The role of pore pressure difusion in a reservoir-induced seismicity site in NE Brazil/
The Açu Dam (Barragem Eng. Armando Ribeiro Gonçalves) is a 34m high carth-filled dam, which is 2.4xl09m3 in volume, and is located ncar thc city of N. São Rafael, in NE Brazil. This is an arca of the Precambrian shield composed of a paragneiss, biotitc gneiss, gneiss-migmatic complex of Archcan form...
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| Endereço do item: | https://app.bczm.ufrn.br/home/#/item/60390 |
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| Resumo: | The Açu Dam (Barragem Eng. Armando Ribeiro Gonçalves) is a 34m high carth-filled dam, which is 2.4xl09m3 in volume, and is located ncar thc city of N. São Rafael, in NE Brazil. This is an arca of the Precambrian shield composed of a paragneiss, biotitc gneiss, gneiss-migmatic complex of Archcan formation and biotite granite of Neoproterozoic age. Scismie monitoring, around this water reservoir, began in 1987, two years after impoundment. The largest magnitude earthquake so far (magnitude 2.8mb) occurred in August 1994. Previous workers have shown, using a smoked-drum seismograph network operated during two field campaigns (1989 and 1990/91), that the scismic activity occurred preferentially with strike-slip dextral focal mechanisms on NE oriented faults. Their work concluded that the scismicity was a typical case of pore pressure triggcring of seismic events in NE oriented faults undcr rcgional E- W compression. From August 1994 until May 1997, a network of 8 three-component digital scismographs operated at the Açu reservoir. A detailed analysis of this data forms the first part of this thesis. The carthquake locations revealed remarkably well-defined NE trending fault struetures beneath the reservoir due to the#$&simple seismic-wave velocity struture of the area. This digital data is shown here to be very suitable for focal mechanism determination and also to investigate scismic anisotropy in the area. This is possible due to the low attenuation of the scismic waves and good signalto-noise ratio in the seismograms provided by the erystalline rocks of the area. The foca lmechanism determination and the shear-wave study of this seismicity confirm that the area is in E-W compression and also that the shear-wave splitting is controlled by the scismic anisotropy associated with the Precambrian foliation. Moreover, this study shows that the seismicity migrates between different faults and within individual faults over different time periods. To investigate this behaviour and relate it to changing water levels in the reservoir and the resulting groundwater flow beneath, a fully three-dimensional groundwater flow code (PARADIGM) is used. PARADIGM is employed to simulate the pressure field in both the rock matrix and an idealised fault geometry. This ftow modelling forms the second major component of this thesis. The significance of my approach is that I explieit1y consider the physical meaning of hydraulic conduetivity and storativity and their impact on ftuid flow. In this model, both realistic information on the hydrogeological regime of the area and measurements of real fault zone hydraulie properties are#$&included. The combination of the spatio-temporal analysis of the scismic activity and the numerical simulations carried out in PARADIGM provide new insights into the mechanism that causes the migratory behaviour. The integrated investigation present in this thesis (numerical simulations plus the spatio-temporal analysis of carthquakc locations) shows that the timing and location of the seismic events are controlled by a small number of highly permeable, hetcrogeneous fault structures. This means that an homogeneous equivalent medium cannot be derived to explain the scismicity in this area. I also show that the migratory bchaviour is a natural eonscqucnee of thc heterogeneity fault hydraulic properties. Thc pressure variations neeessary to trigger scismicity are minute (0.01 bar) and corroborates the idea that the tectonie stress conditions must be close to the critical value for failure. |
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