THE IMPACT OF FLOW THROUGH HETEROGENEOUS FAULTS AS REVEALED BY 3D FLOW MODELLING OF RESERVOIR-INDUCED SEISMICITY IN NORTHEAST BRAZIL

In NE Brazil, earthquake activity associated with the impoundment of a 4km by 35km wide and 34m deep reservoir shows an example of reservoir-induced seismicity driven by pore pressure diffusion. This seismicity is observed to migrate between different faults and within individual faults over different time periods. To investigate this behaviour due to changing water levels in the reservoir and the resulting groundwater flow beneath, we use a fully three-dimensional groundwater flow code (PARADIGM). PARADIGM is employed to simulate the pressure field in both the rock matrix and an idealised fault geometry, assuming a variety of hydraulic property distributions for both the matrix and fault. A sinusoidal, time-varying lake level is applied in the simulations to investigate the relationship between surface water levels in the reservoir and induced seismicity. The numerical simulations show that the timing and location of the seismicity are controlled by a small number of highly permeable heterogeneous fault structures and therefore, an up-scaled equivalent media can not be derived to explain the pattern of observed seismicity.