Statistical characteristics of Reservoir-Triggered Seismicity in Brazil

The first case of Reservoir-triggered seismicity (RTS) in Brazil was reported in the early 1970s. Nowadays, the data base of RTS is comprised by 26 cases that has observed in all Brazilian geological environment, such greatest magnitude was 4.2 mb and intensity VI (MM). The reservoirs that triggered earthquakes were analyzed in relation to dam height, reservoir volume, geology, seismicity level, and stress provinces. The chance of a reservoir-triggering seismicity clearly increases with dam height, as is well known: the chance is only 2% for heights between 20 and 50 m in Brazil, increases to 8% between 50 and 100 m, and reaches 65% for dams taller than 100 m. The reservoir volume also has a clear influence: 26% of the reservoirs with volumes between 1.0 and 10 km3 triggered earthquakes, and the chance increases to 47% for volumes larger than 10 km3. No clear correlation with the geology was found. Dams built on igneous rocks tend to be slightly more prone to RTS, compared with those built on sedimentary rocks, but the difference is not statistically significant. No difference was found among the three main types of geological provinces (cratonic basement, Neoproterozoic fold belts, and intracratonic Phanerozoic basins). A tentative comparison with the yet poorly defined stress provinces in Brazil did not show any difference between regions of compressional stresses and regions with shear stresses. On the other hand, comparison of the fraction of reservoirs producing RTS in natural seismic and aseismic areas showed that regions with natural seismicity have twice as much potential for RTS than aseismic regions. Although dam height is the most influential characteristics, the maximum magnitude does not show any clear correlation with dam height or reservoir size, similar to other cases worldwide. However, almost all narrow and deep reservoir present seismicity few days after the first impoundment. This work discusses the probable main reasons why this occurs.