## 3 Results
### Tectonic Significance of the Middle Segment of the North Tianshan Mountains  
3.2.1. The Hutubi Earthquake  
Regional tectonic movements are closely related to seismicity. Analyzing the seismogenic structures and focal mechanisms of moderate to strong earthquakes in an area can help in the interpretation of the area's tectonic significance. According to the China Earthquake Networks Center, a magnitude 6.2 earthquake occurred in the Hutubi area on 8 December 2016, Beijing time, with the mainshock epicenter located at 43.83\({}^{\circ}\) N, 86.35\({}^{\circ}\) E, and with a focal depth of 6 km. The earthquake has been the subject of much subsequent research. [PERSON] et al. [57] reported out that the aftershocks were concentrated to the west of the mainshock, and they inferred, based on the focal mechanism and the spatial distribution of the aftershock sequence, that the Junggar South Margin fault was the seismogenic structure of the Hutubi earthquake. [PERSON] et al. [58] suggested that the seismogenic structure of this earthquake was a north-dipping thrust fault, probably formed by fault inversion. [PERSON] et al. [41] concluded that the focus of this earthquake was at a depth of 16.5 km, located near the Horgos-Manas-Tugulu fault; they consequently suggested that both the Hutubi earthquake and the 1906 Manas earthquake occurred on the Horgos-Manas-Tugulu fault. [PERSON] et al. [42] performed cut-and-paste inversion to obtain the focal mechanism of the earthquake and speculated that the seismogenic fault might be the south-dipping Oigu fault. As shown by the inverted equivalent apparent density changes described above, there existed an anomaly around the epicenter of the Hutubi earthquake between August 2016 and August 2017 (Figure 6a). We believe that this feature could be a direct manifestation of the crustal density change induced by the movement of the subsurface tectonic system before and after the earthquake; this is further explored below. Due to the scarcity of GPS stations in the study area, no coseismic signals of this earthquake were recorded, and the brief observation period prevented obtaining a significant transient response of the time-varying gravity field to the earthquake. Therefore, we chose to use the SAR data to comprehensively study the earthquake with the D-InSAR technique in combination with the time-varying gravity field and crustal apparent density with the aim of interpreting the tectonic significance of the entire middle segment of the North Tianshan Mountains. 3.2.2. InSAR Coseismic Deformation Field  
We employed C-band Sentinel-1A TOPS-mode SAR images from both the ascending and descending tracks provided by the European Space Agency (ESA) to obtain the line of-sight (LOS)oseismic ground deformation field using the D-InSAR technique. The parameters of the SAR images are listed in Table 1.  
We used the GMTSAR software (UC San Diego: Scripps Institution of Oceanography) for data processing and made use of ESA high-precision orbit data, SRTM DEM data published by NASA (with a spatial resolution of 30 m), and GACOS atmospheric data for orbital correction, topographic phase removal, and atmospheric phase weakening, respectively. The interferograms were processed by Gaussian filtering to raise their signal-to-noise ratio. Then, we applied the minimum-cost flow method for phase unwrapping [59] and established bilinear trend surfaces for the orbits based on the far-field data in the coseismic deformation field to remove the trend of residual phases generated by the errors in the high-precision orbital data [60]. After geocoding and phase transformation, the coseismic deformation fields from the ascending and descending tracks covering the entire study area were obtained (Figure 7). Figure 7 shows that the coseismic deformation field of the Hutubi earthquake had an irregular elliptical shape. It was located on the north side of the Junggar South Margin fault, crossed through by several fold belts.