Improving the characterization of the seismic source in Bebedouro, Paraná Basin, Brazil: further evidence of seismicity triggered by hydraulic stimulation in water wells.

We investigate a long series of small earthquakes (magnitude < 3) with annual cycles between 2004 and 2010 that occurred in the northeastern Paran'a Basin, Brazil. These events were attributed to pore pressure increase in a fractured aquifer caused by the drilling of several water wells down to depths of ~200 m. Because of the poor depth constraints of the initial study, we relocated ~1000 microtremors recorded by the local seismographic network using a more accurate velocity model. To better relocate the events, we based the velocity model on geophysical survey data obtained using several techniques, including seismic refraction, surfacewave dispersion, vertical electrical sounding, high-frequency receiver functions and the time-domain electromagnetic method. The best 1-D modelwas calibrated using a simultaneous inversion of hypocentres and a velocity model including station corrections. The resulting focal depths, which are mostly between 100 and 175 m, place the events at depths consistent with the confined aquifer within the basaltic pack. This result confirms that the earthquakes are related to geological stress disturbances (pore pressures) in pre-existing fracture zones within the basalt layer caused by the perforation of water wells used for irrigation. We used a combination of cross-correlations and arrival times to analyse a cluster of 19 earthquakes. The improved hypocentre distribution of this cluster allowed the determination of a clear faultplane solution, which indicates a normal fault striking WNW-ESE and dipping to the north, with an NNE-SSW extension (T-axis). This mechanism complements stress data collected in southeastern Brazil and is consistent with the regional stress regime. [ABSTRACT FROM AUTHOR]