Aftershock warnings continue to shake up China

The death toll of the May 12 magnitude-7.9 earthquake that devastated China’s Sichuan province has continued to rise, with 40,000 people dead, more than 32,000 still missing and some 5 million people left homeless, according to Chinese authorities. Meanwhile, aftershocks continue to rattle the region.

As rescue workers continued to search the rubble for survivors, the region was sent into a new panic this week following a public warning about further quakes. The statement, issued on Sichuan Television Monday night by the Sichuan Seismological Bureau, warned that a strong, magnitude-6 to -7 aftershock would strike the same region in the next day or two. The announcement caused widespread alarm, as thousands of residents of Chengdu, the capital of Sichuan province, fled their homes and camped outside, fearing that another devastating earthquake was imminent, according to the London Times Online.

Sichuan has already been battered by thousands of small to powerful aftershocks, including a 5.9 quake that shook the region Friday and temporarily disrupted rescue efforts.

The warning of an even more powerful aftershock, however, should have been just that — a warning, not a prediction, scientists say. There is certainly an increased likelihood of strong aftershocks following a strong earthquake, which could be as large as “a magnitude less than the main shock,” says David Applegate, head of the U.S. Geological Survey’s (USGS) Earthquake Hazards Program. But, he says, geologists cannot predict so precisely where and when such an aftershock might occur.

What they can do is estimate the hazard. In California, for example, USGS created an aftershock probability map, updated hourly, that gives the likelihood of strong shaking anywhere in the state for the following 24 hours. “It’s not a prediction as such, but this is our state of understanding,” Applegate says.

“My guess is that they are just relying on the fact that it is common to have aftershocks that are one order of magnitude lower than the main shock,” says Kerry Sieh, a seismologist at Caltech in Pasadena, Calif. “This is reasonable, it seems to me, although the specific timing of large aftershocks would not be predictable.”

Indeed, on Tuesday, news reports suggested that China's National Seismological Bureau had issued a statement in response to the panic, noting that while the likelihood of a powerful aftershock is heightened, “any ‘prediction’ of an earthquake at a certain time and in a certain place is certainly a rumor," as it is currently impossible to predict earthquakes with that level of accuracy.

The Sichuan earthquake was the result of motion along a fault known as the Longmenshan Fault, which is in a “massive crumple zone” marking the collision between the India and Eurasian plates, Applegate says. In 2007, a team of Chinese and U.S. scientists published their assessment of the seismic hazards from the Longmenshan and other faults in the region in the journal Tectonics. Although there have not been many powerful earthquakes there in historic times, the authors wrote, geologic evidence of earlier activity on the faults in this region suggests they could represent “a significant seismic hazard to the densely populated Sichuan Basin.”

Carolyn Gramling

Links:
“Quake rocks central China," Geotimes online, Web Extra, May 12, 2007 
ShanghaiDaily report on Sichuan Seismological Bureau announcement
Times Online story
USGS 24-hour aftershock forecast map

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