## 4 Discussion  
The time-varying gravity field in the study area over the past 6 years was characterized by apparent zoning of positive and negative anomalies, and the transition between anomalous zones was found to be smooth. Positive and negative gravity changes appeared alternately but at a moderate magnitude. A comparison with the seismic activities in the study area shows an apparent correlation between earthquake clusters and gravity anomalies; earthquakes mostly occurred around areas in the high strain zones, in the four quadrants of gravity anomalies, or in the anomalous regions showing alternating positive and negative gravity changes. Before the 2016 Hutubi earthquake, the time-varying gravity field around its epicenter appeared as a transition zone between positive and negative gravity anomalies. The apparent density variations obtained by equivalent source inversion are matched well with the time-varying gravity field changes. The positive and negative equivalent apparent density anomalies show a prominent zoning characteristic, with an evolving and repeating spatial distribution pattern (Figure 6). The source variation features exhibit a good correlation with the seismicity pattern. Seismic activities mostly occurred in regions with dramatic apparent density changes, such as the transition between positive and negative changes and rapid apparent density increases or decreases. The crustal apparent density inversion in this study fulfils the research objective. The examined evolutions of the time-varying gravity field and spatial and temporal crustal apparent density distribution can serve as a basis for analyzing the seismic activity and determining future seismic risk in the study area. We used the 2016 Hutubi earthquake as a case study to thoroughly explore the tectonic significance of the deep part of the study area by examining the geometric parameters of the earthquake's seismogenic fault. The fault model of the earthquake was obtained by inversion based on the SAR-derived coseismic deformation field, and the focal mechanism of this earthquake was investigated to explore the transient response of the subsurface tectonic system to the earthquake. Finally, the tectonic structures were extended to the entire study area. This was done to discuss the regional tectonic background and interpret the tectonic significance of the area's deep layers. The aim was to provide a basis for elucidating local seismogenic mechanisms and enabling early earthquake warnings. The regional time-varying gravity field was obtained by THRO to extract and analyze the long-term features involved in the earthquake preparation process. Further, the coseismic deformation field was obtained from the SAR data to invert the fault model and synthetically reflect the transient response of the tectonic system to the occurrence of an earthquake. This approach can not only make up for the shortcomings of the THRO system, which is not able to capture the transient features of earthquake events, but also provides detailed background information on regional seismic risks and tectonic evolutions for coseismic deformation studies. For example, [PERSON] conducted inversion algorithms and simulation experiments on a spherical model of the time-varying gravity field, and [PERSON] used the gravitational field changes before and after the Lushan earthquake in 2013 to reveal the transfer of deep matter. Previous studies have shown that the Junggar block is subducted beneath the Tianshan block under near north-south compressive stress, forming a multi-layered wedge-shaped thrust system in the shallow crust of the Tianshan piedmont in the study area [84]. Although earthquakes in the study area have mainly occurred on shallow blind thrust faults or fault-related folds, moderate to strong earthquakes on deep faults can also trigger ruptures at shallow depths, resulting in even more serious consequences than the mainshock. The deep blind faults and fault-related folds present in the study area should therefore also receive attention.