We present the spatio-temporal evolution of seismicity recorded by eight three-component digital seismographs in operation continuously during a 3 yr period (1994 August to 1997 May) at Açu reservoir, NE Brazil. The Açu dam is a 34 m high earth-filled dam constructed in 1983 May on an area of Precambrian shield. Based on seismic monitoring between 1987 and 1989 using single-component analogue seismographs, previous workers concluded that the seismic activity was a case of reservoir-induced seismicity (RIS) associated with diffusion of pore fluid pressure beneath the reservoir. The digital data presented here reveal the seismic activity in remarkable detail with vertical and horizontal location errors ≈0.1 km. A total of 286 events were recorded by three or more stations and all occurred at a depth of <5 km. Using these data we demonstrate that the majority of the earthquake activity is clustered within several well-defined zones and that individual zones are active over discrete periods of time. Over the entire period of seismic monitoring between 1987 and 1997 there is no simple correlation between reservoir level and number of seismic events. Lateral migration of the locus of seismic activity in an unpredictable fashion is shown to be partly responsible for the poor correlation, as event detection is not uniform through time. We also show that the time delay between maximum water level and a subsequent increase in seismic activity varies systematically; longer time delays correspond to activation of an earthquake cluster with a greater average hypocentral depth. However, within any one cluster there is no correlation between time delay and depth. The 3-D distribution of seismic activity through time may only be explained in terms of triggering by the diffusion of pore fluid pressure if the rock properties (e.g. permeability, strength) are heterogeneous.
- civil engineering