Study On The Influence Of The Impounding Of The Three Gorges Project On The Geophysical Field And Seismicity In The Reservoir Area

The Three Gorges Project of the Yangtze River is a super large water control project attracting worldwide attention.It has been more than 19 years since the impoundment in June 2003.With the rise of the water level in the reservoir area,the annual fluctuation of the water level in the reservoir area of 30-40 meters,the underground fluid field and seismic activity in the reservoir area have had a far-reaching impact.Therefore,the impact of the impounding of the Three Gorges Project on the comprehensive geophysical field and seismicity in the reservoir area is not only an important scientific issue of great concern to the society,but also an unavoidable practical issue to ensure the safety of the Three Gorges Project and the social security of the reservoir area.Based on the multi-source observation data of earthquake,deformation and gravity obtained by the "reservoir induced earthquake monitoring system of the Three Gorges Project" for many years,and taking the key monitoring area of the Three Gorges(110.2-111.1° E,30.6-31.2° N)as the research area,this paper calculates the comprehensive geophysical field and seismic activity characteristics of the study area in the four stages of the Three Gorges impoundment,135m impoundment,156m impoundment and 175m experimental impoundment.Firstly,based on the relative gravity observation data of the Three Gorges seismic gravity monitoring network from October 2001 to October 2015,the difference results of the earth’s gravity field are gridded by using Kriging interpolation method to obtain the dynamic variation characteristics of the difference of the gravity field in the study area.Secondly,using the GPS observation data of the Three Gorges GPS observation network from October 2001 to October 2016,the crustal horizontal stress-strain field in the study area is calculated by using the block global rotation and uniform strain model(REHSM).Thirdly,based on the catalogue of earthquakes with magnitude M≥0.0 from January 1,1959 to December 31,2016,the seismic activity parameters are calculated by using the Zmap toolbox,and then the correlation between the reservoir water level and the monthly earthquake frequency is tested by using the cross-correlation test method.Combined with the results of the reservoir earthquake source mechanism solution,the impact of water storage on seismic activity is discussed.Finally,based on the water rock coupling theory,the finite difference model of the Three Gorges Reservoir area is established.Using the gravity field difference image,crustal surface strain rate field,seismicity characteristics and related numerical results,the influence of the Three Gorges Project impoundment on the comprehensive geophysical field and seismicity of the reservoir area is analyzed,the correlation between reservoir water,comprehensive geophysical field and induced earthquake is studied,and the genetic mechanism of the Three Gorges reservoir earthquake is discussed.The main understandings and conclusions are as follows:(1)The rise of reservoir water level during the impounding process of the Three Gorges reservoir has had an important impact on the gravity field,crustal stress-strain field and seismicity in the head area of the reservoir.The spatial-temporal distribution characteristics of seismicity in the reservoir area are closely related to the geometric shape of the changes of gravity field and crustal stress-strain field.The crust in the head area of the reservoir has undergone a complex process of"continuous loading,seepage and saturation,rebound and rebalancing",and accompanied by obvious differences in the characteristics of seismic activity stages.(2)During the continuous loading stage of 135m impounding,156m impounding and 175m experimental impounding,with the continuous rise of the reservoir water level,a huge strip-shaped positive anomaly of the earth’s gravity field centered on Zigui and the compression characteristics of the crustal horizontal plane appeared on both sides of the reservoir bank in the study area,which formed a good spatial envelope for the intensive seismic activities within 10km of the reservoir bank,indicating that the direct reservoir water load brought by impounding has a great impact on the earth’s gravity field,the crustal stress-strain field and seismic activity.The seismicity is obviously related to the elastic loading caused by the rapid rise of the reservoir water level,showing small magnitude,shallow focal point,high b value(about 0.91),and showing the "instantaneous" response characteristics to the rise of the reservoir water,that is,most of the seismic events occurred in the process of the rapid rise of the reservoir water.The focal mechanism solutions of seismic events in the same period mostly show the characteristics of strike slip and normal fault,and the directions of P axis,T axis and horizontal maximum principal compressive stress are not consistent,indicating that there is no relatively consistent seismogenic structure,and it is less controlled by the regional tectonic stress field and the active fault zone in the study area.(3)During the seepage saturation stage and rebound rebalancing stage of the 175m experimental impounding,the obvious expansion of the positive gravity anomaly on both sides of the reservoir bank not only outlines the scope of the outflow water,but also shows the strengthening of the indirect gravity effect caused by the outflow water seepage.With the reservoir water level entering the high-level periodic operation stage,a large area of negative gravity anomalies and the characteristics of crustal horizontal plane tension indicate that the crust in the study area has entered the rapid rebound and rebalancing stage.Under the continuous infiltration of reservoir water,the earth’s gravity field and crustal stress-strain field are gradually affected by regional tectonic stress field and active faults,and reservoir water infiltration begins to play a leading role.During the same period,the seismic activity began to show the characteristics that the frequency of earthquakes in the low magnitude scale decreased,and the number of seismic events in the high magnitude scale with M≥3.0 increased.For example,the M5.1 earthquake occurred in Badong on December 16,2013.As the largest earthquake event since the Three Gorges impoundment,the occurrence of Badong M5.1 earthquake has important symbolic significance.Since then,the occurrence of seismic events in the study area usually lags behind the change of reservoir water level by about 4-5 months,showing a lag response feature,that is,seismic events of large magnitude mostly occur during the flood discharge period of the Three Gorges,such as the Zigui M4.5 earthquake in March 2014,M4.9 earthquake and Zigui M4.0 earthquake in May 2014.The focal mechanism solution results show that the correlation between the larger earthquake events and active faults in the study area is more and more significant,and the principal compressive stress field is gradually consistent with that before impounding.Xiangxi reservoir section is a triangle structure where Xiannvshan fault,Jiuwanxi fault and the Yangtze River intersect.There may be a risk of moderate earthquake around this area.(4)In terms of genetic mechanism,during the continuous loading stage of 135m impounding,156m impounding and 175m experimental impounding,the instantaneous elastic loading caused by the rapid rise of the reservoir water level and the instantaneous elastic increase of pore water pressure at the shallow part of the reservoir bottom led to the instability of ore caves,karst caves,shallow strata and faults within 10km along the river and near the reservoir bank and induced earthquakes,which are called non-tectonic(induced)reservoir earthquakes.During the seepage saturation stage and rebound rebalancing stage of the 175m experimental impounding,the water level of the Three Gorges reservoir is in a high-level operation state,and the fluctuation of the reservoir water level up to 30-40 meters has a clear annual variation cycle.With the deepening of the water storage process,the pore water pressure of the crust and rock mass in the study area increases due to the reservoir water infiltration and the friction at the seismogenic fault further decreases.Under the joint action of physical and chemical actions such as the softening and expansion of the reservoir fault or rock mass,the fault instability triggers the seismic event,which is called the tectonic(triggered)reservoir earthquake.