New in situ measurements of the pressure dependence of permeability at the Continental-Deep-Drilling Project in Windisch-Eschenbach/Germany (KTB) indicate a much stronger sensitivity compared to measurements of core samples. The non-linear pressure-dependent permeability combined with the oscillating tidal forces provide the basis for a pumping mechanism in porous rocks. Numerical simulations of the interaction between tidal strain and fluid pore pressure show a residual flow beside fluid oscillations for the simplified model of a u-shaped fault system. The magnitude of the residual flow is dependent on the permeability and the scale of the area considered. A residual Darcy velocity of 4⋅10-14 m/s was calculated with KTB permeabilities. ¿ 1998 American Geophysical Union