Seismic activity triggered by water wells in the Paraná Basin, Brazil

Abstract

[1] Triggered seismicity is commonly associated with deep water reservoirs or injection wells where water is injected at high pressure into the reservoir rock. However, earth tremors related solely to the opening of groundwater wells are extremely rare. Here we present a clear case of seismicity induced by pore-pressure changes following the drilling of water wells that exploit a confined aquifer in the intracratonic Paraná Basin of southeastern Brazil. Since 2004, shallow seismic activity, with magnitudes up to 2.9 and intensities V MM, has been observed near deep wells (120-200 m) that were drilled in early 2003 near the town of Bebedouro. The wells were drilled for irrigation purposes, cross a sandstone layer about 60-80 m thick and extract water from a confined aquifer in fractured zones between basalt flow layers. Seismic activity, mainly event swarms, has occurred yearly since 2004, mostly during the rainy season when the wells are not pumped. During the dry season when the wells are pumped almost continuously, the activity is very low. A seismographic network, installed in March 2005, has located more than 2000 microearthquakes. The events are less than 1 km deep (mostly within the 0.5 km thick basalt layer) and cover an area roughly 1.5 km × 5 km across. The seismicity generally starts in a small area and expands to larger distances with an equivalent hydraulic diffusivity ranging from 0.06 to 0.6 m 2 /s. Geophysical and geothermal logging of several wells in the area showed that water from the shallow sandstone aquifer enters the well at the top and usually forms waterfalls. The waterfalls flow down the sides of the wells and feed the confined, fractured aquifer in the basalt layer at the bottom. Two seismic areas are observed: the main area surrounds several wells that are pumped continuously during the dry season, and a second area near another well (about 10 km from the first area) that is not used for irrigation and not pumped regularly. The main area displays cyclic annual activity, but the second area does not. We explain the earthquake swarms as being triggered by pore pressure diffusion in the fractured basalt layer due to additional pressure from the newly connected surface aquifer. This reaches critically prestressed areas up to a few kilometers away from the wells. During periods of continuous pumping, the reduction of pore pressure in the confined aquifer stops the seismic activity. Our study suggests that this kind of activity may be more common than previously thought and implies that many other cases of small tremors associated with the drilling of water wells may have gone unnoticed.

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“…Vidale and Shearer, 2006). Other theories such as anthropogenic overexploitation of aquifers, seasonal changes of piezometric levels and heavy rainfall with associated hydraulic diffusion could also explain earthquake triggering in areas of low seismic activity (Hainzl et al, 2006;Assumpcao et al, 2010).…”

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“…The studied region was defined according to the previous seismicity study carried out by Assumpção et al [24] at this area and according to further seismic events registered by the Seismology Laboratory from IAG/USP. The soundings were allocated in order to have the best match possible with the most significant seismic activity registered.…”

“…This seismic activity is concentrated mainly at two areas: Andes, where the activity was first noticed and is more frequent, and Botafogo [17] [24]. Both seismic areas are shown in Figure 1.…”

“…Some of them produced water at very high flow rates (160 m 3 /h to 190 m 3 /h) compared to the other wells in the same region (up to 88 m 3 /h). Studies in that area using seismic, geoelectric and electromagnetic methods showed a relation between the seismic activity and a changing groundwater flow due to over exploitation of the Serra Geral aquifer [24].…”

“…Both seismic areas are shown in Figure 1. In this figure, they were delineated based on information from the epicenters located by Assumpção et al [24]. The wells at these areas are 120 to 200 meters deep and were drilled into the basalt of Serra Geral Formation, characterized by a confined fractured aquifer [25].…”