## 3 Results
### Equivalent Source Inversion
#### 3.1.3 Time-Varying Gravity Field  
In this study, we selected the mobile gravity observations recorded by the monitoring network in the study area from August 2016 to July 2022 and further adjusted and inverted the gravity data to analyze the dynamic changes at the annual scale in the gravity field and equivalent crustal apparent density. Figure 5 demonstrates the dynamic changes in the gravity field in the study area, with the rectangles of different colors representing the gravity differences at the monitoring stations after adjustment. The contours of the gravity field changes are also plotted. The focal mechanisms of the 2016 Hutubi earthquake, 2018 Urumqi earthquake, 2018 Hutubi earthquake, and 2020 Toks  
Figure 4: Inverted checkerboard pattern of the gravity monitoring network. Solid black lines represent faults. The squares indicate the gravity points. The black and white beach ball indicates that earthquakes of magnitude 5 or greater occurred in the study area between July 2016 and July 2022. Figure 3: Checkerboard model for testing the resolution of the gravity monitoring network in the study area. Solid black lines represent faults. The squares indicate the gravity points. The black and white beach ball indicates that earthquakes of magnitude 5 or greater occurred in the study area between July 2016 and July 2022.  
by black beach balls. Solid black lines delineate faults, and yellow circles of different sizes denote felt earthquakes during the research period. Figure 5a shows the gravity change pattern in the study area from August 2016 to August 2017. The gravity field in the entire region exhibited apparent zoning characteristics. The Urumqi piedmont degression region was dominated by negative gravity variations, and the foothills of the North Tianshan Mountains all showed positive gravity changes. On 8 December 2016, a magnitude 6.2 earthquake occurred in Hutubi. As shown in the figure, this earthquake occurred at the junction of positive and negative gravity changes, confirming the migration of subsurface materials in this region. The foothills of the North Tianshan Mountains are subjected to intense uplift, and the gravity field change should thus be negative. The Urumqi piedmont degression is undergoing mild subsidence, and its gravity  
Figure 5: Gravity field variations after adjustment. (**a**) August 2016 to August 2017; (**b**) August 2017 to August 2018; (**c**) August 2018 to July 2019; (**d**) July 2019 to August 2020; (**e**) August 2020 to July 2021; (**f**) July 2021 to July 2022. Solid black lines represent faults. The black and white beach ball indicates that earthquakes of magnitude 5 or greater occurred in the study area between July 2016 and July 2022. The solid yellow circle represents information about earthquakes that occurred in the study area from July 2016 to July 2022, with different magnitudes represented by different sizes. The red and blue rectangles represent changes in gravity differences at ground gravity monitoring points. field change should be a positive value of relatively small magnitude. However, we found exactly the opposite (Figure 5a), with the strongly uplifting foothills of the North Tianshan Mountains showing dramatic positive variations, indicating that subsurface medium migration did occur in this region. The regional tectonic stress around the epicenter had changed, and the subsurface media were migrating from the north to the south, eventually triggering the Hutubi earthquake. We believe that these factors probably constituted the main causes of the Hutubi earthquake. Figure 5b shows the gravity change pattern in the study area from August 2017 to August 2018. Compared with the pattern from August 2016 to August 2017 (Figure 5a), the regions with positive gravity changes increased throughout the study area, and the gravity field variation around the epicenter turned from negative to positive.