## 5 Discussion  
Comparison of the Difference and Efficiency of Multi-Direction Phase Gradient Stacking and SBAS for Typical Landslide Detection  
The Suoertou landslide is an earth-flow landslide which has been active since its reactivation in the 1970s, and the entire length of the landslide body is about 3000 m [43]. A1 in Figure 7a,c and D1 in Figure 7b,d represent the Suoertou landslide, which shows a wide upper part and a narrow middle and lower part. There are two main deformation zones in the whole slope, with a significant deformation in the upper and lower parts, while the middle part is more stable. In the SBAS results, the deformation rate of the slope is about \(-81.9\) mm/yr, and according to the distribution of deformation points, the deformation is greatest in the upper part of the slope bordering with the middle part of the slope. This leads to the fact that not all of the deformation points can be detected in this area, but using this paper's method, the area is detected. This limitation is most evident in the A2/D2 landslide, where a clear signal exists throughout the landslide, but effective deformation points are difficult to detect in the SBAS results. This situation cannot be avoided during the phase deconvolution of conventional time-series InSAR methods, where deformation is too large to cause incoherence [10; 44], and therefore the only way to remove low-coherence regions is to mask them during phase deconvolution. However, the method in this paper applies differential interferograms for phase gradient calculation and stacking, so these low-coherence regions may retain signals in some interferograms, and all are able to be detected in a sufficiently large number of regions. Figure 8 depicts another significant landslide accumulation area, namely the Nanyu landslide group, known for its historical presence. While the overall slope has maintained relative stability in recent decades, localized small landslides, such as the Mentou Ping landslide and the Jiangdingya landslide [37], have emerged within certain sections. It is important to note that the Xieliupo landslide is a large landslide located north of the Nanyu landslide on an active fault line [40]. Continuous erosion at the base of the Bailong River sustains its active state. Studies indicate significant deformation within this landslide slope, posing challenges in effectively monitoring adequate deformation points [45; 46], evident in the SBAS results shown in Figure 8c,d. The SBAS method primarily captures signals at the base of the slope, but struggles to detect effective deformation points in the upper regions. Conversely, the multi-direction phase gradient stacking method, showcased  
Figure 7: Suoertou landslide (region 1 in Figures 5 and 6). **(a)** Multi-direction phase gradient stacking results of the ascending track; **(b)** Multi-direction phase gradient stacking results of the descending track; **(c)** SBAS deformation rate of the ascending track; **(d)** SBAS deformation rate of the descending track. in the ascending and descending results in Figure 8a,b, reveals activity across the entire slope. This contrast highlights the challenge faced by the SBAS in capturing comprehensive deformation signals along the entire slope. In other small landslides, such as A8 in Figure 8a,c, the landslide boundaries detected by this paper's method are in high agreement with the areas with deformations obtained by the SBAS. This is also true of D27, D28, and D29 in Figure 8b,d. Comparing the results of the method in this paper with the SBAS method, it is easy to find that the results of phase gradient stacking are positively correlated with the magnitude of deformation in the detectable region. Landslides with large deformations, such as D28, also show relatively high values in the results of phase gradient stacking. In contrast, landslides with smaller deformations, such as D27, also show relatively low values in the results of phase gradient stacking.