This study main aim was to analyse the spatio-temporal trend in seismicity recorded in the proximity of the Pirrís Reservoir (central Costa Rica), where impoundment for the purposes of flling the reservoir to its total volume (3,6*107 m3) started in 2011. We diferentiated between the events that occurred before, during and after this flling operation. Using a seismic analysis, we sought to defne and understand the efects which such reservoir operations have on seismic activity in the area. To this end, we evaluated the spatio-temporal evolution of Coulomb failure stress (ΔCFS) changes due to surface water load, and its correlation with seismicity. Overall, the results of this study provide a perspective of how the water load in the reservoir can afect the stress state in the close area. In our study case, we have detected: an increase in b-value after impoundment, an increment of rate for shallowest events (h≤10km), an increasing trend of higher magnitude events and a possible trigger efect on local faults. All these aspects could be useful to control the reservoir operations and to help in decision making in order to guarantee the safety of these critical emplacements.
