After the completion of the Three Gorges Dam, it increases the risk of inducing an earthquake. We use the GRACE Gravity Field Model to analyze the relationship between the operation of the Three Gorges Reservoir and the change of gravity field in western Sichuan. The research results indicate that the reservoir water level and the western Sichuan gravitational field are positively correlated. In the early stage of rising water level, the change of gravity field is not apparent, and the change of gravity field gradually increases with time. Therefore, the change of reservoir water level affects the gravity field in western Sichuan. The dynamic changes of the gravity field can reflect the Earth’s material change and deformation process and are closely related to earthquakes. Consequently, the Three Gorges Dam will indirectly affect the seismicity in western Sichuan by affecting the gravity field. The research provides valuable information for studying regional reservoir earthquake disasters and supports related policy decisions.
Nowadays, water resources have become more important [1,2,3]. As the most common water retaining structure, dams have a long history and are of great significance for regulating river runoff. Half of the world’s dams are located in China, and southwest China is a vital hydropower development area . Although dams bring many benefits, dams may induce geological disasters: including earthquakes . So far, as the world’s largest water conservancy project, the Three Gorges Project’s average water level is 175 m, total storage capacity is 39.3 billion m³, total length is over 600 km, the average width is 1.1 km, and reservoir area is 1,084 km [6,7]. With the completion of the large-scaled Three Gorges Dam and the rise of reservoir water, the underground pressure of the upstream and the surrounding areas increase. As a result, the existing balance is broken, and the risk of induced earthquakes significantly increases [8,9].
Due to seismic-formative conditions and mechanisms’ complexity, randomness, and suddenness, seismic activity is considered an infinitely complex system that cannot be accurately predicted . Reservoir-induced earthquake seismicity (RIS) refers to the seismicity in or near the reservoir area induced by the reservoir water storage and release activities [9,11]. Many vital aspects must be considered in analyzing the complexity of the seismic activity .
In studying geological conditions of reservoir-induced earthquakes, Gupta summarized the typical characteristics of RIS . The tectonic stress state of the reservoir area directly determines the maximum intensity of reservoir earthquakes [14,15]. Therefore, it is not difficult to find that RIS has drawn lots of attention and been studied thoroughly. McGarr believed that the tectonic stress state of the reservoir area directly determines the maximum intensity of the reservoir earthquake. For example, a reservoir-induced earthquake has an intensity of 5, so there must be an appropriate size fault near the reservoir that can generate an earthquake of this intensity . Mao has studied RIS data for almost 50 years. In his opinion, there are many reasons for RIS, but the fault response phenomenon that induces earthquakes is evident. This phenomenon is caused by the uneven distribution and concentration of RIS. Reflected by the uncertainty of the relationship between the seismic zone and the location of the reservoir and the distribution of certain faults along the reservoir area of the seismic zone .
At present, due to the development of data processing [18,19,20,21] and deep learning [22,23,24,25], scholars have further deepened their seismological research on RIS. In terms of the characteristics of the earthquake sequence, Hainzl and Ogata  carefully studied the relationship between RIS activities and the action of the reservoir fluid and proposed the idea of using the infectious aftershock sequence model (ETAS model) to quantitatively detect the fluid-induced action. In his opinion, the combined action of internal and external stresses causes the reservoir fluid to induce earthquakes. In terms of focal mechanism, Talwani  has studied many RISs in the Carolinas of the United States and believes that the occurrence of earthquakes is related to water pressure. The pore penetration of reservoir water pressure causes reservoir water to penetrate into the focal point of the earthquake, which leads to the occurrence of reservoir earthquakes. In terms of the frequency spectrum characteristics of RIS, Jain et al.  analyzed the abnormal characteristics of the seismic stress drop of the reservoir and the abnormal characteristics of the corner frequency.
This research studies the impact of the Three Gorges Dam on seismic activity in western Sichuan using the GRACE Gravity Field Model, which can provide a new reference direction for earthquake-related research and provide reference material for earthquake prediction, which can promote the development of research in this area. In addition, western Sichuan is not within the Three Gorges Reservoir area, so this article studies the impact of the Three Gorges Reservoir on seismic activities outside the reservoir area. So far, research on the impact of the reservoir on earthquakes outside the reservoir area is relatively rare, so this article is not only for economic development. Moreover, the practical significance of earthquake prevention and disaster reduction also has essential significance in scientific research.
2 Study area and data
The Three Gorges Project is currently the largest concrete gravity dam globally . The normal storage level of the reservoir is 175 m, the total storage capacity is 39.3 billion m3, the total length of the reservoir is more than 600 km, and the reservoir area is 1,084 km2. Therefore, the Three Gorges Reservoir brings greater benefits to our lives. However, many problems may also arise. This article will conduct an in-depth study on the impact of the construction of the Three Gorges Dam on seismic activity in a certain area.
This article selects the western Sichuan region as the research area because Sichuan is the most severely affected area of geological disasters around the Three Gorges Dam. In addition, it is one of the high earthquake-prone areas in China. Most of the earthquakes in Sichuan occur in western Sichuan [30,31]. Western Sichuan is located at the junction of the three plates of the Qiangtang block, the Yangzi block, and the Songpan-Garze block. It is the middle-south section of the central axis tectonic belt of the Chinese mainland and is prone to earthquakes. We have counted the earthquake data in Sichuan and displayed it on the map, as shown in Figure 1.
The UTCSR Center releases the gravity field data selected in this study, and currently, the monthly data of the Level-2 RL05 version is commonly used by everyone . Selected 128 months of data from 2002.04 to 2013.07, of which 8 months of data such as 2002.06, 2002.07, 2003.06, 2011.01, 2011.06, 2012.05, 2012.10, and 2013.03 are missing. The bit model of the Level-2 RL05 version data is solved to the 60th order. This article uses the GSM static gravity field model. The data of this model not only cover the 60th order bit coefficients but also include the spherical harmonic coefficient error and standard deviation.
There are several assumptions about why regional or global gravity fields change [21,33]. One of the assumptions is that the crustal stress changes cause the vertical changes of observation points; the change of the stress field causes the change of the density of the crustal medium and the source medium, which leads to the changes of the observation points’ gravity; the change of the regional stress field makes the underground cracks increase, and the cracks penetrate, which lead to the gush of heat mantle material and the change of gravity field; the dislocation and movement of the fault can cause the gravity field change. Therefore, the dynamic changes of the gravity field can reflect the change and deformation of inside material and close contact with the earthquake in the deep part of the Earth’s crust.
When grace satellites orbit the Earth, the gravity changes will show in the distance change between the GRACE A and GRACE B [23,34,35]. When the coordinate origin of the selected reference ellipsoid coincides with the Earth’s centroid, the gravitational force of the outer space of the Earth can be represented by spherical harmonic coefficients [36,37]:
In formula (1), V is gravitational potential, W is gravity potential, Z is the potential of centrifugal force, which can calculate the coefficients of spherical harmonics. Moreover, r is the radial distance between the earth center and the calculation point. R is the equatorial radius of the Earth. G is the gravitational constant, M is the total mass of the Earth, n is the order of the spherical function, m is a series of spherical harmonics, N max is the maximum orders of spherical function, λ is the longitude, and θ is the colatitude, P nm is n order m normalized associated Legendre function, and are normalized spherical function coefficients. Usually, gravity anomalies can be divided into two kinds [38,39]. We select the mixed gravity anomaly as the research object. The relation between the gravity anomaly and the disturbance position coefficient can be obtained by the Molodensky boundary value problem [40,41]. By calculating Legendre function by standard forward nematic method  and determining Gaussian smoothing filter function as the space smoothing function, the recurrence formula can be as follows:
To better show the regional gravity anomaly changes, it is necessary to filter out errors and other physical signal interference generated during GRACE satellite observations. The commonly used method is Gaussian filtering. The filtering effect of Gaussian smoothing filter radius of different sizes is different. The experiment shows that with the increase of the Gaussian smoothing radius, the band interference in the gravity change becomes less and less, and the gravity change becomes more apparent. The effect is best when the radius is 789 km, and it is easier to observe the information of the gravity field change. According to the literature, when the gravity field model is intercepted to order 60, the filter radius of the Gaussian filter has an upper limit. The maximum is 789 km . Therefore, the filter radius of the Gaussian filter in this article is 789 km.
The gravity change of the western Sichuan gravity field can be calculated according to formula (2). The gravity field change of western Sichuan is studied for two cases: water storage and reservoir drainage. Research stages are shown in Table 1. Moreover, the gravity change of storage and drainage periods is shown in Figures 2–13. The legend is on the right side of the figure, and different colors represent different gravity field variation values, and the unit is 10−8 m s−2. The period when the water level rises from 68 to 135 m is marked as XL1, XL2, and XL3. The period when the water level rises from 145 to 175 m is marked as XH1, XH2, XH3, XH4, and XH5. The period when the water level descends from 175 to 145 m is marked as F1, F2, F3, and F4.
|Water level rises (water storage)||Water level falls (water release)|
|68–135 m||145–175 m||175–145 m|
|Serial number||Time||Serial number||Time||Serial number||Time|
Figures 2–4 show the changes in the gravity field in western Sichuan when the water level rises from 68 to 135 m. The ranges of changes are 0.6–2.6, 2.0–2.8, and 3.5–4.0, respectively, and the changes are all positive. Figures 5–9 show the low water level from 145 to 145 m. The gravity changes at the high water level of 175 m (from the trough to the continuous peak), the ranges of changes are 1.7–2.6, 0–2.3, 1.1–1.7, 1.0–1.9, and 0.6–1.7, respectively, and the changes are also positive. Figures 10–13 show the gravity changes from the high water level from 175 m to the low water level 145 m (peak to continuous wave trough), the change ranges are −0.7 to −0.1, −0.6 to 0.4, −1.0 to −0.6, and −2.3 to −1.5, and the changes are all negative.
The gravity field change is positive when the water level rises, or the contrary. Therefore, it is easy to tell that the gravity field change is more evident as time passes. Moreover, the reason is that the western Sichuan is far away from the Three Gorges Reservoir, and the water level’s effect on gravity is delayed. Therefore, the water level of the Three Gorges Reservoir will affect the gravity field of western Sichuan. According to research, some scholars think the earthquake usually happens during the reverse recovery of the gravity field.
Furthermore, some scholars conclude that several years before the earthquake, the regional gravity field shows the change tendency from positive to negative. Therefore, there is a close relationship between the earthquake and the gravity field. Moreover, the water level of the Three Gorges Reservoir can influence the earthquake activity of western Sichuan by changing the gravity field.
The seismic data used in this study come from the China Seismological Network, the Three Gorges water level data come from the official website of China Three Gorges Corporation, and the gravity field data come from the official website of the German Potsdam Geoscience Research Center (GFZ). The source of the data is authentic and reliable. This article studies the impact of the construction of the Three Gorges Dam and its impoundment and release on the earthquake activity in western Sichuan. The influence of other factors on the earthquake in western Sichuan cannot be ruled out. The occurrence of the earthquake in western Sichuan is the result of the combined effect of many factors . This study uses Fourier transform and correlation analysis to quantitatively explore the correlation between the two and explain the impact of the Three Gorges Dam on the seismic activity in western Sichuan from the perspective of the changes in the gravity field of the western Sichuan. To a certain extent, it affects the seismic activity in western Sichuan. The Three Gorges Dam affects the seismic activity in western Sichuan by affecting the changes in the gravity field in western Sichuan. The Three Gorges Dam impounds water. As a result, the gravity field in western Sichuan changes to positive, and the Three Gorges Dam releases water, and the gravity field in western Sichuan changes to negative.
In the initial stage of the rise of the water level, the gravitational field changes insignificantly. As time goes by, the gravitational field changes gradually increase. The change of reservoir water level affects the gravity field in western Sichuan. The dynamic change of the gravity field can reflect the change and deformation process of the Earth’s internal material and is closely related to earthquakes. Liu et al.  built a high-resolution Boussinesq water-load model for the Badong County section of the Three Gorges Reservoir by referring to Google Earth image data for different years. First, they explained the seismogenic mechanism of earthquakes (from 2003 to now) in Badong County. Then, they traced the evolution of earthquakes in the reservoir area before and after impoundment on a NE–SW transect. Compared with this study, our study did not build a model. We can get the result only by relying on the dark flowers on the other side of the gravity field, significantly saving time wastage.
Besides, using the data of four wells with similar borehole logging, aquifer lithology, and water chemistry type around the Three Gorges Dam, Ma et al.  examined the relationship between earthquake-induced hydrologic changes and faults. The results indicate that two of the wells that penetrate the fault damage zones recorded sustained water level changes and experienced changes in tidal response and hydraulic properties following the Wenchuan M W 7.9 earthquake. Compared with this, our study gets the result only by relying on the changes in the gravity field without too many elements, where the changes are more apparent, and the conclusions are more convincing.
This article studies the impact of the Three Gorges Dam on the seismic activity in the long-distance area outside the reservoir area. The current research on the impact of the reservoir on the seismic activity is generally carried out on the seismic activity in the reservoir area and the study on the impact on the long-distance area outside the reservoir area. It is still in the early stage, this article is still exploring, and there are still many shortcomings in the research process. This is also the direction for this research to continue to work in the future:
This article explains the influence of the Three Gorges Dam on the seismic activity in western Sichuan from two aspects: the changes in the gravity field in western Sichuan and the position of the plate where the Three Gorges Dam is located. The influence of the Three Gorges Dam on seismic activity in western Sichuan may be in many ways. Yes, this article only selects two of them for the explanation. In the future, scholars can study how the Three Gorges Dam affects seismic activity in western Sichuan from other angles .
This article only analyzes the distribution characteristics of earthquakes in western Sichuan from a statistical point of view combined with plate structure. The distribution of earthquakes may be related to local geological structural conditions, lithological conditions, and hydrogeological conditions [34,46]. Therefore, the reservoir impacts the distribution of earthquakes outside the reservoir area. The impact needs to be further studied.
In the future, scholars can also establish a three-dimensional geological model of the study area according to the actual topography and geological structure to study the impact of water-loading effects on faults. They can also carry out special studies on the effects of water on rock masses, focusing on the study of groundwater effects on deep parts [30,47]. The effects of infiltration and softening of fault materials, pore pressure diffusion, and its effects are the dominant factors.
This article uses the earthquake catalog from 1970 to 2013 and the water level data of the Three Gorges Reservoir from 2002 to 2013 as the primary research data, using statistical methods according to the discovery phenomenon – exploring the cause – explaining the idea of focusing on the Three Gorges Dam to the long-distance area outside the reservoir area, studied the impact of the earthquake, taking western Sichuan as an example.
This correlation is explained from the perspective of the Three Gorges Dam’s influence on the changes in the gravity field in western Sichuan and thus the seismic activity. This article has concluded that the changes in the water level of the Three Gorges Reservoir affect the gravity field in western Sichuan, and some scholars have concluded that earthquakes are closely related to the changes in the gravity field. Finally, the article combines the geographical location of the Three Gorges Dam and the Chinese mainland plate map to explain the characteristics of earthquakes in the western Sichuan region before and after the construction of the Three Gorges Dam and in different impounding periods.
Funding information: This work was supported by the Sichuan Science and Technology Program (2021YFQ0003).
Author contributions: Wenfeng Zheng contributed to the conception of the paper and supervision. Yaxiang Wang performed the formal experiment. Ziyi Cao and Yan Liu contributed significantly to analysis and manuscript preparation. Jiawei Tian and Zhaojun Pang performed the data analyses and wrote the manuscript. Juan Li and Lirong Yin helped perform the analysis with constructive discussions. Shan Liu, Lirong Yin performed the formal analysis and revised the manuscript.
Conflict of interest: Authors state no conflict of interest.
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