Constraints from magnetotellurics on the geological structure across a seismic area associated with water wells in the Paraná basin, Brazil

Abstract

The occurrence of earthquakes associated with the opening of groundwater wells is a very rare phenomenon. We present the results from a magnetotelluric profile of about 4 km-long that crosses a seismic zone near the town of Bebedouro, in the Northeastern part of the intra-cratonic Paraná basin, in Brazil. The seismic activity is thought to be caused by a raise in pore-pressure within a fractured basalt aquifer due to the natural injection of water from the shallow aquifer into the basalt. Regularized 2D inversions of the magnetotelluric data show that the basalt layer presents both electric resistivity (60-300 ohm-m) and thickness (400-600 m) within the expected ranges for the study area. However, an unforeseen abrupt change (~200 m) in the basalt thickness has been observed. This variation occurs close to the Western limit of the seismic zone and it nearly coincides with a geoelectric basement deepening of about 600 m. We interpret these lateral contrasts as a fault system that probably controlled the deposition of the Permian to Jurassic volcano-sedimentary sequence under the study area. Our study suggests that a preexisting structure affecting the basement and the basaltic layer is determinant in favoring seismicity associated with the opening of water wells.