Theories at this time
Semiconductor effect [Freund, 2000], in which rock starts to fracture, releasing charge carriers (p-holes), which in turn, generate large ground currents and corresponding magnetic field disturbances, as well as IR signatures in 8 and 11µm bands when the charges are neutralized on the earth’s surface.
Piezomagnetism: the magnetic properties of rock are known to change as a function of applied stress [Revol et al., 1977] and have been combined with stress changes from dislocation models of fault rupture [Johnston, 1978; Banks et al., 1991] to produce magnetic field fluctuations of a few nanoTesla, which are readily observed [Johnston, 1997].
Resistivity changes: the electrical resistivity of porous rocks is known to change as a function of compression and shear [Brace et al, 1965; Yamazaki, 1965], which can be measured using passive (telluric or magnetotelluric) or active experiments [Park et al., 1993].
Streaming potential: when a conducting fluid is forced to flow past a stationary charged surface, an electrokinetic effect can cause currents to begin flowing either in the fluid itself or in the surrounding rock [Fitterman 1979], which, under realistic crustal parameters can generate surface magnetic fields of a few nanoTesla [Fenoglia et al., 1995].
Magnetic signal generation by either conducting fluid flowing in the presence of the Earth’s magnetic field or by the magnetohydrodynamic conversion of a seismic signal into an electromagnetic signal during propagation in a conductive medium [Molchanov et al., 2001]. While these mechanisms have been proven to work in laboratory settings, it is unclear how they behave in conditions more closely approximating those of the core (wet rocks under confining pressure) due to the absence of detailed measurements in active fault zones.
Other proposed mechanisms include piezoelectric effect [Finkelstein et al., 1973], triboelectricity [Gokhberg et al., 1982], fluid vaporization [Chalmers, 1976],
