## 3 Results
### Equivalent Source Inversion
#### 3.1.3 Time-Varying Gravity Field
Negative gravity anomalies appeared in the eastern part of the Horgos-Manas-Turpan fault, north of Urumqi City, and in other local areas, suggesting that the tectonic stress field and subsurface medium distribution in the Urumqi piedmont depression region had recovered after the earthquake. On 20 January 2018, a magnitude 4.9 earthquake occurred at the junction of positive and negative gravity changes in the Urumqi area, presenting apparent correlations with the positive gravity variation anomalies at the foothills of the North Tianshan Mountains. Figure 5c shows the gravity change pattern in the study area from August 2018 to July 2019, which presents as completely changed, with the foothills of the North Tianshan Mountains and the Urumqi piedmont depression region showing negative gravity variations and the southern Junggar Basin turning into a positive gravity change zone. On 18 August 2018, another magnitude 4.9 earthquake occurred in Hutubi; at this time the gravity field anomaly was changing from positive to negative, which further indicates that subsurface medium migration or exchange might occur in the study area. It has been frequently reported that moderate to strong earthquakes tend to occur in the four quadrants of gravity changes, anomalous regions showing alternating positive and negative gravity changes, or areas with a strong gravity gradient [1; 53; 54]. Figure 5d demonstrates the gravity change pattern in the study area from July 2019 to August 2020. Compared with Figure 5c, most of the study area was dominated by negative gravity changes, and the region with positive change was migrating southward in the horizontal direction. The degree of negative gravity variation increased in the western part of the study area, including the Cholma region, indicating that migration of subsurface materials from the west to the east was again occurring in the study area. There was an isolated point of positive gravity change in the Shihezi area, suggesting that tectonic stress might be accumulating in this region. Figure 5e shows the gravity change pattern in the study area from August 2020 to July 2021. Regions with positive gravity changes were distributed in Dushanzi, Shihezi, and Cholma and extended along the Horgos-Manas-Turpan fault to the north of Urumqi. The foothills of the North Tianshan Mountains still showed negative gravity changes. On 8 August 2020, a magnitude 4.9 earthquake occurred in Toksun County, a transition zone between positive and negative gravity anomalies. This seismic signal was once again correlated with the gravity field anomaly. Figure 5f shows the gravity change pattern in the study area from July 2021 to July 2022. The gravity field in the study was dominated by positive changes, with negative gravity variations only locally distributed in Urumqi. The enhanced degree of gravity field changes in the study area indicates that the energy level in this region increased and that tectonic stress was accumulating. The time-varying gravity field in the study area exhibited clear zoning features with smooth changes. Although the magnitude of the gravity variations was not drastic, the site values showed alternating positive and negative changes. Comparison with the seismic events and fault activities in the study area suggests an apparent correlation between the earthquake clusters and gravity anomalies. These earthquakes mostly occurred in the regions with a high gravity gradient or the anomalous zones with alternating positive and negative gravity changes [55]. Considering the regional tectonic background, it can be concluded that the study area is still at risk of moderate to strong earthquakes.