Numerical modelling of pore-pressure diffusion in a reservoir-induced seismicity site in northeast Brazil

Abstract

A 3-D fluid-flow model is used to investigate pore-pressure diffusion as a mechanism for reservoir-induced seismicity (RIS) at the Acu reservoir in NE Brazil. The Acu dam is a 34-m high earth-filled dam constructed in 1983 on an area of Precambrian shield. Seismic activity in this area has been monitored over a 10-yr period (1987-1997). The frequency of earthquakes clearly varies with seasonal fluctuations of the reservoir level. Information on the hydrological regime of the Acu reservoir (rainfall, vegetation, surface and subsurface storage, etc.) is used to set up a regional groundwater-flow model in order to obtain boundary conditions for a more detailed study of the area of seismic activity. To explain the observed time lag between maximum reservoir level and peak seismic activity we calculate the magnitude and timing of the maximum piezometric head in the depth range of observed seismic activity. By assuming that individual earthquake ruptures occur when the local piezometric head is at a maximum, values of bulk permeability, K, and storativity, S, are derived. If a 3-D homogenous subsurface permeability structure is assumed then the values of K and S obtained are not self-consistent and are physically unrealistic. However, if a high-permeability fault is embedded into, and explicitly coupled with, the surrounding lower-permeability matrix, then our estimates of subsurface hydraulic properties agree well with other field and laboratory measurements. The inclusion of a discrete fault plane in the model is consistent with the results of high-resolution seismic monitoring using a local digital network of stations, which show that the earthquake hypocentres define a set of steeply dipping NE-striking fault planes beneath the reservoir.