Seismic monitoring of the Açu reservoir (31-m depth, 2.4 × 109 m3 in volume), in Rio Grande do Norte State, Northeastern Brazil, started in 1987 about 2 years after impoundment. The largest earthquake so far (magnitude 2.8) occurred in August 1994. From 1987 to 1989, the monthly number of induced events had a clear correlation with the water level, with a 3-month delay, and the activity occurred mainly inside the reservoir. Although no preimpoundment monitoring had been carried out, the correlation of the water level and the seismicity strongly suggests the activity was induced by pore pressure diffusion. Since 1990, the activity migrated toward the border of the reservoir, and the number of events no longer correlated with the water level. A seismographic network was deployed around the reservoir during two field campaigns (1989 and 1990/91), showing that the activity occurred preferentially with strike-slip mechanisms on NE oriented faults. The E-W orientation of the P axes, parallel to the regional maximum horizontal stress, and the presence of many NE trending faults and fractures in the Precambrian basement near the reservoir, suggest that the probably induced seismicity was a typical case of water pore pressure facilitating earthquake occurrence in pre-existing zones of weakness under high regional stresses.

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