China earthquake survey doubles Beijing's high-risk fault lines
Beijing has 15 major active fault lines, more than double the previous count, according to a new geological survey commissioned by the government. A third were "strongly active", suggesting a high earthquake risk, but when the next big jolt may hit remains uncertain.
"The formation and development of most geological hazards in the Beijing plain are closely related to the activity of fault structures," the researchers said in a paper published this month by the Chinese Journal of Geophysics.
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One fracture, dubbed F1-1, was identified beneath the city's northeastern Huairou district where some of the country's most expensive research facilities are under construction, including the world's most powerful wind tunnel for hypersonic weapon development.
Also under threat, according to the study, is the thriving information technology zone in Changping, Daxing's hi-tech industry zone, and the upper-class residential area in Shunyi district.
Beijing's old town, including the Forbidden City and Temple of Heaven, were also built on or near fault lines, but these fractures were generally very old, with some having been quiet since the extinction of the dinosaurs.
The researchers found relatively low stress building up on these old fault lines, which they believed were no longer active.
Beijing, which sits on a basin with mountains on three sides, is one of the most studied areas for earthquake risk due to its political importance. The city has suffered numerous big earthquakes through history which caused massive destruction and many deaths.
The area is squeezed by opposing forces from the Eurasian and Pacific plates, with most of the stresses ending up in the active faults.
Previous surveys relied on traditional methods such as drill sampling and seismic waves to locate the fractures. The resolution was low, and the results could not tell how the stress changed over time.
Lei's team used gravity mapping to detect the fault lines and measure their stress levels. The higher the stress at a fracture, the denser the rocks become, which can cause a gravitational anomaly that can be picked up by sensitive instruments on the ground or in space.
Liu Yanhui, professor of civil engineering with Guangzhou University's Earthquake Engineering and Research Testing Centre, said old buildings sitting on fault lines could be strengthened by quake-proof technology. Various engineering approaches were now available to improve the chances of structures surviving an earthquake, he said.
The technology could also be applied to new buildings. "But for the sake of safety, building plans should avoid the fault zones as much as they possibly can," Liu said.
Beijing has experienced nearly 600 earthquakes since records began 1,700 years ago, with dozens estimated to have exceeded magnitude 7. On September 2, 1679 - at the peak of the Qing dynasty under Emperor Kangxi - an estimated magnitude 8 earthquake struck the Pinggu district in northeastern Beijing killing tens of thousands, including some government officials.
Beijing is surrounded by other major fault lines. In 1976, a huge earthquake destroyed the city of Tangshan, less than 200km (124 miles) south of Beijing. More than 240,000 people were killed, making it the second deadliest earthquake in the 20th century.
Why Beijing and its surrounding areas were so prone to earthquakes was a puzzle to scientists. The northern China plain sits on an ancient geological structure known as a craton, which should be earthquake-proof because of its stability.
In recent decades, Chinese researchers found a large amount of evidence suggesting the northern China craton is severely damaged from the inside.
How and why that damage occurred remains one of the biggest unsolved problems in geophysics.
This article originally appeared on the South China Morning Post (SCMP).
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