Study On Reservoir Induced Earthquake In Nyingchi Section Of Yarlung Zangbo River Based On Gravity Data

This thesis uses high-precision gravity/GNSS hybrid observation data to study the Yarlung Zangbo River at the Eastern tectonic knot of Himalayas.The crustal density structure of the valley is inverted,and the isostatic additional force is calculated.Besides,through the change of Coulomb stress caused by the impoundment of the reservoir,the location of the pre-built reservoir and the height of the dam are finally given.The Kriging interpolation model is used to predict the difference between the model data and the fusion data of the free-air gravity anomaly of the Eastern tectonic knot.It is concluded that the free-air gravity anomaly corrected by the Kriging interpolation model is closer to the measured value.The main researches are as follows:1.Based on the measured high-precision gravity/GNSS data,the process includes adjustment treatment,topography correction,stone slab correction,etc.,and obtained the free-air and Bourger gravity anomaly field.The bouguer gravity anomaly reaches its smallest value near Sangri,about -500 m Gal;the highest value appears in the west of Milin,about -330 m Gal.2.Inverted the density structure of the upper and lower reaches of the Yarlung Zangbo Grand Canyon.In the Sangri-Nyingchi section in the upper reaches of the Yarlung Zangbo Canyon,the density of the upper crust varies between 2.6 g/cm~3 and 2.8 g/cm~3,while the density of the lower crust is 2.85 g/cm~3;the crustal density of the Medog section in the lower reaches of the canyon gradually increases from shallow to deep,and its density varies between 2.6 g/cm~3 and 2.85 g/cm~3.In addition,the inverted Moho of the Yarlung Zangbo River valley has a relative uplift on the west side of Milin.It is found that it is caused by the upwelling of mantle material through the fragile part of the crust.3.The isostatic surface depth of the Sangri-Nyingchi section in the upper reaches of the canyon varies from 55 km to 65 km,and the isostatic surface depth of the Medog section in the downstream varies from 37 km to 45 km.Under the layered crustal model,when the isostatic depth and Moho depth are known,the isostatic additional force can be obtained.The isostatic additional force is upward from Sangri to 230 km of the profile,while it is downward from 230 km of the profile to the Nyingchi section,and it is upward in the Medog section.In the vicinity of Lang County,100 km upstream of Milin,the isostatic additional force is upward.Building a large reservoir here will make the lithosphere more balanced,thereby reducing the risk of reservoir-induced earthquake.4.Large reservoirs will form a huge body of water after impounding,which will affect the Coulomb stress on the fault in the reservoir and the surrounding area.When the physical parameters of the fault are determined,the size and shape of the water storage determine the magnitude of the Coulomb stress change on the fault.Through the elevation data,the water forms with the levels of 3100 meters,3150 meters and3180 meters were obtained,and the Coulomb stress changes on the Yarlung Zangbo River fault were calculated under these three water storage conditions.The results show that when the water level reaches 3180 meters,the Coulomb stress change on the Yarlung Zangbo River fault has a value of 0.11 bar.Therefore,it is finally recommended that the water level of the reservoir should not exceed 3,180 meters,so as to reduce the risk of reservoir-induced earthquake.5.The free-air gravity anomaly field at the Eastern tectonic knot of Himalayas is obtained by remove-and-restore prediction method,and its value varies between-350 m Gal and 550 m Gal.The difference between the fusion gravity data and the model gravity data is mainly concentrated near the on-site gravity observation point.The difference between the two is up to 200 m Gal.However,the gravity anomaly far away from the on-site observation point has not been corrected too much.The Kriging interpolation model is used to predict the difference between the fusion data of the unobserved points and the model data.The final result shows that the free-air gravity anomaly corrected by the Kriging interpolation model is closer to the measured value.