Shanghai sank by 7.76 millimeters last year on average, continuing a trend of declining land subsidence since 2002, engineers at Shanghai Institute of Geological Survey said yesterday.

The body said that the city had spent 35 million yuan (US$4.2 million) over recent years to build a citywide supervision network to oversee land subsidence. Its goal was to keep the annual average subsidence within 5 millimeters by 2010.

The average land subsidence in 2004 was less than 10.48 millimeters recorded in 2003 and 11.16 millimeters in 2002, engineers said.

"Once our new system notices any accelerating subsidence in a certain area, we will take emergency measures to counteract it," Yan Xuexin, the institute's chief engineer, said yesterday. He said the network comprises more than 40 "supervision stations" around the city where researchers insert steel tubes into "basic rocks" underground to measure the subsidence by observing vertical movement.

Normally, the depth of the basic rocks ranges from 60 meters to 300 meters underground.

Geologists said the city's land subsidence was primarily caused by the overpumping of underground water and the rapid construction of skyscrapers. The city's primary method to counteract subsidence is to control the pumping of underground water.

(Shanghai Daily April 6, 2005)