## 2 Materials and Methods  
### Gravity Monitoring Network in the Middle Segment of the North Tianshan Mountains  
Mobile gravity monitoring has been conducted by the Seismological Bureau of the Xinjiang Uygur Autonomous Region since the 1980s, with two periods of repeated observation at each station every year. The gravity monitoring network in Northern Xinjiang had been modified twice by 2015, with the number of monitoring stations increased to 97 and network coverage extended to the middle segment of the Tianshan Mountains and the Yili Basin. This enhanced the effectiveness of surface gravity measurements and allowed for better capture of the gravity anomalies in the Xinjiang region. The study area (Figure 1) is located at the southern margin of the Junggar Basin and the middle segment of the North Tianshan Mountains, and features complex geological structures and frequent seismic activities [19]. The monitoring network covers densely populated cities, including Urumqi, Changji, Huttubi, Shihezi, and Dushanzi, which has enhanced the seismic monitoring capability of the region and laid a solid foundation for obtaining data to reveal the spatiotemporal variation characteristics of the local gravity field. Mobile gravity observation in the study area was carried out using CG5 and CG6 high-precision gravimeters, and the personal digital assistant (PDA) system provided by the Institute of Seismology, China Earthquake Administration, was used for data recording and processing. The observations featured a self-deviation within 25 \(\times\) 10\({}^{-8}\) m\(\cdot\)s\({}^{-2}\), a mutual deviation within 30 \(\times\) 10\({}^{-8}\) m\(\cdot\)s\({}^{-2}\), and a unit weight variance of 10.0 \(\times\) 10\({}^{-8}\) m\(\cdot\)s\({}^{-2}\), and therefore fulfilled the specification requirements. In the classical constrained adjustment calculations, segment differences with large errors were rejected or down-weighted. After determining the instruments' precision, the measurement results were re-weighted accordingly to ensure data accuracy. The mobile gravity records were then gridded to obtain the dynamic spatiotemporal variation patterns in the study area.  
Seismic activities are frequent in the study area, with more than one hundred felt earthquakes occurring during our research period. The largest event, with magnitude 6.2, occurred in Huttubi on 8 December 2016. Here, we used mobile gravity observations from August 2016 to July 2022 to explore gravity field changes at the annual scale during these six years. We performed equivalent source inversion based on the time-varying gravity field to obtain the characteristics of the crustal apparent density distribution in the study area and further explored the physical variation features of seismic sources within the crust. Figure 1 was drawn by the Generic Mapping Tools (GMT), in which the topographic relief data are the data of GMT, and the seismic data are from the China Earthquake Networks Center (CENC). Fault data and ground gravity point data are from the Earthquake Agency of the Xinjiang Uygur Autonomous Region.  
Figure 1: Seismological gravity monitoring network in the middle segment of the North Tianshan Mountains. Solid black lines delineate faults; the black beach ball symbol shows the focal mechanism of the 2016 Huttubi earthquake; red circles are felt earthquakes during the last decade; yellow triangles are the gravity observation stations; solid blue lines are gravity traverses. The solid cyan line marks the location of the geological profile.