## 2 Materials and Methods
### Tectonic Background
The maximum thickness of the Mesozoic-Cenozoic sediments in the Manas region within the depression reaches 12,000 m [31]. Subsidence centers of different ages have migrated from the east to the west. The Early Tertiary subsidence center was located around Shawan and Anjihai, and the deposition thickness was the greatest around Wusu in the Late Tertiary. The subsidence center in the Quaternary had migrated to Wusu and west of this location [30]. Three sets of nearly east-west trending thrust fault-anticline belts developed within the Urumqi piedmont depression. From south to north, these are the Oigu piedmont, Horgos-Manas-Tugulu, and Dushanzi-Halaande-Anjihai thrust fault-anticline belts, separated by synclines [30]. In the following, these are referred to as the Q, M, and D thrust fault-anticline belts, respectively. The Q anticlines are parts of an arcuate structure, consisting of six anticlines in an echelon arrangement (i.e., the Oigu, Changji, Kalazha, Xishan, Qidaowan, and Nanfukang anticlines), which were formed by the northward thrusting of the Bogda block [32]. The north limb of the Xishan anticline lies in the Wangjiagou-Toutunhe area, where the Upper Pleistocene gravel bed is tilted with the folds. Its southern limb is faulted by the Xishan fault, with the Badaowan anticline folded into the Upper Pleistocene strata. The Nanfukang folds in the North Bogda Mountain in the east were formed under intense folding activities involving the Upper Pleistocene strata [33]. The M anticlines consist of the Horgos anticline, Manas anticline, Turpan anticline, and Beisantai uplift, which are associated with more intense folding activities than the Q anticlines, and the Upper Pleistocene sequences are generally deformed and folded. The hinge sections or two limbs of these anticlines are typically faulted, and these folds are all active in the Holocene. The intense folding activities have lasted from the end of the Early Pleistocene to the present, controlling the occurrence of large earthquakes, e.g., the Manas earthquake of magnitude 7.7 in December 1906 in the North Tianshan region [34; 35; 36]. The D anticlines are younger folds involving the Upper Pleistocene strata, and the Holocene active faults are developed along the anticline structures. Although the D thrust fault-anticline belt was formed much later, roughly from the end of the Early Pleistocene to the beginning of the Middle Pleistocene [37], the associated tectonic activities were very intense, with an average shortening rate of 1.7-1.9 mm\(\cdot\)a\({}^{-1}\)[30; 38]. The thrust fault-anticline belts in the Urumqi piedmont depression are mainly asymmetrical anticlines with steep north limbs and gentle southern limbs (Figure 2), with active vertical faults parallel to the fold axes on their north limbs and hinge zones and north-dipping backthrust faults on their south limbs. The activity of the multiple sets of piedmont fold belts gradually increases from the south to the north (Figure 2). These folds mostly have a thin-skinned structure with a shallow depth, forming a pattern of closely packed stripes trending east-west [23]. The study area is one of the most seismically active regions, with frequent moderate-to-strong earthquakes. It has been shown that the frequent earthquakes in this area may be caused by differential block movements [39]. Since the twentieth century, the seismicity in the study region first increased, followed by a calm period, and then increased again [23]. Two earthquakes of magnitude 7 or above have occurred in this area, with the largest one being the Manas earthquake of magnitude 7.7 in December 1906, followed by a magnitude 7.2 event at Xinyuan in 1944 and a magnitude 6.6 earthquake northeast of Urumqi in 1965. No earthquakes of magnitude six or above occurred during the last few decades, and the faults remained locked until the calm period was broken by the 2012 Xinyuan-Hejing earthquake of magnitude 6.6.