Study On Seismogenic Environment And Mechanism In The Jinping I Reservoir And Its Surrounding Areas

The Jinping I Reservoir in Sichuan Province is tallest hyperbolic arch dam,which began impound in 2012.Half a year after impoundment,a large number of earthquakes occured in Muli County.On November 22,2013,ML4.1,ML4.4,andML3.9,occurred successively.In order to comprehensively understand the seismogenic process and seismogenic mechanism of earthquake occurred in Jinping I reservoir,it is necessary to study the seismicity,earthquake distribution,velocity structure,focal mechanism solution,stress field before and after the impoundment,and compare these research contents with other reservoirs and tectonic earthquakes.These studies are of great significance for disaster reduction,and reservoir safety.This paper mainly conducted the following research.Analyzed and compared the changes in seismic activity before and after impoundment in the Jinping I Reservoir in Sichuan.The doubledifference location method was used to conduct seismic precise positioning research on 7213 earthquakes with magnitudes above ML0.5 occurring before and after impoundment in the reservoir and its surrounding areas,obtained precise location results for 6664 earthquake events.The spectral analysis moment tensor inversion method was applied to calculate the source mechanism solution of 76 earthquakes with magnitudes aboveML3.0 occurring before and after impoundment in the reservoir and its surrounding areas,and the results of 71 earthquakes with good quality were selected.The grid search method was used to invert the regional stress field of different parts of the study area before and after impoundment and calculate the R value.The double-difference tomography method was used to invert the P-wave and S-wave velocity structures in study area through body wave travel time data,and chessboard resolution tests were conducted.The obtained results of seismic activity,earthquake distribution,velocity structure,source mechanism solution,stress field,and other aspects of the Jinping Reservoir were compared with those of other reservoirs,and similarities and differences were analyzed to summarize different types of reservoirs and analyze their seismogenic environments and mechanisms.This study also analyzed and compared the similarities and differences and the causes of the results of the Jiping Reservoir and the LushanMS7.0 earthquake sequence,and comprehensively studied the causes of these results,the seismogenic environment and mechanism of the Jiping Reservoir.The research results of seismicity,earthquake distribution,velocity structure,focal mechanism solution,stress field and other aspects obtained in this study were compared with those of other reservoirs to study the seismogenic environment and mechanism of Jinping I reservoir.The Jinping I Reservoir,Longtan Reservoir,and Zipingpu Reservoir have all experienced delayed response during the initial impoundment with different manifestations.The focal depths are relatively shallow,and the epicenters of earthquakes are mainly distributed within 10 kilometers from the bank.The types of earthquake focal mechanisms that occur after the impoundment of these reservoirs are mainly strike and thrust type.The occurrence time of the Sichuan Lushan Ms7.0 earthquake is very close to that of the Jinping I reservoir earthquake swarm.In order to study the difference between the research results of Jinping I reservoir and the tectonic earthquake,relocated the Lushan earthquake sequence,studied the temporal and spatial evolution characteristics of seismicity,and compared it with the Jinping I reservoir.By comparing and analyzing similarities and differences,further analysis was conducted on the seismogenic environment and mechanism of the Jinping I Reservoir.The innovation of this paper mainly lies in four aspects:(1)There is a lack of systematic comparison of reservoir earthquakes before and after impoundment,this paper systematically conducted research on the Jinping Reservoir earthquake from multiple aspects and compared the research results before and after impoundment.(2)The seismic activity of the Jinping Reservoir changed greatly before and after impoundment,.The area where the earthquake swarm occurred was very calm before impoundment,and no earthquakes above ML0.5 had occurred in several years.After impoundment,the frequency of earthquake was very high,with thousands of earthquakes above ML0.5 occurring in a single month,and the epicentral range was highly concentrated,which is different from most reservoir earthquakes.The research on Jinping Reservoir earthquake is unique.(3)By comparing the research results of the Jinping Reservoir with those of other reservoirs such as Xinfengjiang Reservoir,Three Gorges Reservoir,Zipingpu Reservoir,Longtan Reservoir,and Shanxi Reservoir,this paper analyzed the similarities and differences in earthquake activity changes,earthquake distribution,mechanism solutions,stress field distribution,body wave velocity structure,etc.,and summarized the characteristics of different types of reservoir earthquakes and analyzed their seismogenic environment and mechanism.(4)The research results of the Jinping Reservoir were compared with those of the Lushan Ms7.0 earthquake sequence to analyze and compare the similarities and differences in various aspects and causes of reservoir earthquakes and tectonic earthquakes.The achievements and important findings of this paper mainly include:(1)The seismic response time of the Jinping Reservoir is relatively long,which is related to the delayed response caused by the location of earthquake swarm and pore pressure diffusion.After the water level rises rapidly,the seismic activity increases significantly,and there is a good correspondence in different stages of impoundment between water level and seismic activity.The seismic response time and the correspondence between seismic activity and water level change are different for different reservoirs,which is not determined by the water level change but mainly by the different seismogenic environments and mechanisms.Compared with tectonic earthquake sequences,the decay of seismic activity in reservoirs is usually slower,and a significant characteristic of reservoir seismic sequences is that the seismic activity decreases and then increases again after the water level changes dramatically.(2)This study collected earthquake catalog and phase observation reports of 7213 earthquakes with magnitudes above ML0.5 in the Jinping Reservoir and its surrounding areas,and obtained precise location results for 6664 earthquakes by using the double-difference location method.The precise location results showed that after impoundment,the distribution of earthquake swarm near Hulugou became more diffuse and arranged in a divergent pattern along the NS,NNW-SSE,and NW-SE directions,which is different from the NW-SE and NNW-SSE seismic belt distribution in the study area before impoundment.The source depth of earthquakes in study area before impoundment was concentrated between 6 and 20km,while the source depth of earthquakes in swarm after impoundment became shallower,ranging from 4 to 14km.In most cases,reservoir earthquake with initial seismic response time relatively slow and relatively far from the reservoir bank is deeper than that of other reservoir earthquakes.Compared with tectonic earthquake sequences,the source depth of reservoir earthquakes is shallower,mostly above 14km,and the epicentral distance from the river bank is mostly within 10km.(3)The source mechanism of earthquakes that occur after impoundment in the Jinping Reservoir is mostly strike-slip type,with a small amount of thrust type distribution.Strike-slip mechanisms account for the largest proportion in many reservoir earthquakes,and pore pressure plays an important role in the seismogenic process of this type of reservoir earthquakes.As pore pressure increases,the effective normal stress on the fault decreases,leading to a reduction in the shear strength of the fault and ultimately causing earthquakes.(4)After impoundment,the distribution of P and T axes near Hulugou is more complex,and the majority of P and T axes are close to horizontal,which is different from other areas in the study area.After impoundment,the average principal stress field take deviation,and the maximum principal stress axis near Hulugou is closer to the N direction,and the minimum principal stress axis is closer to the E direction.The maximum and minimum principal stress axes are nearly horizontal,and the intermediate stress axis is nearly vertical.Such stress field environment is conducive to the sliding of oblique-slip and strike-slip faults.(5)After impoundment,the R value of the earthquake swarm near Hulugou increased compared to the R value in the study area before impoundment.The intermediate stress value became closer to the maximum principal stress value,and the eigenvalues of the stress tensor showed an arithmetic progression.Other reservoir earthquakes that observed an increase in R value after impoundment indicate the influence of the additional stress field caused by pore pressure diffusion superimposed on the regional tectonic stress field.(6)The results of body wave velocity structure research show that there are significant lateral differences in the velocity structure between 4 and 14 km in the study area,with multiple small areas of high-speed and low-speed anomalies.The earthquake swarm near Hulugou are mostly located at the edge or within the high-speed body,and the seismogenic environment of the earthquake swarm is different from that of the surrounding tectonic earthquakes.The seismogenic environment of the Jinping Reservoir earthquake is more similar to that of other reservoirs such as Xinfengjiang Reservoir and Three Gorges Reservoir,and also has similarities with the seismogenic environment of larger magnitude tectonic earthquakes.Unlike reservoirs with a seismogenic environment that is mostly large-scale low-speed bodies dominated by infiltration,the seismic mechanism of the Jinping Reservoir earthquakes focuses more on the influence of pore pressure.It is speculated that the high-speed anomaly body near Hulugou has accumulated a large amount of strain energy,and the water storage process of the reservoir promotes the release of accumulated strain energy from the high-speed area to the low-speed area.The seismogenic environment and mechanism of the Jinping Reservoir earthquake mainly result from the superposition of the additional stress field caused by pore pressure diffusion and the already high regional stress field,causing the release of strain energy from the high-speed anomaly body below the earthquake swarm to adjacent low-speed anomaly bodies.(7)Due to the different degrees of influence of load,infiltration,and pore pressure,different rock types and permeability conditions,geological structural conditions,and regional stress situations,there are differences in the seismogenic environment and mechanism of different reservoir earthquakes.The seismogenic environment and mechanism of reservoir earthquakes are significantly different from those of tectonic earthquakes but also have some similarities.