We determined the electric and magnetic fields generated during failure of faults containing sealed compartments with pore pressures ranging from hydrostatic to lithostatic levels. Exhumed fault studies and strain measurement data limit the possible size of these compartments to less than 1 km in extent. Rupture of seals between compartments produces rapid pore pressure changes and fluid flow and may create fractures that propagate away from the high-pressure compartment, along the fault face. Nonuniform fluid flow results from pressure decrease in the fracture from crack-generated dilatancy, partial blockage by silica deposition, and clearing as pressure increases. A direct consequence of this unsteady fluid flow may be associated transient magnetic signals caused by electrokinetic, piezomagnetic, and magnetohydrodynamic effects. Models of these processes for fault geometries with 1-km-high pressure compartments show that electrokinetic effects are several orders of magnitude larger than the other mechanisms. The electrokinetic signals produced by this unsteady flow are comparable in magnitude and frequency to the magnetic signals observed prior to the ML 7.1 Loma Prieta earthquake of October 18, 1989, provided fracture lengths are less than 200 m. ¿ American Geophysical Union 1995