Study On Mechanism Of Reservoir-induced Seismicity Based On Theory Of Fluid-solid Coupling In The Danjiangkou Reservoir Area

Reservoir-induced seismicity (RIS) is formed due to the alteration of the original seismic activities’frequency and intensity in reservoir area resulted from impoundment. In general, the epicenter of RIS is very close to important hydraulic facilities. The RIS with medium intensity often occurs at reservoir area with deep water near the dam of reservoir and its periphery, which has the characteristics of shallow hypocenter and high epicentral intensity. Therefore, the RIS with medium and high intensity in particular not only damages the hydraulic facilities but also results in severe secondary disasters which may pose risks to the downstream area. The emergence of RIS throughout the world drew the extensive attentions of researchers, engineering community and the public. A lot of research work had been carried out, in which many qualitative findings were yielded but there is no mature theory to explain the mechanism of RIS at present. Danjiangkou reservoir is one of the largest reservoirs built in our country, and its reclaiming project is the source of south-to-north water diversion, the grand project across the century in our country. Since the impoundment in 1967, the seismic activities at the reservoir area have been more active (for example, a 4.7-magnitude earthquake with the epicentral intensity of 7 occurred at Songwan in 1973), which undoubtedly poses threats to the security of the dam and people around the reservoir area. In the reclaiming project, the water level arose to 170m, which could induce earthquake with higher intensity or not is an important issue concerned by many people. In regard to this, this paper took Danjiangkou reservoir as an example to study the mechanism of RIS by way of fluid-solid coupling theory, which was funded by National Science Foundation of China. The main work of this paper is presented as follows:1. Based on the experimental results of mechanical properties of rocks at Danjiangkou reservoir, the variation of the mechanical parameters of rocks at Danjiangkou reservoir with the depth smaller than 20 km was studied with reference to the relationship between the horizontal principal stress deviator and the depth obtained from the KTB continent deep drilling in Germany and the variation trend of the measured horizontal average ground stress deviator with the depth in our country. It was concluded that mechanical properties of rock vary with the depth, in which elastic modulus, cohesion strength, internal friction angle and yield strength rise first and then drop with the increase of depth ranging from 0 to 10 km, but poisson ratio has the reverse observation.2. According to the geological conditions of the Danjiangkou reservoir, the computational model was firstly set up considering fracture zone and the variation of mechanical properties of rock with the depth. Adopting this model, the initial ground stress field of the reservoir area was derived, which shows that:1) the critical depth of ground stress filed of the reservoir area is about 1km; 2) the horizontal principal compression stress at the surface with the depth smaller than 500m falls in the range from 13MPa to 23MPa and the horizontal principal tension stress varies from 3 MPa to 12 MPa; 3) when the depth is smaller than 10km, the difference between the two horizontal principal stresses has nearly the linear relation with the depth. However, it reduces suddenly at the depth of 10km and disappears at the depth over 20km with the rock in hydrostatic pressure.3. By way of adding the perimeter of the reservoir to the model set up previously to derive the initial ground stress field and then applying the body load of water, the numerical simulations of hydraulic pressure, strain field and displacement field resulted from the phase-Ⅰand phase-Ⅱimpoundment of the reservoir were performed. It shows:1) the hydraulic pressure and displacement field resulted from the phase-Ⅰimpoundment is the main cause inducing a series of minor earthquakes around the periphery of the reservoir,2) the increase of hydraulic pressure and displacement resulted from the phase-Ⅱimpoundment and the stress accumulated for 30 years will activate a series of minor earthquake again at Houpo, Jiuzhong, Pengqiao, Changfang and Tiancuan around the periphery of the reservoir. The comparison analyses between the tectonic stress field corresponding to before the phase-Ⅰimpoundment, after the phase-Ⅰimpoundment and after the phase-Ⅱimpoundment respectively were carried out, which indicates that the tectonic stress field after impoundment is roughly same as that before impoundment, the stress resulted from the hydrostatic pressure changes little and earthquake with high intensity is not likely to be activated by the stress resulted from the water body load.4. Based on fluid-solid coupling theory, Danjiangkou reservoir is simulated taking into account the heavy stress field, the tectonic stress field, hydraulic stress field and seepage field. Mechanism of reservoir-induced seismcity is revealed. (1) Fracture or crack rock in the surface of Danjiangkou reservoir area is in the critical state of tension under the tectonics normal stress, broken rock in the depth of 9km about is in the critical state of compression under the tectonics normal stress. (2) Normal stress caused by watering of phase-Ⅰand phase-Ⅱmake the zone in width 2-12km around reservoir in the tension state, which accelerate the damage of fracture or crack rock, and make the rock in the tension state about in the 9km depth below the reservoir. This tension accelerate the rock in this depth to damage with the technical stress. (3) Pore water pressure increases and the depth of the largest pore pressure decreases under the reservoir with the raising of the water level. Greater is the pore water pressure, the effective stress and shearing strength of the rock is smaller.(4) The raising of the water level increases coupling. Then it makes the stability of the rock around the reservoir in the depth of 5-12km drop. it is predicted that a series of moderate earthquakes would be occurring in the place of songwan-guanfangtan-jiuxichuan, shangsi-heitaoyuan, linmaoshan-danjiangkou-qingshangang and zishanao-jiujunxian in the three years after phase-Ⅱimpoundment of the reservoir, the depth of earthquake epicenter is in the range of 5 to 11km.Lastly, the problems to be solved for this research and some suggestions for the future work are given based on the summary of this paper.