Theoretical calculations coupled with present-day knowledge of the dynamic tensile behaviour of rock indicate that the propagating shock wave, and accompanying rarefaction wave, from a large meteoritic impact event may fracture the crust down to the Moho. Diapiric action by trapped upper mantle fluids could lead to annealing of these sub-vertical faults by material of high carbon content. Are these faults observable on the surface using geophysical methods? One possible means proposed is to try to monitor current flowing down them by an appropriate natural-source electromagnetic experiment. An example is discussed for which such current flow may be occurring.