# The Impact of Rising Reservoir Water Level on the Gravity Field and Seismic Activity in the Reservoir Area: Evidence from the Impoundment of the Three Gorges Reservoir (China)

^{1}

^{2}

^{3}

^{4}

^{*}

## Abstract

**:**

^{−8}ms

^{−2}. The seismicity activity dominated by micro-earthquakes after a 135 m water level rose rapidly, and the monthly average earthquake frequency increased from 2.00 before the impoundment to 92.60 after the 175 m stage. (2) From the beginning of the impoundment to the experimental impoundment stage of 175 m, the time series correlation test result between the monthly frequency of earthquakes and the water level of the reservoir also changed from uncorrelated before the water storage to correlated when the time lag was 0 months at a 95% confidence threshold. This indicates that the seismic activity obviously has a direct relationship with the load pressure produced by the rapid rise of the reservoir water level, which causes the instability of the mines, karst caves, shallow rock strata, and faults within 10 km along the river and near the reservoir bank, and consequently induces earthquakes. (3) As the TGR enters the 175 m high-level operation stage, the cross-correlation test confirmed that the seismic activity and the reservoir water level show negative correlation characteristics under the time lag of 4 to 5 months, indicating that the seismic activity has a lagging response to the reservoir water level change. The continued infiltration of the reservoir water, followed by the softening of the faults and other actions, triggered the Xiangxi M4.1 earthquake at the center of the four quadrants of gravity anomalies near Xiangxi on 22 November 2008. The Xiangxi segment of the reservoir and its periphery, a triangular geological region where the Xiannvshan faults, the Jiuwanxi fault, and the Yangtze River meet, might be at risk of having reservoir-induced tectonic earthquakes.

## 1. Introduction

^{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.

## 2. Geological Structure Characteristics of the Reservoir’s Head Area

## 3. Data and Methods

#### 3.1. Gravity Surveying and Data Adjustment

#### 3.2. Gridding of Gravity Data

#### 3.3. Earthquake Monitoring Catalog

#### 3.4. Cross-Correlation Method Based on Reshuffling Tests

## 4. Gravity Field Changes and Seismicity during the Process of Water Impounding

#### 4.1. Before the Water Impoundment

#### 4.2. The 135 m Stage

#### 4.3. The 156 m Stage

#### 4.4. The 175 m Experimental Stage

## 5. Time Series Correlation Test of Seismic Activity and Reservoir Water Level Based on Reshuffling Tests

## 6. Causing Mechanism of Seismicity during the Impoundment

#### 6.1. Influence of Direct Reservoir Water Load

#### 6.2. The Effect of Reservoir Water Infiltration

## 7. Conclusions

- (1)
- The rising reservoir water level has had a crucial impact on the gravity field and seismic activity in the reservoir’s head area. The cumulative changes in the gravity field from October 2001 to November 2008 show that water impounding has led to a huge banded positive anomaly of gravity that reaches $450\times {10}^{-8}{\mathrm{ms}}^{-2}$ along the river near Xiangxi. The seismicity activity dominated by micro-earthquakes rose rapidly after the 135 m water level, and the monthly average earthquake frequency increased from 2.00 before the impoundment to 92.60 after the 175 m stage. The massive amount of water load caused the study area to have a positive anomaly of banded gravity along the river, which is centered on Badong county and Xiangxi Estuary of Zigui county. It also formed a good spatial envelope due to the sudden increase in seismic activity during the corresponding periods, which means that the variation of gravity can reveal the influence range of direct reservoir water load.
- (2)
- From the beginning of the impoundment to the experimental impoundment stage of 175 m, the time series correlation test result between the monthly frequency of earthquakes and the water level of the reservoir also changed from uncorrelated before the water storage to correlated when the time lag was 0 months at a 95% confidence threshold. Further gravity observations and numerical simulation results showed that the seismic activity obviously has a direct relationship with the load pressure produced by the rapid rise of the reservoir water level, which causes the instability of the mines, karst caves, shallow rock strata, and faults within 10 km along the river and near the reservoir bank, and consequently induces earthquakes.
- (3)
- As the TGR entered the 175 m high-level operation stage, the cross-correlation test confirmed that the seismic activity and the reservoir water level showed negative correlation characteristics under the time lag of 4 to 5 months, suggesting that the seismic activity has a lagging response to the reservoir water level change. Gravity and numerical simulation results showed the continued infiltration of the reservoir water, followed by the softening of the faults and other actions, triggering the Xiangxi M4.1 earthquake at the center of the four quadrants of gravity anomalies near Xiangxi on 22 November 2008. The Xiangxi segment of the reservoir and its periphery, a triangular geological region where the Xiannvshan faults, the Jiuwanxi fault, and the Yangtze River meet, might be at risk of having reservoir-induced tectonic earthquakes.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Acknowledgments

## Conflicts of Interest

## Appendix A. An Example of Gridding the Gravity Observation Data

#### Appendix A.1. An Overview of Kriging Interpolation

#### Appendix A.2. Gridding of Gravity Observation Data before and after 135 m Water Impounding Stage

**Figure A1.**The difference results of the gravity observations from October 2002 to October 2003. (

**a**) All stations, (

**b**) remove the riverside Area A, and (

**c**) remove the mountain Area B.

**Figure A2.**Kriging interpolation of the dynamic changes of the gravity field from October 2002 to October 2003. (

**a**) All the stations, (

**b**) after removing the stations in Area A, and (

**c**) after removing the stations in Area B.

**Figure A3.**Error analysis of Areas A and B: (

**a**) observed value and Kriging interpolation result and (

**b**) distribution of relative error.

## Appendix B. Three-Dimensional Finite-Difference Model Based on Fluid–Solid Coupling Theory

#### Appendix B.1. Model Building

**Figure A4.**The finite-difference model of the head area of the Three Gorges Reservoir: (

**a**) free surface, (

**b**) geometry model of fault zone and block, and (

**c**) mesh division.

**Table A1.**Underground stratification and lithology related parameters, Adapted with permission from Ref. [30]. 2020, Journal of Geodesy and Geodynamics.

Number | Lithology | Thickness$/m$ | $\mathrm{Bulk}\mathrm{Modulus}/GPa$ | $\mathrm{Shear}\mathrm{Modulus}/GPa$ | $\mathrm{Internal}\mathrm{Friction}\mathrm{Angle}/\left(\xb0\right)$ | $\mathrm{Cohesion}/MPa$ | $\mathrm{Tensile}\mathrm{Strength}/MPa$ | $\mathrm{Permeability}\mathrm{Coefficient}/{m}^{2}\xb7{\left(Pas\right)}^{-1}$ | $\mathrm{Density}/kg\xb7{\left(m\right)}^{-3}$ |

1 | Gravel, hard clay | 1500 | $8.33\times {10}^{-3}$ | $3.85\times {10}^{-3}$ | 32.00 | $3.00\times {10}^{-3}$ | 110.00 | $1.00\times {10}^{-13}$ | 1600 |

2 | Sandstone, mudstone | 1500 | 26.80 | 7.00 | 27.80 | 27.20 | 223.00 | $1.00\times {10}^{-15}$ | 2630 |

3 | Siltstone, shale | 1000 | 15.60 | 10.80 | 32.10 | 34.70 | 197.00 | $1.00\times {10}^{-16}$ | 2570 |

4 | Limestone | 1000 | 22.60 | 11.10 | 32.00 | 42.00 | 134.00 | $1.00\times {10}^{-17}$ | 2680 |

5 | shale bookstone | 2000 | 8.80 | 4.30 | 27.80 | 14.40 | 202.00 | $1.00\times {10}^{-18}$ | 2660 |

6 | Limestone | 5000 | 43.90 | 30.20 | 32.10 | 51.00 | 10.34 | $1.00\times {10}^{-19}$ | 2690 |

#### Appendix B.2. Calculation Steps and Boundary Condition Settings

#### Appendix B.3. Post-Processing

## References

- Carder, D.S.; Isaacks, W.H.; Greenwood, W.P. Influence of reservoir loading on local earthquake activity. Geol. Soc. Am. Bull.
**1947**, 58, 1264. [Google Scholar] - Gupta, H.K. A review of recent studies of triggered earthquakes by artificial water reservoirs with special emphasis on earthquakes in Koyna, India. Earth-Sci. Rev.
**2002**, 58, 279–310. [Google Scholar] [CrossRef] - Drakatos, G.; Papanastassiou, D.; Papadopoulos, G.; Skafida, H.; Stavrakakis, G. Relationship between the 13 May 1995 Kozani-Grevena (NW Greece) earthquake and the Polyphyto artificial lake. Eng. Geol.
**1998**, 51, 65–74. [Google Scholar] [CrossRef] - Jimenez, A.; Tiampo, K.F.; Posadas, A.M.; Luzon, F.; Donner, R. Analysis of complex networks associated to seismic clusters near the Itoiz reservoir dam. Eur. Phys. J.-Spec. Top.
**2009**, 174, 181–195. [Google Scholar] [CrossRef] - Al-Saigh, N.H. The Mechanism of Induced Seismicity at Mosul Reservoir Based on the First Motion Analysis. J. Geol. Soc. India
**2010**, 76, 399–402. [Google Scholar] [CrossRef] - Cheng, H.; Zhang, H.; Shi, Y. High-resolution numerical analysis of the triggering mechanism of M(l)5.7 Aswan reservoir earthquake through fully coupled poroelastic finite element modeling. Pure Appl. Geophys.
**2016**, 173, 1593–1605. [Google Scholar] [CrossRef] - Grasso, J.R.; Karimov, A.; Amorese, D.; Sue, C.; Voisin, C. Patterns of reservoir-triggered seismicity in a low-seismicity region of France. Bull. Seismol. Soc. Am.
**2018**, 108, 2967–2982. [Google Scholar] [CrossRef] - Xuan, N.B.; Trong, D.C.; Long, Q.N.; Bach, X.M.; Le, H.T.; Hien, P.L.; Goyal, R.; Than, A.T.; Hung, N.P. Assessment on maximum magnitude of natural and triggered earthquake when water is impounded in the mining pit: A case study in Nui Nho quarry, Vietnam based on gravity and magnetic data. Russ. J. Earth Sci.
**2020**, 20, 1. [Google Scholar] - Zhang, H.; Cheng, H.; Pang, Y.; Shi, Y.; Yuen, D.A. Influence of the impoundment of the Three Gorges Reservoir on the micro-seismicity and the 2013 M5.1 Badong earthquake (Yangtze, China). Phys. Earth Planet. Inter.
**2016**, 261, 98–106. [Google Scholar] [CrossRef] [Green Version] - Yao, Y.S.; Wang, Q.L.; Liao, W.L.; Zhang, L.F.; Chen, J.H.; Li, J.G.; Yuan, L.; Zhao, Y.-N. Influences of the Three Gorges Project on seismic activities in the reservoir area. Sci. Bull.
**2017**, 62, 1089–1098. [Google Scholar] [CrossRef] [Green Version] - Jiang, H.K.; Song, J.; Wu, Q.; Li, J.; Qu, J.H. Quantitative investigation of fluid triggering on seismicity in the Three Gorge Reservoir area based on ETAS model. Chin. J. Geophys.-Chin. Ed.
**2012**, 55, 2341–2352. [Google Scholar] - Jiang, F. Role of Gravimetry in Monitoring the Crustal Deformation of Three Gorges Reservoir Area. Geomat. Inf. Sci. Wuhan Univ.
**2003**, 28, 679–682. [Google Scholar] - Xing, C.; Gong, K.; Du, R. Crustal deformation monitoring network for three gorges project on yangtze river. J. Geod. Geodyn.
**2003**, 23, 114–118. [Google Scholar] - Tu, P.; Cen, Z.; Chen, H. Monitoring landslides deformation in three gorges reservoir area by using the repeat-pass spaceborne InSAR. Remote Sens. Technol. Appl.
**2010**, 25, 886–890. [Google Scholar] - Zhang, S.; Chen, Z.; Wang, Q.; Liu, J.; Zhang, P. The anomaly feature extraction of mobile gravity in the sichuan-yunnan region using wavelet transform methodthe case study of the ludian m_s6.5 and jinggu m_s6.6 earthquakes in 2014. J. Geod. Geodyn.
**2020**, 40, 87–93. [Google Scholar] - Zhu, Y.; Liu, F.; Zhang, G.; Xu, Y. Development and prospect of mobile gravity monitoring and earthquake forecasting in recent ten years in China. Geod. Geodyn.
**2019**, 10, 485–491. [Google Scholar] [CrossRef] - Sun, S.; Xiang, A.; Zhu, P.; Shen, C. Gravity change and its mechanism after the first water impoundment in three gorges project. Acta Seismol. Sin.
**2006**, 28, 485–492. [Google Scholar] [CrossRef] - Yang, Y.; Gao, Y.; Wang, W.; Kang, S.; Yang, G. Analysis of the influence of water impoundment and drainage in the Three Gorges Reservoir on the ground gravity change. Sci. Surv. Mapp.
**2018**, 43, 66–70. [Google Scholar] - Yang, G.; Shen, C.; Wang, X.; Sun, S.; Liu, D.; Li, H. Numerical simulation of gravity effect of water-impoundment in three gorges reservoir. J. Geod. Geodyn.
**2005**, 25, 19–23. [Google Scholar] - Li, X.; Yao, Y.; Zeng, Z.; Liu, L. Analysis of the formation system of the present tectonic stress field in the head area of the three gorges reservoir. J. Geomech.
**2006**, 12, 174–181. [Google Scholar] - Liu, J.Z.; Wang, T.Q.; Chen, Z.H.; Zhang, P.; Zhu, C.D.; Zhang, S.X. Analyzing gravity anomaly variations before the 2016 Ms 6.4 earthquake in Menyuan, Qinghai with an interpolation/cutting potential field separation technique. Appl. Geophys.
**2018**, 15, 137–146. [Google Scholar] [CrossRef] - Wiemer, S.; Wyss, M. Minimum magnitude of completeness in earthquake catalogs: Examples from Alaska, the western United States, and Japan. Bull. Seismol. Soc. Am.
**2000**, 90, 859–869. [Google Scholar] [CrossRef] - Telesca, L. Analysis of the cross-correlation between seismicity and water level in the koyna area of india. Bull. Seismol. Soc. Am.
**2010**, 100, 2317–2321. [Google Scholar] [CrossRef] - Schultz, R.; Telesca, L. The cross-correlation and reshuffling tests in discerning induced seismicity. Pure Appl. Geophys.
**2018**, 175, 3395–3401. [Google Scholar] [CrossRef] - Little, M.A.; McSharry, P.E.; Moroz, I.M.; Roberts, S.J. Testing the assumptions of linear prediction analysis in normal vowels. J. Acoust. Soc. Am.
**2006**, 119, 549–558. [Google Scholar] [CrossRef] [Green Version] - Zhang, L.; Yao, Y.; Shen, X.; Wei, G.; Chen, J. Study on type and focal mechanism of the earthquakes in three gorges reservoir. J. Geod. Geodyn.
**2014**, 34, 77–82. [Google Scholar] - Telesca, L.; ElBary, R.E.F.; Mohamed, A.E.-E.A.; ElGabry, M. Analysis of the cross-correlation between seismicity and water level in the Aswan area (Egypt) from 1982 to 2010. Nat. Hazards Earth Syst. Sci.
**2012**, 12, 2203–2207. [Google Scholar] [CrossRef] [Green Version] - Dvorkin, J.; Nur, A. Dynamic poroelasticity—A unified model with the squirt and the biot mechanisms. Geophysics
**1993**, 58, 524–533. [Google Scholar] [CrossRef] - Xie, X.D. Study on Mechanism of Reservoir-Induced Seismicity Based on Theory of Fluid-Solid Coupling in the Danjiangkou Reservoir Area. Ph.D. Thesis, Wuhan University, Wuhan, China, April 2010. [Google Scholar]
- Yu, J.; Wang, Q.; Zhao, Y. Stress field of the three gorges based on fluid-solid coupling simulation. J. Geod. Geodyn.
**2020**, 40, 928–930, 941. [Google Scholar] - Hundelshaussen, R.; Costa, J.F.C.L.; Marques, D.M.; Bassani, M.A.A. Localised kriging parameter optimisation based on absolute error minimisation. Appl. Earth Sci.-Trans. Inst. Min. Metall.
**2018**, 127, 153–162. [Google Scholar] [CrossRef] - Ryu, J.S.; Kim, M.S.; Cha, K.J.; Lee, T.H.; Choi, D.H. Kriging interpolation methods in geostatistics and DACE model. KSME Int. J.
**2002**, 16, 619–632. [Google Scholar] [CrossRef]

**Figure 1.**Comprehensive information of the head area of the TGR. (

**a**) The location of the study area; (

**b**) overview of the geological structure in the head area of the TGR and the reservoir-induced earthquake monitoring system for the Yangtze River Three Gorges Project. (1. Quaternary strata; 2. Cretaceous~Upper Triassic upper caprock structural sublayer; 3. Sinian~Middle Triassic lower caprock structural sublayer; 4. Middle Proterozoic-upper basement structural sublayer; 5. Lower Proterozoic-lower basement structural sublayer; 6. Jingningian period intermediate-acid igneous rocks; 7. The Three Gorges key monitoring area; 8. Gravity Survey Line; 9. Gravity observation network; 10. Seismic stations; 11. The Three Gorges Dam; 12. Normal fault; 13. Reverse fault; 14. Strike–slip fault. Faults: F1, Xinhua faults; F2, Gaoqiao faults; F3, Shuitianba faults; F4 Wuduhe, faults; F5, Badong faults; F6, Tianyangping faults; F7, Xiannushan faults; F8, Jiuwanxi faults; F9, Longwangchong faults).

**Figure 2.**Codes of the gravity station and area selection for the interpolation algorithms: A represents riverside and B hills.

**Figure 3.**Data accuracy and integrity: (

**a**) Mean-squared error of the gravity observation network from 2002 to 2009. (

**b**) Magnitude of completeness calculated by using Zmap.

**Figure 4.**Reservoir water level, monthly seismic frequency, and the earthquakes above M4.0 during the impounding process of the TGR (January 2001–December 2009, M ≥ 0).

**Figure 5.**Spatial distribution of seismicity in the study area during the impoundment of the Three Gorges Project (January 2001–December 2009, M ≥ 0). (

**a**) Before the water impoundment; (

**b**) 135 m stage; (

**c**) 156 m stage; (

**d**) 175 m experimental stage.

**Figure 6.**Gravity field differential dynamic variation images in the study area during the impoundment of the Three Gorges Project (October 2001–October 2009). (

**a**) October 2001–October 2002; (

**b**) October 2002–October 2003; (

**c**) October 2003–November 2004; (

**d**) November 2004–October 2005; (

**e**) October 2005–November 2006; (

**f**) November 2006–November 2007; (

**g**) November 2007–November 2008; (

**h**) November 2008–October 2009.

**Figure 7.**Correlation test between monthly frequency of seismicity and reservoir water level in the study area during the impoundment of the Three Gorges Project (January 2001–December 2009). (

**a**) Before the impoundment; (

**b**) 135 m stage; (

**c**) 156 m stage; (

**d**) 175 m stage.

**Figure 9.**The maximum shear–strain field at 500 m below the ground in the study area: (

**a**) before the impoundment, (

**b**) 135 m stage, (

**c**) 156 m stage, and (

**d**) 175 m stage.

**Figure 10.**Changes in the gravity field after November 2008 (November 2008–October 2009) and the focal mechanism of the Zigui M4.1 earthquake.

**Figure 11.**Pore pressure of A-A′ profile under different water levels: (

**a**) 135 m stage, (

**b**) 156 m stage, and (

**c**) 175 m stage.

**Table 1.**The percentage of relative error between different interpolation results and the observed value.

Method | Area A | Area B |
---|---|---|

Kriging | 5.34% | 17.12% |

RBF | 5.73% | 17.86% |

TLI | 6.69% | 18.88% |

**Table 2.**Average monthly earthquake frequency in different magnitude grades of the study area and in different impoundment stages of TGR.

Impounding Stages | Beginning and End Dates | Average Monthly Earthquake Frequency | ||||||
---|---|---|---|---|---|---|---|---|

0.0–0.9 | 1.0–1.9 | 2.0–2.9 | 3.0–3.9 | 4.0–4.9 | 5.0–5.9 | All | ||

Before the impoundment | 1 January 2001–24 May 2003 | 0.97 | 0.90 | 0.07 | 0.07 | 0.00 | 0.00 | 2.00 |

135 m | 25 May 2003–19 September 2006 | 16.43 | 2.20 | 0.25 | 0.03 | 0.00 | 0.00 | 18.90 |

156 m | 20 September 2006–27 September 2008 | 65.08 | 8.58 | 0.63 | 0.00 | 0.00 | 0.00 | 74.29 |

175 m | 28 September 2008–31 December 2009 | 82.67 | 8.73 | 1.13 | 0.00 | 0.07 | 0.00 | 92.60 |

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. |

© 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Meng, Q.; Yao, Y.; Liao, W.; Zhang, L.; Dang, X.
The Impact of Rising Reservoir Water Level on the Gravity Field and Seismic Activity in the Reservoir Area: Evidence from the Impoundment of the Three Gorges Reservoir (China). *Appl. Sci.* **2022**, *12*, 4085.
https://doi.org/10.3390/app12084085

**AMA Style**

Meng Q, Yao Y, Liao W, Zhang L, Dang X.
The Impact of Rising Reservoir Water Level on the Gravity Field and Seismic Activity in the Reservoir Area: Evidence from the Impoundment of the Three Gorges Reservoir (China). *Applied Sciences*. 2022; 12(8):4085.
https://doi.org/10.3390/app12084085

**Chicago/Turabian Style**

Meng, Qingxiao, Yunsheng Yao, Wulin Liao, Lifen Zhang, and Xuehui Dang.
2022. "The Impact of Rising Reservoir Water Level on the Gravity Field and Seismic Activity in the Reservoir Area: Evidence from the Impoundment of the Three Gorges Reservoir (China)" *Applied Sciences* 12, no. 8: 4085.
https://doi.org/10.3390/app12084085