Numerical modelling of pore pressure variations due to time varying loads using a hybrid technique: the case of the Itoiz reservoir (Northern Spain)

Abstract

In this work, we present a hybrid technique to estimate the pore pressure variations at the neighbourhood of dams due to time varying water loads in their reservoirs. When considering flow boundary conditions, the solution of the equations of the problem can be obtained as the superposition of (1) the part computed from a homogeneous diffusion equation with the Dirichlet boundary condition (this is due to the pore pressure diffusion) and (2) the solution of an initial value problem in an inhomogeneous diffusion equation in which the inhomogeneous term is related with the stress variations due to the water loads (the solution due to the compression in the medium). Here, two different techniques are joined to calculate each one of these partial solutions: the pore pressure diffusion term is obtained by using the Green's function of the problem, whereas the second contribution due to stress time changes is computed with a finite difference method.

This hybrid technique has been used to compute the pore pressure variations produced by the initial impounding of the Itoiz reservoir, northern Spain. The possible relation between the reservoir and a close seismic series occurred on 2004 September, 8 months after the beginning of its impounding, is investigated. We pay special attention to the pore pressure changes at the hypocentre location of the main shock (with magnitude Mw= 4.5), and also evaluate the change of the Coulomb Failure Stress (ΔCFS) produced by the water loads in the reservoir over the fault responsible of this main shock, obtaining a maximum change of 0.5 kPa in the best of the cases. Accordingly, it seems that the role of the impounding of the reservoir to the main shock was marginal, and that the main load on the origin of the triggered seismicity could well be related to the regional state of stresses of the Pyrenees range and adjacent zones.