A warning system went online Thursday in Beijing to monitor sinking ground, a threat to many areas across the country.

The Ministry of Land Resources and the Beijing municipal government set up the system, which can monitor, research and control ground subsidence episodes. A citywide geological survey was also begun.

Ground sinking poses a major threat to the capital, said Beijing Vice Mayor Lu Hao. The warning system will help to ensure safety for the city and for the 2008 Olympic Games.

About 50 cities in China, covering 48,655 square kilometers in total, have sunk. The value of damage has run as high as 100 million yuan (US$12 million).

Earth subsidence is caused by depletion of water in underground aquifers, said Zhang Hongtao, deputy director of the China Geological Survey Bureau. Excessive pumping of ground water, exploration of geothermal resources and large-scale construction are all contributing factors.

The cities of Shanghai, Tianjin and Taiyuan have been the worst hit. Each has sunk by more than two meters since the early 1900s.

To date, more than 1,800 square kilometers in Beijing have been affected. The biggest drop was 722 millimeters, said an official with the Beijing Institute of Geological Survey.

The problem has damaged or destroyed about 50 locations in Beijing, including factories, buildings and ground pipelines, said Wei Lianwei, director of the Beijing Institute of Geological Survey.

In 1996, earth subsidence caused a small crack in the subway at Chongwenmen.

In June 1997, it caused a blast at the Beijing Tongzhou Dongfang Chemical Plant.

In 1998, a crack caused by sinking ground forced a rubber factory in Shunyi District to stop production. The crack is about 800 meters wide and 25 kilometers long and will eventually threaten the safety of the Beijing Capital International Airport, experts said.

Shanghai is also taking steps to control subsidence, said Zhang A'gen, deputy director of the Shanghai Municipal Bureau of Housing, Land and Resources. The city has earmarked more than 10 million yuan (US$1.2 million) to prevent and control the problem, and has established 38 monitoring stations.

(China Daily July 23, 2004)