## 1 Introduction  
There have been many reports about earthquakes triggered/induced by the rising reservoir water level since [PERSON] introduced the concept of reservoir earthquakes in the study of the earthquake event in the Mead Lake area of the United States [1]. According to studies, these reservoir-induced earthquakes were found in 95 locations around the world, and there were four destructive reservoir earthquakes above M6.0, such as the Koyna M6.3 earthquake in India in 1967 [2]. The construction of various special monitoring networks has increasingly enhanced the monitoring capability in many reservoir areas, which has continuously captured more and more reservoir earthquake events [3; 4; 5; 6; 7; 8]. About 1.12 km in width and 185 m in depth, covering the western part of Hubei Province and the eastern part of Chongqing City, the Three Gorges Reservoir (TGR) is the world's largest hydropower station. It has a capacity of \(39.3\times 10^{9}\) m\({}^{3}\) and a water surface area of approximately 1045 km\({}^{2}\)[9]. With the impounding process of the TGR, the water level before the dam in the reservoir area gradually rose from around 60 m to 135 m, 156 m, and 175 m. Thereafter, it fluctuated between 30 and 40 m each year. Such long-term and cyclical water level changes have profoundly impacted the gravity field, crust strain and stress fields, and seismic activity in the TGR area and its outer limit. As of the end of 2020, the Three Gorges Monitoring Area has monitored about 13,963 earthquakes above M0, including 8063 earthquakes of 0 to 0.9 magnitude, 5308 of 1.0 to 1.9 magnitude, 536 of 2.0 to 2.9 magnitude, 45 of 3.0 to 3.9 magnitude, 10 of 4.0 to 4.9 magnitude, and 1 earthquake of above 5.0 magnitude. The most intense one was the Badong county M5.1 earthquake on 16 December 2013. [PERSON] et al. studied many such seismic activities using the earthquake catalogue of small earthquakes with precise positioning that systematically gives the temporal and spatial distribution and activity characteristics [10]. The ETAS model that can detect the triggering effect of the fluid showed that the impoundment played an important role in triggering the 2013 Badong M5.1 earthquake sequence [11]. The change in Coulomb Failure Stress calculated by the coupled three-dimensional poroelastic finite-element model further suggested that the procedure or occurrence of the earthquakes was facilitated by impoundment [9]. However, aside from the tectonic reservoir earthquakes, such as the Badong M5.1 earthquake, the collapsing of a large number of mines and kasts in the area along the river also caused the earthquakes. They have no obvious seismic faults, and the vertical crustal compression created by water impoundment is the leading cause. The most direct observational data were obtained from the precision leveling observation to the ground subsidence of the reservoir area. On the other hand, as the observation distance increases, the steep mountains and complex terrains in the Three Gorges Reservoir area cause the heavy accumulation of precision leveling error [12; 13]. In contrast, the InSAR data are primarily used for monitoring the displacement of landslides in the area [14]. Therefore, we have yet to see any large-scale vertical deformation studies of the Three Gorges' crust being published. However, it is encouraging that [PERSON] and [PERSON] gave the solution integration of the change of elevation \(\delta\)h and the change of gravity \(\delta\)g using the model of gravity change caused by elastic deformation in a uniform non-weighted half-space and the principle of virtual work. Therefore, gravity observation can directly reflect the crustal movement in the vertical direction [12]. At the same time, because mobile gravity is easy to use and can quickly produce measurements, it can identify the gravitational effect of faults caused by material migration and tectonic deformation [15]. In recent years, there have been several strong earthquakes that happened after substantial mobile gravity anomalies were detected.