## 3 Results
### Equivalent Source Inversion
#### 3.1.4 Apparent Density Changes of the Sources
This phenomenon suggests that the crustal density deficit at the Urumqi piedmont depression was restored after the earthquake. On 20 January 2018, a magnitude 4.9 earthquake occurred at the junction of positive and negative gravity field changes in the Urumqi area, with an epicenter also located at the junction of positive and negative changes in apparent density. We suggest that the migration of subsurface materials altered the tectonic stress field, thereby triggering the fault activity. Figure 6c shows the apparent density change pattern in the study area from August 2018 to July 2019. The regions with positive and negative apparent density changes in the study area were completely interchanged, with the foothills of the North Tianshan Mountains and the Urumqi piedmont depression showing a decrease in apparent density and the north of Urumqi City and the Hutubi area showing an increase. During the crustal density change, a magnitude 4.9 earthquake occurred again in Hutubi on 18 August 2018. Figure 6d shows the apparent density change pattern in the study area from July 2019 to August 2020. The zoning features in the study area became more prominent, with an enhanced negative apparent density change in the Dushanzi area, and the region with an increase in apparent density shifted to the south, indicating that crustal material  
Figure 6: Equivalent apparent density variation features in the study area. (**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. migration occurred again from the west to the east in the study area. Figure 6e shows the apparent density change pattern in the study area from August 2020 to July 2021, displaying obvious zoning features. The regions with an increase in apparent density expanded around Dushanzi, Cholma, and Shihezi but shrunk around Urumqi. The degree of apparent density decrease at the foothills of the North Tianshan Mountains was reduced. On 8 August 2020, a magnitude 4.9 earthquake occurred in Toksun County, where the apparent density change was turning from positive to negative, once again corresponding to the crustal material migration feature in this region. Figure 6f shows the apparent density change pattern in the study area from July 2021 to July 2022, displaying a predominantly increasing trend, while an apparent density decrease was only locally distributed in Urumqi. This increase in apparent density in the study area suggests that the tectonic stress had been accumulating and that the energy level in the study area may be increasing. In summary, the positive and negative equivalent apparent density anomalies in the study area show zoning characteristics, and their spatial distribution pattern has been evolving and repeating. The source variation features correspond well to the seismicity pattern, and their measurement accuracy is sufficient for further studies on the magnitude of equivalent apparent density changes obtained by inversion. For our study area, the inversion results of the sources' apparent density can serve as the basis for analyzing local seismicity and determining the sources of future seismic risk.