Study On Change Of Stress Field Before And After Filling Water In Nuozhadu Reservoir

The Yunnan Nuozhadu Reservoir studied in this article is one of the typical reservoir with high dam and large capacity in the western part of mainland China.The reservoir is located in the seismically active Yunnan region with complicated geologic structure.Comparing to low-intensity regions,more small earthquakes happen before reservoir filling,which provides more possibility for researching the characteristics of geophysical field changes before and after reservoir filling.This article discussed the seismic activity characteristics before and after reservoir filling using the secondary earthquakes time recorded by the reservoir network from June 2008 to December 2016,obtained their focal mechanism solutions based on the inversion of the first motion of the p-wave and the amplitude ratio of the p-wave and s-wave and obtained the changes in the stress field within the reservoir region before and after reservoir filling by inverting the spatial distribution of the focal mechanism solutionsThe main conclusions and results in this article are as follows1.Based on the seismic event statistics before and after reservoir filling inside the reservoir region which was defined by extending both sides 10km north from the reservoir along the backwater zone of the river,seismic activities after reservoir filling were increased obviously compared to that before reservoir filling and there was high correlation between the seismic activities and the water level after reservoir filling inside the reservoir region.The b value decreased after reservoir filling based on the Gutenberg-Richter law,which indicated that the stress field inside the reservoir region was changed.However,the b value outside the reservoir region did not change.We speculated that the earthquakes happened in this region were due to the normal geological activities.2.The catalog of the focal mechanism solutions was built for the earthquakes bigger than 3 magnitude inside and adjacent to the Nuozhadu Reservoir Region3.The focal mechanism solutions of the medium and small earthquakes with 3.0≤MS≤5.0 inside and adjacent to the Nuozhadu region showed that the earthquakes happened inside and outside the reservoir region before reservoir filling were mainly the strike-slip earthquakes.It indicated that horizontal movement was the main geological activity in this region.The focal mechanism did not have significant change after reservoir filling outside the reservoir region,while various focal mechanisms were shown inside the reservoir region.This change was possibly caused by the local stress field changes due to reservoir filling.4.The research results of the stress field before and after the reservoir filling inside and outside the reservoir region indicated that structural settings did not change much before and after the reservoir filling inside and close to the reservoir region and the plunge angle of the principal compressive stress axis inside the reservoir region changed from nearly horizontal before reservoir filling to approximately 50 degree after reservoir filling,while the plunge angle of the principal compressive stress axis outside the reservoir region did not have significant change.Through analysis,we thought that the increased gravity due to reservoir filling increased the shear stress on the fault plane inside the reservoir region and weakened the fault,which contributes to trigger earthquakes.