Crustal deformation processes occur with relative rapidity when short-term precursors appear at the final preparatory stage of a tectonic earthquake. This paper contains calculations of crustal bulk strain before an earthquake, based on the model of a rigid inclusion. The data give initial conditions for the problem of fluid percolation in a two-phase medium accompanied by electrokinetic effects. The solution provides a description of systems of currents arising in the vicinity of the source region. An analysis of this theoretical evidence yields information on the behaviour of short-term precursors in telluric currents and the geomagnetic field.