## 3 Methods
### Data Processing  
Used as an authoritative time series InSAR processing approach [37], SBAS-InSAR possesses the capabilities of yielding a high-quality differential interferogram based on flexible combinations of a certain amount of SAR images covering the study area and overcoming decoherence by setting appropriate thresholds, achieving further precise surface subsidence monitoring results. The detailed flowcharts of the SBAS-InSAR technique are shown in Figure 2.  
Figure 2: Flowchart of the SBAS-InSAR technique (formally adapted from [34]).  
Descriptions of several key steps with respect to data processing are as follows:  
1. Preprocessing: The entire data processing workflow was performed using the SBAS-InSAR module in SARscape (version 5.2.1), developed by samap SA in Cascina, Switzerland. After configuring the initial environmental parameters, all SAR images were preprocessed (including cropping and registration) to prepare for the SBAS-InSAR analysis. Note that the polarization mode was set to the \"VV\" option due to its superior resistance to interference.
2. Connection graph: As the first step of SBAS-InSAR processing, 64 SAR images are supposed to be divided into a number of interference image pairs according to the thresholds of the time and space baselines. Specifically, the more image pairs there are, the more reliable the deformation results become, and the longer the data processing takes accordingly. In this study, we set the threshold of space baseline to 45% of the max space baseline and the threshold of time baseline to 120 days. Eventually, a SAR image from 26 February 2019 was selected as the super master image and 605 interference image pairs were acquired in total (Figure 3).
3. Interferometric process: In this stage, the above-mentioned interference image pairs were all subjected to an array of interference processing. Initially, all interference image pairs were aligned with the super master image. Afterwards, the SRTM DEM covering the study area was imported as assistance data, accomplishing the steps of interferogram generation, terrain phase flattening, Goldstein filtering, coherence calculation, and Minimum Cost Flow (MCF) phase unwrapping, in consecutive order [38].
4. Refinement and re-flattening: According to the Ground Control Points (GCPs) and Precise Orbit Determination (POD) ephemeris data, this step aims to estimate and eliminate the residual terrain phase and phase ramp after unwrapping. Crucially, at least 20 GCPs distributed uniformly in the study area were supposed to be selected at zones with few deformations [39].
5. Inversion: There are two steps to SBAS-InSAR inversion [40]. In the first step, deformation velocity and residual terrain phase were evaluated, which is the core of inversion. On the basis of deformation velocities, time series deformation results were captured, having experienced the estimation and elimination of the atmospheric phase by the Goldstein filtering method in the second step.  
Figure 3: Connection graph. (**a**) Time-position plot; (**b**) time-baseline plot (the yellow dots represent the super master image, the green dots represent slave images, and the line segments connecting the dots represent an interference image pair).