Abstract: Previous research was focused mainly on land subsidence during decline of groundwater levels. However, there have been few studies for land subsidence during the recovery of groundwater levels. This paper analyzes the deformation of each individual aquifer or aquitard after 1998, when groundwater levels in aquifers continued to rise due to reduced pumpage and increased artificial recharge. From 1998 to 2012, the average groundwater level in the second, third, fourth and fifth confined aquifers rose on average by 2.1 m, 3.6 m, 12.4 m and 12.7 m, respectively. Because of the recovery of groundwater levels in Shanghai, the land subsidence rate gradually decreased from 12.2 mm/a in 1998 to 1.83 mm/a in 2011. And the strata below the second confined aquifer even became uplifted after 2009. Land subsidence was primarily contributed by compaction of the first soft layer with a compaction rate of 2-4 mm/a in recent years. During the recovery of groundwater level, two kinds of deformation characteristics were recognized. One is thatdeformation basically instantaneously responds to groundwater level change in the confined aquifer, and residual deformation is small. This type of deformation occurs in the first, second, third and fifth confined aquifer and the fifth and sixth aquitard. The second type is that deformation is delayed and residual deformation is obvious, but the deformation rate decreases gradually. This type of deformation occurs in the second, third and fourth aquitard. Deformation of the fourth confined aquifer is complex and includes both kinds of deformation characteristics. The large residual deformation resulted from the fact that non-recoverable skeletal specific storage is greater than the less-recoverable skeletal specific storage. The delayed deformation resulted from the delayed response of water level within aquitard to the change in the adjacent aquifers.

Key words: deformation characteristics, residual deformation, delayed deformation, land subsidence