## 4 Results: Spatial--Temporal Evolution of Subsidence  
### Rates of Land Subsidence  
The average vertical velocity map of the LZP was generated by the processes described in Section 3 with three kinds of SAR images acquired from 1992 to 2010. They are shown in Figure 5 with the order of their acquisition times. Figure 5a shows the velocity map of JERS. The main subsidence areas distribute along the coastline (the black dashed elongated lines). The peak land subsidence occurred in the southeast of Zhanjiang with a maximum velocity 26 mm/year. In urban areas the land subsidence occurred mainly in Zhanjiang city, with a velocity of 6-15 mm/year. Figure 5b is the velocity map of ENVISAT. Compared with the JERS data, the ENVISAT data has a much larger coverage but with comparatively poorer coherence due to the serious temporal decorrelation of the C-band satellite. There are three obvious subsidence areas shown in Figure 5b, such as the Zhanjiang city represented by red dashed rectangle, and two insets named 'Nansan Island' and 'ZhaoZiLin'. The maximum subsidence rate (19 mm/year) appears in the coastal areas of Nansan Island. In addition to the land subsidence, there are also 1 to 2 mm/year uplifts in the southwest of the LZP. For the land  
Figure 4: Exponential covariance function of velocity of each interferogram before (black) and after (red) atmosphere phase screen (APS) correction with our modified stacking method. (**a–c**) show results of the frame covering Zhanjiang city with JERS, ENVISAT, and ALOS1, respectively. Every black curve represents the statistical result of an interferogram in the stacking for each platform. subsidence from 2006 to 2010, we collected the ALOS1 images to map. As shown in Figure 5c, the main subsidence zones also distribute along the coastline (about 19 mm/year) defined by a black dashed line. Another feature of the subsidence is its discrete distribution in agricultural areas, see Figure 2c, such as farmlands in Leizhou and Suixi. Land uplift was observed in the southwest of the peninsula ranging from 1-3 mm/year, in accordance with results of ENVISAT data. Based on the results in Figure 5, we can conclude that the major area of the LZP was stable during the period from 1992 to 2010, in spite of a small scale land subsidence in the urban and agricultural areas along the coastline. The subsidence in agricultural areas was moderate with a rate of \(\sim\)19 mm/year. Furthermore, the subsidence area was spreading instead of being still during the image coverage time. For example, the subsidence center in the overlapped coverage area of JERS and ALOS1 was moving from the northeast of Xuwen to the junction areas of Leizhou and Xuwen. Also, the subsidence rate and range in urban areas were changing. This is further discussed in Section 4.2. The subsidence in coastal areas, increased both in rate and range, especially the area marked with black dashed lines (from 8-24 to 12-32 mm/year) in Figure 4a,c. This subsidence area is general consistent with that of the compressible deposit (Q4) distribution in Figure 2a. In order to quantitatively analyze the relationship between the depth of compressible sediments and land subsidence [28], we generated the distribution of recent soft soil thickness of the Holocene age with 22 boreholes by the method of [29], see Figure 2b. The statistical results of mean subsidence rate and pixel numbers of ALOS1 are shown in Figure 6 due to a more complete coverage compared with the other two satellites. We can see that the majority of the area is underlain by soft-soil thicknesses of 15-20 m, and the mean vertical velocity is growing with the increase of thickness, which agrees with the results of [30], i.e., maximum average deformation velocities are found on the thickest soft soils (>35 m), conversely, the mean velocity values calculated for the thinnest soft soils are satisfactorily within the precision range. The relationship between these two factors can help to interpret the mechanism of land subsidence.