# Characterizing Micro-Displacements on Active Faults in the Gobi Desert with Time-Series InSAR
## 2 Study Area and Geological Setting  
Beishan lies in northwest Gansu Province in China (Figures 1 and 2), within 40\({}^{\circ}\)00\({}^{\prime}\) to 42\({}^{\circ}\)00\({}^{\prime}\) N and 96\({}^{\circ}\)40\({}^{\prime}\) to 98\({}^{\circ}\)40\({}^{\prime}\) E and covering an area of more than 12,000 km\({}^{2}\) (Figure 3). The area has a semiarid climate, where average annual evaporation significantly exceeds precipitation (average precipitation for Gansu is 37-734 mm pa). The dry climate produces ephemeral rivers and very limited vegetation growth, with the Gobi landscape characterised by flat plains and small hills with an elevation between 1400 m and 2000 m above sea level [16; 17]. Tectionically, Beishan is part of Tarim Basin, northeast of the Aerjin Fault system, and it has an annual uplift rate of 0.6-0.8 mm a\({}^{-1}\)[18]. The Aerjin Fault system (Altyn Tagh Fault, Figure 2) is a strike-slip fault zone that has been active since the early Cenozoic era but, due to the buffer impact of the Qilian Mountains, which absorb most of the energy generated by the fault zone, Beishan has been described as \"the most stable region within the active zone\" [19]. This is the reason for the preselection of this region as a potential HLW repository. The regional in situ stress analyses indicate that the maximum horizontal stress is oriented NE-SW, explaining the NW-SE trend of the reverse faults in Figure 2.  
Figure 1: Location of Beishan area in Gansu Province, northwest China (highlighted by red box). Figure 2: Structural geology map around Beishan [20] highlighting the locations of Beishan, Tarim Basin, Aerjin Fault System (Altyn Tagh Fault), and Qilian Mountains. The study area is indicated by the blue dashed outline, labelled Figure 3.  
Tectonic processes are the most significant factors contributing to the elevation differences within the rock sections of the area. This is mainly driven by a compressive stress field in the NE-SW direction. Based on the in-situ stress measurements at borehole BS01, the orientation of the average maximum compressive stress is N35\({}^{\circ}\)E, as shown in Figure 3, but it is understood to vary with depth [20,23]. The horizontal stress orientation is governed by the tectonic movements [24], with trends of maximum horizontal compression reflected in the displacement direction of the shifting granite regions. There are two groups of faults that divide the Jiujing region into rhombus-shaped blocks, as shown in Figure 3. The first group is a set of northeast-trending (around 50\({}^{\circ}\)N) oblique-slip faults with clear sinistral shearing properties, which includes Erdaojing Fault (F\({}_{14}\)), Xijanshan Fault (F\({}_{15}\)), Bantan Faults (F\({}_{16-1}\), F\({}_{16-2}\)), Shiyuyejing-West Fault (F\({}_{17}\)), Shiyuyejing Fault (F\({}_{18}\)), Ba'antan-West Fault (F\({}_{19}\)), and Jiujing Faults (F\({}_{20-1}\) and F\({}_{20-2}\)), which were most active in the late Pleistocene era (126,000-12,000 years ago) [16,18,25,26]. The second group includes Erdaojing-Hongqishan Fault (F\({}_{1}\)) and Jinmiaogou Fault (F\({}_{6}\)). These two east-west-trending fault zones control the south and north boundaries of the Jiujing region, respectively, and are widely considered to be south-dipping reverse faults [16,18,21,24]  
Figure 3: The map of the Jiujing granite region shows the distribution of faults, Quaternary sediments, exposed formations, and the granite, where BS01, BS02, and BS03 refer to borehole number. Information is summarised from literature [21,22]. (Table 1). Due to the complex structural geology along the fault zone, the characteristics of these two faults are unclear. The Jiujing granite region can be divided into four units, which are Jiujing (middle Proterozoic), Bantan (middle Proterozoic), Jiazijing (Silurian), and Shimenkan (Permanian) [29]. The Jiujing unit is composed of tonalite with an area of 220 km\({}^{2}\) and the Bantan unit is composed of porphyritic-monzonitic granite with an area of 53 km\({}^{2}\)[30].