**1. Introduction**

A landslide is the downslope movement of soil, rock, and debris under the action of gravity and the landform that results from such movement [1]. The factors triggering slope failure mainly include rainfall, earthquakes, fluvial erosion, excavation, and construction activities [1]. Slope failure occurs frequently in rainy and mountainous areas, often resulting

**Citation:** Chen, J.; Zhang, J.; Wu, T.; Hao, J.; Wu, X.; Ma, X.; Zhu, X.; Lou, P.; Zhang, L. Activity and Kinematics of Two Adjacent Freeze–Thaw-Related Landslides Revealed by Multisource Remote Sensing of Qilian Mountain. *Remote Sens.* **2022**, *14*, 5059. https://doi.org/10.3390/ rs14195059

Academic Editors: Paolo Mazzanti and Saverio Romeo

Received: 31 August 2022 Accepted: 30 September 2022 Published: 10 October 2022

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in casualties and property damage [1,2]. Slope failure at high latitudes and high altitudes in cold regions has attracted significant attention for decades. Climate warming and the consequent varied freeze–thaw states in cold regions may accelerate the occurrence and development of landslides [3,4]. Thaw-related landslide activities (such as retrogressive thaw slumps and active layer detachment) are increasing extensively in the permafrost regions of both the Arctic [5–8] and the Qinghai-Tibet Plateau (QTP) [9–11], a phenomenon that is mainly related to extremely warm summers and extensive summer precipitation. Even in very cold permafrost regions, extreme warming events can cause slope failure due to the thawing of ice-rich permafrost [12]. However, slope failures on the boundaries of permafrost and seasonally frozen ground remain little characterized or understood.

Optical and radar remote sensing techniques are becoming one of the most important tools for investigating the activity and kinematics of landslides, especially in rural and mountainous regions [13,14]. The occurrence and development of landslides can be obtained from optical remotely sensed images [15–19]. Rapid and abrupt slope movements can be quantified according to the differences in DEM from two successive orthorectified images [20–22] or from single-pass InSAR observations [23,24]. However, optical images are prone to the effects of atmospheric conditions and revisit time, which impede the quantification of landslide activities. In addition, optical-based slope movement is insensitive to gradual slope movement or creep, which is often a sign of slope stability. Interferometric synthetic aperture radar (InSAR) has been increasingly explored and successfully applied to identify and monitor gradual slope deformation with centimeter-to-millimeter accuracy [13,25–28]. The advantage of InSAR is that it is less affected by cloudy weather and works regardless of whether day or night conditions. Recently, the development of unmanned aerial vehicle (UAV) cameras allows the generation of very-high-resolution optical images and digital surface models (DSMs). The combination of InSAR and UAV data has advanced the understanding of crack development, landslide evolution, and susceptibility [29–34].

In this study, we combine multisource optical and radar remote sensing images to investigate the activity and kinematics of two adjacent landslides (hereafter referred to as "twin landslides") located in the boundary regions of permafrost and seasonally frozen ground on Qilian Mountain in the QTP. High-resolution optical satellite and UAV-based images are used to investigate the activity of the twin landslides. InSAR measurements are used to map and quantify the gradual slope movement before and after the failure of the twin landslides. The triggering mechanisms of the twin landslides and their potential risks are analyzed.

### **2. Study Area**

In this study, we investigate two adjacent landslides occurring on the slope of the middle and lower reaches of Datong River on the southeast slope of Qilian Mountain. For convenience, we named the two adjacent landslides as twin landslides and refer to them as "QLDT01" and "QLDT02" throughout the paper (Figure 1C). The study area is located between Tuolai Mountain in the north and Datong Mountain in the south, both of which belong to branches of the Qilian Mountains. The Wari Gaqu River rises from the Tuolai Mountains and ultimately joins the Datong River. A number of brooks from the Datong Mountains = discharge into the Datong River (Figure 1A). Therefore, the study area is a place where many rivers converge. The Tuolaishan and Datongshan faults are distributed in the northwest and southwest of the study area, respectively. The altitude in the study area ranges from 3400 to 3600, which is in the lower boundary of permafrost and seasonal frozen ground on the QTP. The permafrost distribution map is shown in Figure 1B [35].

Qilian Mountain is dominated by a continental alpine semi-humid mountain climate, which is characterized by long, cold, and dry winters and short, cool, and moist summers. Annual average air and ground surface temperatures are approximately −2.4 and 2.9 ◦C, respectively [36]. The coldest January averages below −11 ◦C, and the warmest July

averages below 25 ◦C; most of the Qilian Mountains are below 0 ◦C from December to March, and the highest temperatures range from 4 to 15 ◦C from April to October. Precipitation ranges from 300 to 500 mm annually and is mainly concentrated during May to September in the form of rainfall [37,38].

**Figure 1.** Study area and field photos. (**A**) The locations of the twin landslides (QLDT01 and QLDT02) are marked by yellow dots within the black rectangle. The background map is the hill-shaded DEM. The mountains, rivers, and roads in our study area are marked. (**B**) The distribution map of permafrost and seasonal frozen ground in the QTP [35]. The black rectangle presents the location of our study area. The inner map in the bottom right-hand corner shows the location of the QTP in China. (**C**) An unmanned aerial vehicle (UAV) photograph of the twin landslides taken in April 2021. The white arrow denotes the direction of the landslide movement. The yellow polygons are the boundaries of the twin landslides.
