on important faults within and around the Sichuan basin," write the authors.
But Parsons told SciDev.Net that analysing static stress only looks at long-lasting changes. Earthquakes can also be triggered by shorter 'dynamic stress changes'. These happen as the seismic waves from a mainshock pass through the Earth's crust, which results in transient changes.
Qu Guosheng, chief engineer at China National Earthquake Response Support Service, says that the accuracy of such mapping depends on the quality of the model, rather than which type of stress is used to make the calculations.
"Different people have a different understanding of how to use past data to build an effective model to predict aftershocks," Qu Guosheng told SciDev.net.
To create an automatic forecast system to judge aftershocks using mainshock information, all the faults of the areas of interest need to be programed into a model. Researchers don't always agree with each other's models, explaining why aftershocks are not often predicted, adds Qu.
Parsons emphasises that we could only exactly pinpoint where and when the next earthquake would happen by knowing the total stress in the region and the threshold at which such stress causes an earthquake.
But knowing where earthquakes are more likely to happen could help in diaster management, the authors write.
Nature doi:10.1038/nature07177 (2008)