A subsurface stress analysis and its possible relation with seismicity near the Itoiz Reservoir, Navarra, Northern Spain

We study the effects of surface water loads and moderate-magnitude earthquakes on the subsurface state of stress and its relation with seismicity, near the newly constructed Itoiz reservoir in the western Pyrenees, northern Spain. To do this, we initially compute the evolution of the stress changes in the subsoil due to the water load distribution during the impounding of the reservoir, and relate it with the main seismicity occurred after the beginning of impoundment. After this, we compute the coseismic and postseismic stress changes produced by the main events of the seismic series and study its influence on the triggering of the aftershocks. Stress changes due to the water loads are computed by means of the Boussinesq solution for a homogeneous elastic half-space whereas the coseismic stress changes due to earthquakes are obtained using the equations for an internal elastic dislocation in a half space. The effects over the possible seismogenic faults are in both cases, accounted by means of the Coulomb Failure Stress criterion (Delta CFS). We considered different threshold values and apparent friction coefficients of Delta CFS for the effective static triggering. Then, we calculate the pore pressure changes produced by the time evolution of the surface water loads, as also the pore pressure changes generated by the elastic dislocations due to the largest earthquake of the series (mainshock). The fault parameters for the computation of the Delta CFS of the perturbing seismic events were selected from published focal solutions. To define the target fault parameters we took into account the focal solutions of the earthquakes and aftershocks themselves, the parameters of mapped faults based on the tectonic setting and those of optimally oriented faults. Due to the absence of reliable estimations for the slip distributions of the main earthquakes, we assumed for stress computations a semi-elliptical distribution of slips over the fault. Results show the existence of a mainly positive stress influence of the surface water loads on the main earthquake of the series, in two different senses: the elastic Coulomb failure stress and the change in pore pressure. However, due to different uncertainties on the possible values of the apparent coefficient of friction (mu') for the zone, there is not a clear indication of an effective earthquake triggering of the mainshock due to the elastic stress changes by the surface water loads. On the other hand, a more clear effect is found on the possible triggering of aftershocks due to the largest earthquakes of the series. It is also found a positive pore pressure influence of the two largest events on most of the aftershocks. (C) 2009 Elsevier B.V. All rights reserved.