## 5 Discussion  
Before this study, land subsidence measurements were almost absent in the LZP. With three kinds of SAR images acquired from 1992 to 2010, we achieved the first deformation map of the whole peninsula with a modified stacking method. The InSAR measurement indicates that the land subsidence is mainly distributed along the coastline and the arable lands with the maximum velocity as 32 and 19 mm/year, respectively. The land subsidence evolution in Zhanjiang city moved from urban areas to suburban areas with an increasing trend along the coastal areas. We can say with proper evidence that the land subsidence in the LZP will continue if the groundwater extraction is not regulated. Similarly, land subsidence caused by over-extraction of groundwater occurs in many delta areas in China, such as the Yellow River Delta and the Yangtze River Delta [37; 38]. Thus regular land subsidence monitoring using the InSAR technique is very necessary to regulate groundwater extraction and land subsidence. Land subsidence is a common problem in most peninsulas and delta areas over the world [39]. Similar to the LZP, over-exploitation of groundwater is also the main reason for land subsidence in the Chalco basin located southeast of Mexico City and Jakarta in Indonesia (see Table 3). The obvious difference is the speed of groundwater extraction and the water level declining in the urban areas, although the use of groundwater in these cities is similar. The extraction speed and drawdown velocity of groundwater in the three cities are 7.8 m\({}^{3}\)/s, 1.5 m/year, 1.5 m\({}^{3}\)/s, 0.1-1.9 m/year and 6.7 m\({}^{3}\)/s, 0.1-0.8 m/year, respectively [4; 8; 10; 40; 41; 42]. So we can see that the groundwater level in LZP is comparatively steady although the groundwater extraction speed is high. Interestingly, the maximum velocity of Mexico City and Jakarta exceeds that of the LZP by an order of magnitude (see Table 3). Based on these differences, we can conclude that the following two factors will help understand the mechanism of surface deformation: (1) Geology and aquifers. The LZP has a wide range (\(\sim\)3694 km\({}^{2}\)) of a mixture of confined basalt and volcanic, which has stronger water storage capacity. The strong water storage capacity can promote leakage between aquifers. More importantly, lots of dead craters located at these areas with local depth up to 310 m, can directly reach to the deep aquifer and help leakage between the shallow and deep aquifers directly as described in Section 2. Furthermore, the LZP has much thicker aquifer (500 m) than the other two places. (2) Compressible sediments determine the land subsidence velocity in agricultural areas. The subsidence of agricultural area in the LZP (<25 mm/year) is much smaller than that of the other two places (>100 mm/year), although the former has a larger agriculture area (see Table 3). In Mexico City and Jakarta, the agricultural lands are mainly located above the highly compressible sediments, which leads to the compaction of aquifers and finally causes land subsidence. In the LZP, the compressible sediments are mainly distributed along the coastline \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Comparison** & & & & & \\
**(mm/year)** & & & & & \\ \hline Leveling1989–1999 & \(-5\) & \(-3.2\) & \(-3.6\) & \(-2.5\) & \(-3.1\) \\ InSAR\_JERS1992–1998 & \(-7\) & \(-5\) & \(-6\) & \(-4\) & \(-5\) \\ Difference & 2 & 1.8 & 2.4 & 1.5 & 1.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison between Interferometric Synthetic Aperture Radar (InSAR) and leveling results. with large thickness, whereas in agricultural lands, especially in Xuwen, the thickness is less than 10 m, see Figure 2b. Furthermore, wide range arable lands overlie the thick basalt and volcanic whose compressibility is low. Because of this, the LZP has luckily enjoyed comparatively slow subsidence.