INVESTIGATING THE RELATIONSHIP BETWEEN FAULT PERMEABILITY AND EFFECTIVE STRESS USING CONSTRAINTS FROM RESERVOIR INDUCED SEISMICITY Rebecca J Lunn¹, Aderson F do Nascimento², Patience Cowie 3 ¹) School of the Built Environment, Heriot-Watt University, Edinburgh ²) Universidade Federal do Rio Grande do Norte, Programa de Pós-Graduação em Geodinâmica e Geofísica 3 ) Department of Geology and Geophysics, University of Edinburgh Abstract: This research uses observations of RIS beneath Açu Reservoir, NE Brazil, to investigate the damage zone permeability of geological faults. High-resolution digital seismic monitoring of the reservoir has provided unusually detailed information on the locations of seismic events. The temporal distribution of these events shows them to be directly related to annual fluctuations in the reservoir level. Model simulations, using a decoupled hydromechanical formulation, show that for pressure-diffusion to be a hydrogeologically consistent mechanism for RIS, preferential flow must occur within 2D fault planes embedded in a 3D low permeability matrix. Further, the observed spatial and temporal variability of earthquakes indicate that these faults must have heterogeneous permeability fields with significant spatial structure; pockets of high and low permeability of the order of 0.5 - 1.5 km in diameter. Predictions of the maximum pressure change in these faults at hypocentral depths indicate <0.05 kPa is required to trigger seismic events. 1. INTRODUCTION This research uses observations of reservoir induced seismicity (RIS) at Açu reservoir, NE Brazil, to investigate the spatial and temporal evolution of effective stress in the region and its relationship to fault permeability. Açu reservoir was constructed in 1983 and has a capacity of 2.4 × 10 9 m 3 maintained by a 34 m high earth-filled dam constructed on Precambrian shield. Annual reservoir variation is 3-6 m which results in annual seismic activity due to a proposed mechanism of pore pressure diffusion (Ferreira et al. (1995), do Nascimento et al. (2003a)). Digital data at Açu Figure 1. Monthly variation of reservoir water depth (top line) and monthly seismic activity (histograms) at Açu reservoir. have revealed the seismic activity in remarkable detail (do Nascimento et al. 2003a). The majority of earthquake activity is clustered within several well-defined zones and individual zones are active over discrete periods of time. In this paper we present the main features of the seismicity data and propose that the observed spatial and temporal evolution of seismicity at Açu is defined by pressure diffusion through local fault planes with heterogeneous permeability structures. Table 1. Activation periods of clusters recorded by the digital seismic network and permanent station BA1. Period of activation Active seismic cluster Nov. 1994 - 15/Aug./1995 a, BA1 cluster 15/Aug./1995 - 21/Oct./1995 a 1/Oct./1996 - 22/Nov./1996 b 25/Nov./1996 - 30/Dec/1996 c,b 30/Dec./1997 - May/1997 a,b 2. SEISMIC ACTIVITY AT AÇU Seismic activity in the Açu area has been monitored over a ten year period from 1987-1997. However, accurate hypocentral information is only available from 1994 to 1997, when a network of three-component digital seismographs were