Frequent Glacial Hazard Deformation Detection Based on POT-SBAS InSAR in the Sedongpu Basin in the Himalayan Region

Abstract

The Sedongpu Basin is characterized by frequent glacial debris movements and glacial hazards. To accurately monitor and research these glacier hazards, Sentinel-1 Synthetic Aperture Radar images observed between 2014 and 2022 were collected to extract surface motion using SBAS-POT technology. The acquired temporal surface deformation and multiple optical remote sensing images were then jointly used to analyze the characteristics of the long-term glacier movement in the Sedongpu Basin. Furthermore, historical meteorological and seismic data were collected to analyze the mechanisms of multiple ice avalanche chain hazards. It was found that abnormal deformation signals of glaciers SDP1 and SDP2 could be linked to the historical ice avalanche disaster that occurred around the Sedongpu Basin. The maximum deformation rate of SDP1 was 74 m/a and the slope cumulative deformation exceeded 500 m during the monitoring period from 2014 to 2022, which is still in active motion at present; for SDP2, a cumulative deformation of more than 300 m was also detected over the monitoring period. Glaciers SDP3, SDP4, and SDP5 have been relatively stable until now; however, ice cracks are well developed in SDP4 and SDP5, and ice avalanche events may occur if these ice cracks continue to expand under extreme natural conditions in the future. Therefore, this paper emphasizes the seriousness of the ice avalanche event in Sedongpu Basin and provides data support for local disaster management and disaster prevention and reduction.

1. Introduction

Mountain glaciers are an important part of the cryosphere and an excellent indicator for monitoring global climate change, as they are extremely vulnerable to even small changes in climate [1]. Climate change has caused glaciers around the world to shrink in recent decades; although there are certain regional differences, the overall retreat of glaciers leads to ice collapse in glaciers and glacial lake outburst floods, which further triggers the glacier disaster chain [2,3,4,5,6].

Equally, seismic activity can easily lead to a large amount of loose soil in valleys, which can easily lead to large-scale debris flow disasters in earthquake areas. Although extreme temperature and precipitation predominately trigger glacial lake outburst floods in the cryosphere, seismic activity has intensified the scale of debris flow in valleys [7,8,9,10].

The Qinghai–Tibet Plateau is one of the areas where glaciers are well developed beyond the North and South Poles, and climate-sensitive temperate glaciers are extremely developed [11,12,13]. Temperate glaciers have significantly retreated owing to global warming, leading to a sharp increase in glacial debris movement, which threatens the ecosystem and causes glacial hazards [14,15,16,17]. The Sedongpu Basin is located on the left bank of the Yarlung Zangbo River in Linzhi City, China, and has a developed temperate glacier. This has caused ice avalanche debris flow disasters to occur frequently in this region. At least eight river-blocking events caused by ice avalanches have occurred in the Sedongpu Basin since 1969, including the largest-scale ice avalanche that occurred in 2017 and 2018 on the back edge of the ice shelf, which caused the ice avalanche debris flow to rapidly move to the outlet of the Sedongpu Basin, block the Yarlung Zangbo River, and form a debris dam and dammed lake [17,18,19].

Therefore, it is very important to carry out long-term deformation monitoring of the frequent ice avalanche events in the Sedongpu Basin and analyze the mechanisms causing ice avalanche events. Previous studies have mainly focused on the movement mechanisms of glacier SDP1 in the Sedongpu Basin (Figure 1A) before 2019. Changes in the material sources of ice avalanches and the causes of ice avalanche chain hazards have been discussed based on optical images from multiple periods, Synthetic Aperture Radar (SAR) images, and Digital Elevation Model (DEM) data [20,21,22,23,24,25,26,27]. It has been found that multiple ice avalanche hazards occurring in the Sedongpu Basin could be linked to abnormal precipitation, temperature rise, and seismic activity [19,28,29]. However, long-term deformation monitoring of multiple glaciers and material source regions of the Sedongpu Basin has been lacking until now; in particular, it is still unknown whether abnormal deformation signals occur before an ice avalanche.

Here, we focused on the long-term surface movement in the Sedongpu Basin, particularly for the detection of abnormal deformation signals before an ice avalanche. First, Sentinel-1 SAR images observed between 2014 and 2022 were collected, and the temporal surface deformation in the Sedongpu Basin was extracted using the Pixel Offset Tracking method (POT) and Small Baseline (SBAS) techniques. Furthermore, the long-term glacier movement characteristics were discussed based on the extracted POT deformation and multiple optical remote sensing images. Finally, the mechanism of multiple ice avalanche hazards was analyzed based on the relationship between the surface deformation, temperature variation, abnormal precipitation, and seismic activity around the Sedongpu Basin.

2. Dataset and Method

C-band (wavelength 5.56 cm) IW-mode SAR images with a resolution of 13.97 m × 2.33 m (azimuth × range) captured between 2014 and 2022 by the Sentinel-1 satellite were collected. The image coverage is shown in Figure 1A and the detailed parameters are listed in Table S1. In addition, the SRTM_DEM released by the United States Geological Survey (Sunrise Valley Drive Reston, VA, USA) was used to enhance the registration accuracy of the SAR images. Furthermore, the satellite orbit errors were corrected by utilizing precise orbit data released by the European Space Agency (Paris, France) [30,31].

It is difficult to maintain Interferometric Synthetic Aperture Radar (InSAR) coherence using the InSAR technique because of the rapid movement of glaciers [32,33]. Therefore, we adopted the POT method to extract the temporal deformation of multiple Sedongpu glaciers [34,35,36,37]. The monitoring precision of POT deformation is approximately one-tenth of the image pixel resolution, which is about 1.4 m × 0.2 m (azimuth × range) for the SAR image used in this study [38]. In addition, previous studies have shown that glacier movement in the Sedongpu Basin can reach several meters or even tens per day [18,29], suggesting that the POT technique is sufficiently accurate to extract glacier deformation in the Sedongpu Basin.

2.1. Technique Flow

GAMMA software (GAMMA_SOFTWARE-20150702) was used to carry out POT processing of the collected SAR images [39,40], and the technique flow of POT-SBAS is shown in Figure 2. First, the SAR images were registered to the same geospatial location, and the systematic slope phase error was then removed from the POT deformation derived from the dense matching of the SAR images with sub-pixel precision [41]. The precision of POT deformation is dependent on the matching window size and temporal baseline length. A large matching window size can provide POT deformation with high accuracy; however, it will result in a very low computing efficiency. A small window size could improve the computing efficiency but may cause mismatching and high uncertainty in POT deformation [42,43]. Additionally, it is difficult to estimate the POT deformation from an SAR pair with an excessively long temporal baseline. Therefore, a 60-day temporal baseline and a 125 × 25 pixel matching window size were adopted in this study. Finally, the temporal deformation of the Sedongpu Glacier was calculated based on the POT-SBAS technique, which uses POT to track the single SAR pair of the glacier’s deformation and the SBAS technique for the temporal deformation calculation. The precision of POT-SBAS is about one-thirtieth of a pixel, which is even better than the POT technique [44,45,46,47].

2.2. Calculation of 3D Slope Surface Displacement Based on POT-SBAS Data

The slope gradient and aspect data were extracted from SRTM-DEM data, and the slope surface coordinate system was first constructed around the Sedongpu Basin (Figure 3). A deformation transformation model was developed for the three-dimensional deformation calculations based on the geospatial relationships, and Formulas (1)–(5) were combined to obtain the 3D deformation of the glacier in the Sedongpu Basin.

$$\left[\begin{array}{c}{D}_{LOS}^a\\ D_{AZI}^a\\ D_{LOS}^d\\ D_{AZI}^d\end{array}\right]=\left[\begin{array}{ccc}\sin {\phi }_a\sin {\theta }_a& \cos {\phi }_a\sin {\theta }_a& -\cos {\theta }_a\\ \cos {\phi }_a& -\sin {\phi }_a& 0\\ \sin {\phi }_d\sin {\theta }_d& -\cos {\phi }_d\sin {\theta }_d& -\cos {\theta }_d\\ -\cos {\phi }_d& -\sin {\phi }_d& 0\end{array}\right]\left[\begin{array}{c}{D}_N\\ D_E\\ D_H\end{array}\right]$$

(1)

(1) $0°<\beta \le 90°$

$$\left[\begin{array}{c}{D}_N\\ D_E\\ D_H\end{array}\right]=\left[\begin{array}{ccc}\cos \alpha \cos \beta & -\sin \beta & \sin \alpha \cos \beta \\ \cos \alpha \sin \beta & \cos \beta & \sin \alpha \sin \beta \\ -\sin \alpha & 0& \cos \alpha \end{array}\right]\left[\begin{array}{c}{D}_S\\ D_V\\ D_{Normal}\end{array}\right]$$

(2)

(2) $90°<\beta \le 180°$

$$\left[\begin{array}{c}{D}_N\\ D_E\\ D_H\end{array}\right]=\left[\begin{array}{ccc}-\cos \alpha \left|\cos \beta \right|& -\sin \beta & \sin \alpha \left|\cos \beta \right|\\ \cos \alpha \sin \beta & -\left|\cos \beta \right|& \sin \alpha \sin \beta \\ -\sin \alpha & 0& \cos \alpha \end{array}\right]\left[\begin{array}{c}{D}_S\\ D_V\\ D_{Normal}\end{array}\right]$$

(3)

(3) $180°<\beta \le 270°$

$$\left[\begin{array}{c}{D}_N\\ D_E\\ D_H\end{array}\right]=\left[\begin{array}{ccc}-\cos \alpha \left|\cos \beta \right|& \left|\sin \beta \right|& -\sin \alpha \left|\cos \beta \right|\\ -\cos \alpha \left|\sin \beta \right|& -\left|\cos \beta \right|& -\sin \alpha \left|\sin \beta \right|\\ -\sin \alpha & 0& \cos \alpha \end{array}\right]\left[\begin{array}{c}{D}_S\\ D_V\\ D_{Normal}\end{array}\right]$$

(4)

(4) $270°<\beta \le 360°$

$$\left[\begin{array}{c}{D}_N\\ D_E\\ D_H\end{array}\right]=\left[\begin{array}{ccc}\cos \alpha \cos \beta & \left|\sin \beta \right|& \sin \alpha \cos \beta \\ -\cos \alpha \left|\sin \beta \right|& \cos \beta & -\sin \alpha \left|\sin \beta \right|\\ -\sin \alpha & 0& \cos \alpha \end{array}\right]\left[\begin{array}{c}{D}_S\\ D_V\\ D_{Normal}\end{array}\right]$$

(5)

where $\phi$ represents the satellite flight direction angle, $\theta$ is the satellite incidence angle, $\alpha$ is the slope gradient angle, $\beta$ is the slope aspect angle, $D_{LOS}^a$ denotes the deformation in the range direction of the ascending pass, $D_{AZI}^a$ represents the deformation in the azimuth direction of the ascending pass, $D_{LOS}^d$ is the deformation in the range direction of the descending pass, $D_{AZI}^d$ is the deformation in the azimuth direction of the descending pass, $D_N$ is north–south deformation, $D_E$ is east–west deformation, $D_H$ is the deformation vertical to the ground surface, $D_S$ is the deformation parallel to the slope aspect direction, $D_V$ is the deformation vertical to the slope aspect direction, and $D_{Normal}$ is the deformation in the normal direction of the slope surface.

3. POT-Derived Deformation Field in Sedongpu Basin

Figure 4a–d show the extracted surface deformations along the azimuth and range directions of the satellite in the Sedongpu Basin. As observed in Figure 4a,b, the ascending track POT deformation is sensitive to the movement of glaciers SDP4 and SDP5. Meanwhile, the descending track POT data reveal more detailed movement of glaciers SDP1, SDP2, and SDP3. Furthermore, the surface three-dimensional deformation field (Figure 4e,f) was estimated using the method described in Section 2.2. Figure 4e,f illustrate that the Sedongpu glaciers are primarily controlled by movement along the slope with relatively smaller movement along the normal direction.

The Sedongpu glacier comprises several sub-glaciers, including SDP1–SDP5. The main glacier, SDP1, is located in the central part of the Sedongpu Basin (Figure 1A), which is characterized by a long glacier tongue and two fan-shaped material source areas located to the northeast of SDP1. Ice cracks are well developed in the S-1 material source area, and the S-2 area is characterized by sharp terrain and frequent avalanche events (Figure 1F) [29]. Glacier SDP2 is located in the east of the Sedongpu Basin, with its material source area, S-3, located at the back edge of the glacier (Figure 1G). Glacier SDP3 is located in the north of the basin, with a small material source area and insignificant surface movement. Glaciers SDP4 and SDP5 are located in the northwest of the Sedongpu Basin, and ice cracks are also well developed in these two sub-glaciers.

The information in Figure 4 indicates that the overall movement of the main glacier in SDP1 is primarily due to comprehensive slope surface movement, followed by vertical surface movement. The maximum deformation rate on the slope surface is located in the central flow area of the glacier, where the deformation rate reaches 74 m/a. During the monitoring period from 2014 to 2022, the slope cumulative deformation exceeded 500 m, and the glacier is still in active motion.

We adopted the temporal POT deformation of six points (Figure 4) to reveal the movement features of glacier SDP1. SDP1-P1, located in the material source area of S-1, showed a relatively stable movement velocity during the monitoring period and a slight acceleration after 2021 (Figure 5). This suggests that the source material area continued to move throughout the observed period, providing sufficient ice and rock material to the ice tongue of glacier SDP1. Although there is no direct evidence of ice avalanches occurring in the S-1 area, ice cracks are well developed in this region, indicating that the S-1 area may be at high risk for ice avalanche events under extreme conditions such as earthquakes and/or heavy rainfall [48].

SDP1-P2 to SDP1-P5 are located in the main detrital material flow region of glacier SDP1. SDP1-P2 exhibited a relatively large movement velocity before 22 October 2017, after which the motion signal suddenly decreased but then tended to increase again after June 2021 (Figure 5). The temporal deformations of SDP1-P3 to SDP1-P5 were characterized by an acceleration–deceleration–acceleration tendency before 30 November 2018, followed by a long inactive stage until June 2021, after which another deformation acceleration was detected from June to September 2021. It is worth noting that the optical remote sensing images provide evidence of glacier movement during the period of sudden changes in movement velocity. In particular, ice motion acceleration can be observed before the previous ice avalanche event [20,29].

SDP1-P6 is located on the left side of the glacier SDP1 ice tongue, with a sudden increase in deformation on 17 October 2018 and June–September 2021, but tended to remain stable during the rest periods. The optical remote sensing images (Figure 6i–k,m–o) show significant material loss in both SDP1-P6 and the material source area of S-2 when abnormal deformation signals were observed. This suggests that the aforementioned abnormal deformation of SDP1-P6 may have been caused by the ice avalanche event that occurred at the back edge of the material source of S-2.

Figure 4 demonstrates that the descending track InSAR is more sensitive to the surface motion of SDP2. The glacier movement of SDP2 is primarily in the range direction, with a maximum deformation rate of 57 m/a. The cumulative slope deformation exceeded 300 m during the monitoring period. The temporal deformation of SDP2-P1 (Figure 5) shows no obvious surface motion before 31 December 2018, after which the deformation velocity significantly increased, returning to a stable state after 16 August 2019. Additionally, Figure S8c,d suggest that a glacier avalanche occurred at the back edge of SDP2 from 25 November 2018 to 30 December 2018, and it is hypothesized that this glacier avalanche event caused the continued motion of SDP2-P1 until 16 August 2019. SDP2-P2 is located in front of SDP2-P1. Figure 5 shows that surface deformation occurred in SDP2-P2 between 16 August 2019 and November 2021. It is clear that SDP2-P2 began to move when the motion signal disappeared in SDP2-P1. This suggests that the surface motion caused by the glacier avalanche event at the back edge of SDP2 transferred from SDP2-P1 to SDP2-P2, with ice and rock materials then moving into the main stream of the Sedongpu Basin (Figure 1 and Figure 5).

Figure 5e and Figure S9a,b show that the motion velocity of glacier SDP3 is relatively small, and no abnormal deformation signal was observed during the observed period. This suggests that glacier SDP3 remained in relatively stable motion until August 2022. In addition, the temporal deformation of SDP4-P1 and SDP5-P1 exhibited a significant seasonal variation, with motion velocity slightly increasing from June to September each year. This was likely caused by the periodic freeze–thaw process in this area [17,49]. Moreover, ice cracks are well developed in SDP4 and SDP5 (Figure S9e–i), and ice avalanche events may occur if these ice cracks continue to expand in the future [48].

4. Discussion

Meteorological and historical seismic data were collected to discuss their effects on the frequent ice avalanche events in the Sedongpu Basin.

Figure 7 shows the daily meteorological changes around the Sedongpu Basin from 2014 to 2022. The average temperature was about 9 °C, with most winter temperatures falling below 5 °C. By analyzing the trend line of temperature change, the linear growth rate of temperature from 2014 to 2022 exceeded 0.2 °C/a, which is higher than the global average and has significantly contributed to the glacial retreat (Figure 8) [26]. The Sedongpu region is located in the southeast of the Qinghai–Tibet Plateau and has had a faster warming trend than other regions since 1960. The continuous temperature rise in the Sedongpu region further increases the sensitivity of glaciers to climate change, which easily leads to glacier instability [50,51,52]. In addition, precipitation is mainly concentrated from March to September, and the daily precipitation in 2020, 2021, and 2022 was relatively high, reaching more than 15 mm/d on average, and some days reaching over 30 mm/d. Moreover, the summer precipitation increased significantly after 2018; an increase in rainfall causes glaciers to absorb heat and triggers ice ablation, which reduces the stability of the glacier and allows rainwater to enter the bottom of the glacier through the surface channel, thus accelerating the glacial movement.

The Sedongpu Basin belongs to the Eastern Himalayan tectonic region, located at the subduction front of the Indian Plate and the Eurasian Plate. The well-developed active structure in this area has led to many major seismic events [21]. Figure 9 shows historical earthquake events within 300 km around the Sedongpu Basin between 2014 and 2022. During this period, 129 earthquakes occurred, of which 91 had a magnitude below 4, 32 had a magnitude of Ms 4–5, 3 had magnitude of Ms 5–6, and 2 were greater than magnitude 6. The blue dashed line box in Figure 10 highlights the earthquake-intensive area within 30 km around Sedongpu Basin, which includes a total of 25 earthquakes. Compared to the southern bank of the river, the northern bank experiences more earthquakes, with most occurring near the Sedongpu Basin. In addition, previous studies have shown that frequent earthquakes disrupt the equilibrium state of the glacier. Under the continuous disturbance of earthquakes and aftershocks, glaciers become highly vulnerable to bedrock rupture caused by seismic activity. This allows surface meltwater to flow into the bedrock interface, further reducing the stability of the glacier [53].

Figure 10 shows a sharp increase in temperature in 2017, followed by a significant rise in both annual temperature and precipitation after 2019. The accumulated precipitation from 2020 to 2022 was more than twice that of previous years, reaching over 2500 mm/yr. Figure 9 and Figure 11 both show historical earthquake events around the Sedongpu Basin between 2014 and 2022. The largest earthquake, with a magnitude of Ms 6.9, occurred near the Sedongpu Basin on 18 November 2017. Following this, 14 aftershocks occurred, with the largest having a magnitude of Ms 5.0. All of these aftershocks took place within 30km of the Sedongpu Basin.

Figure 10 shows that there was a sharp increase in average temperature in 2017, which accelerated the freeze–thaw process and led to the ice avalanche disaster that occurred on 22 October 2017 [54,55]. Subsequently, the motion velocity of glacier SDP1 began to increase. In addition, the Ms 6.9 earthquake occurred on 18 November 2017, and the continuous aftershocks had a significant effect on the formation and expansion of ice cracks (Figure 12), which reduced the stability of the glacier and promoted the occurrence of the ice avalanche in Sedongpu Basin on 21 December 2017 [19,56].

Figure S10 shows a period of concentrated precipitation before the ice avalanche occurred in 2018, which highly correlates with the historical ice avalanche disaster in glacier SDP1 that year. Furthermore, after 30 December 2018, the glacier stabilized until the next ice avalanche event. The abnormally high temperatures may have accelerated the freeze–thaw process, and the increasing summer precipitation likely further accelerated the melting of the Sedongpu glacier due to rising annual temperatures in high-elevation areas. Therefore, the sharp increases in both temperature and precipitation after 2019 likely triggered the ice avalanche that occurred between June and September 2021. Finally, it can be confirmed that the multiple ice avalanche hazards in the Sedongpu Basin are linked to local abnormal precipitation, rising temperatures, and seismic activity.

5. Conclusions

In this study, we extracted temporal glacier motion information based on SAR images between 2014 and 2022 captured by the Sentinel-1 satellite in the Sedongpu Basin. In addition, InSAR deformation, optical imagery, and meteorological and historical seismic data were jointly used to investigate the cause of the glacier chain hazard in this area. It was found that glacier SDP1 had an obvious acceleration phenomenon in 2017, 2018, and 2021, which was linked to the ice avalanche hazards that occurred in the Sedongpu Basin; the maximum deformation rate was 74 m/a and the cumulative slope deformation reached more than 500 m. An abnormal deformation signal was found between December 2018 and November 2021 in glacier SDP2, which may have been triggered by an ice avalanche occurring at its back edge, and a cumulative deformation of more than 300 m was detected over the monitoring period. Glaciers SDP3, SDP4, and SDP5 were all in a relatively stable state; however, they may be dangerous under extreme conditions of strong precipitation and seismicity. Due to the lack of field monitoring data, the discussion part of this study was mainly carried out based on some assumptions and qualitative analysis. Therefore, it is still necessary to continue to carry out deformation monitoring work in the Sedongpu Basin, so as to provide information for glacier disaster prediction.