MAGNETIC AND ELECTRIC-FIELDS ASSOCIATED WITH CHANGES IN HIGH PORE PRESSURE IN FAULT ZONES - APPLICATION TO THE LOMA-PRIETA ULF EMISSIONS

We determined the electric and magnetic fields generated during failur e of faults containing sealed compartments with pore pressures ranging from hydrostatic to lithostatic levels. Exhumed fault studies and str ain measurement data limit the possible size of these compartments to less than 1 km in extent. Rupture of seals between compartments produc es rapid pore pressure changes and fluid flow and may create fractures that propagate away from the high-pressure compartment, along the fau lt face. Nonuniform fluid flow results from pressure decrease in the f racture from crack-generated dilatancy, partial blockage by silica dep osition, and clearing as pressure increases. A direct consequence of t his unsteady fluid flow may be associated transient magnetic signals c aused by electrokinetic, piezomagnetic, and magnetohydrodynamic effect s. Models of these processes for fault geometries with 1-km-high press ure compartments show that electrokinetic effects are several orders o f magnitude larger than the other mechanisms. The electrokinetic signa ls produced by this unsteady flow are comparable in magnitude and freq uency to the magnetic signals observed prior to the M(L) 7.1 Loma Prie ta earthquake of October 18, 1989, provided fracture lengths are less than 200 m.