The rate of land subsidence in this city seems to have slowed last year, geologists said.

Shanghai sank only 7.5 millimeters during the period, or 1.21 millimeters less than in 2004 and 4.8 millimeters less than in 2000, according to measurements taken by the local subsidence monitoring system.

Local government officials said they expected to keep subsidence to less than 7 millimeters this year and to less than 5 millimeters by 2010.

Yan Xuexin, an engineer at the Shanghai Institute of Geological Survey, said land subsidence was no longer a major threat to Shanghai because the authorities had taken effective steps to control the problem.

"However, subsidence is not only caused by human beings, but also by natural changes, such as rising sea levels. It is difficult for the city to completely avoid land subsidence," he said.

Shanghai has been suffering from serious land subsidence problems since early in the last century. A survey done by the Ministry of Land and Resource showed that the city had sunk by more than 2 meters since 1921.

Shanghai reportedly experienced its worst land subsidence in the 1960s, when it sank by more than 10 centimeters per year.

Even now, subsidence poses a bigger threat to Shanghai than to any other city in China.

Economic losses

According to Xinhua News Agency, an area of more than 1,000 square kilometers in the city has been affected by land subsidence. Some places have sunk by as much 2.8 meters in recent years. Land subsidence has caused economic losses worth more than 100 billion yuan ($12.9 billion) for the city.

"Subsidence was a much more serious problem a couple of years ago, when the city was sinking at a rate of more than 10 millimeters per year," said Yan.

He attributed the problem to over-exploitation of the city's underground aquifer and the rapid and dense construction of high-rise buildings around the city.

"The overuse of underground water resources is the main culprit for the subsidence," he said.

The city now pours a large amount of water back into its underground reservoirs to fill up the vacant space. The local government has also taken measures to cut down on the exploitation of underground water and has closed up some of the city's wells.

In 2003, the municipal government also passed a regulation to limit the height of new buildings and density of construction in the downtown area in a bid to alleviate land subsidence.

Last October, Shanghai passed an administrative regulation on preventing land subsidence.

It stipulates that special projects that are located in subsiding areas or whose groundwork goes deeper than 7 meters should receive a special appraisal before construction starts.

Efforts paid off

"Such efforts seem to have paid off. Shanghai has taken a leading position in the country in terms of its efforts to control land subsidence," said a source surnamed Zhang at the Shanghai Housing and Land Administrative Bureau.

"But the situation concerning the uneven rate of land subsidence in different parts of the city still poses a serious challenge for the city to tackle," he said.

According to media reports, more than 70 cities in China have land subsidence problems, representing a combined subsiding area of 640,000 square kilometers.

In addition to Shanghai, the cities of Tianjin, Taiyuan and Xi'an have all sunk by at least 2 meters. The affluent cities in East China's Jiangsu Province, Suzhou, Wuxi and Changzhou also suffer from serious land subsidence.

(China Daily February 14, 2007)