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While the coastal city Shanghai is sinking at a slower rate than in the 1980s, local geological experts said efforts to ease the surface subsidence in East China's Shanghai-centred Yangtze-River Delta are still urgent.

Statistics from the Shanghai Institute of Geological Survey show the surface of the city has been sinking around 10 millimeters annually since the late 1990s, the slowest rate in the past two decades and almost one quarter of the average rate between the 1920s and the 1960s.

In 2000, the surface of the city sank 12.12 millimeters, according to the institute.

Wei Zixin, chief engineer with the institute, described the fight against land subsidence as "arduous." The tendency, which can be eased but is almost impossible to reverse, still poses a threat to the city's development, Wei said.

Shanghai started exploiting its underground water in 1860 and the surface of Shanghai has been continuously sinking since 1921.

Shanghai was under sea water 3 million years ago and the city is still in danger of sea water invasion, according to the "Shanghai Geological Bulletin" published by the institute.

The main cause of the city's subsidence is the over-use of underground water, according to the institute.

The Shanghai municipal government has taken strict measures against such over-exploitation since 1995 by requiring each deep well in the city to have an official permit.

In addition, underground water usage is limited to less than 10 million cubic meters per year under the measures.

Such measures against the over-exploitation of underground water are "long-term" and require "consistent" support by the government, said Wei.

In 1996, the city government began to invest in a global positioning system (GPS) to monitor land subsidence in the city with a coverage of 700 square kilometers.

Wei also remarked on the "grim" land subsidence situation in neighboring Jiangsu and Zhejiang provinces.

"The situation there is even more severe than that in Shanghai," said Wei.

China Environmental News reported earlier this year that experts think Shanghai suffers a loss of more than 10 million yuan (US$1.2 million) for every millimeter it sinks.

The country's total land subsidence losses are estimated to exceed 100 million yuan (US$12.1 million) annually, according to the report.

(China Daily September 10, 2002)