Numerical solutions of Maxwell equations for a model earth with a reasonably conducting channel indicate that the electric field values are intensified within a certain region only (i.e., above the end of the channel), thus explaining the observed selectivity effect. In this region, the electric field may reach detectable values (5--10m V/km), while the magnetic field still remains low (10-2 nT). The results are compatible with those obtained recently by analytical solutions (Varotsos et al. [1998>). Both the numerical and the analytical solutions lead to a natural explanation of the ΔV/L≈const criterion. This criterion, however, should not be applied over an area with strongly inhomogeneous electrical structure. ¿ 1999 American Geophysical Union