## 5 Discussion
### Correlation between Land Subsidence and Faults  
Fault-related subsidence has been recorded worldwide, such as in some regions of China, and Houston, USA [6; 7; 39; 40; 41]. The faults in YRD are composed of the Yishu fault (NNE 10\({}^{\circ}\)-30\({}^{\circ}\)) and its secondary faults, including the Shangwujing fault and Yidu fault (Figure 7a). The Yishu fault zone is conjugated spatially with its secondary faults, where earthquakes mostly occurred [42; 43]. Figure 7a demonstrates a strong correlation between the land subsidence and fault distribution in the YRD region, which indicates that land subsidence in YRD was affected by the localized faults significantly.  
Figure 6: Horizontal east-west and vertical deformation time series for feature points (i.e. points **a**–**i**), whose positions are shown in Figure 5a.  
Figure 7: Land subsidence rate map over YRD and the deformation results along three profiles. (**a**) Land subsidence rate map, where black lines indicate the surface projections of faults. Graphs (**b**), (**c**) and (**d**) the deformation rates along the profiles aa’, bb’ and cc’, whose positions are labeled on (**a**), respectively. The locations of the faults are adopted from [44].  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**City** & **Location** & **Lat/Lon** & **Corresponding** & **Period** & **Maximum** & **Size** & **(km2)** & **Cause** \\ \hline Dongying, \begin{tabular}{c} YRD \\ (coastal \\ area) \\ \end{tabular} & / & A, B, C, D, E, F & 201204–201907 & 29 & 522 & \begin{tabular}{c} Brine \\ withdrawal \\ \end{tabular} \\ \hline Yantai, \begin{tabular}{c} Dingga Bay, \\ Qingdao \\ \end{tabular} & \begin{tabular}{c} Dingzi Bay, \\ Jiaozhou \\ Bay \\ \end{tabular} & N36.6E120.8 & H, I & 201610–201904 & 7 & 166 & Aquaculture \\ \hline Yantai, \begin{tabular}{c} Qingdao, \\ Rizhao \\ \end{tabular} & / & N37.6E120.2/ & \begin{tabular}{c} N37.6E120.2/ \\ N35.6E119.7/ \\ N35.3E119.5 \\ \end{tabular} & G, J, K & 201610–201904 & 6 & 15 & \begin{tabular}{c} Land \\ reclamation, \\ Sediment \\ consolidation \\ \end{tabular} \\ \hline Dongying \
\begin{tabular}{c} Guangrao \\ \end{tabular} & N37.05E118.4 & / & 201610–201907 & 14 & 730 &
\begin{tabular}{c} groundwater \\ withdrawal, \\ fault \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 2: List of localized land subsidence and causes.  
Three profiles, perpendicular to the faults, are chosen to analyze the spatial correlation between land subsidence and faults, where profile aa' is located in Region F, crossing the Yishu fault, and profiles bb' and cc' are located in Guangzhou, crossing the Yidu fault and Shangwujing fault, respectively. Deformation rates along three profiles are shown in Figure 7b-d, from which it can be seen that different displacements are observed across three faults. Based on the InSAR measurements, the differences between the mean deformation rates on two sides of the faults from 2016 to 2019 are 40 mm/a for the Yishu fault, 110 mm/a for the Yidu fault and 70 mm/a for the Shangwujing fault.  
In general, the fault constrains the flow of groundwater, the extraction of groundwater can cause a decline in pore stress within the groundwater reservoir and alters the stress state near the fault section, which therefore aggravates the localized subsidence [6; 41].