Several studies have been conducted to try and understand the source of electromagnetic signals observed prior to a seismic event. These phenomena are the result of stress variations which occur before an earthquake, but the exact physical processes at play have yet to be identified. Assuming the source of these waves is located at the epicentre creates a major problem concerning the attenuation of wave emitted in the ground, which is function of exp(-αx) where:
α = (πfµσ)1/2
f is the wave’s frequency,
µ is the magnetic permeability,
σ is the electric conductivity,
x is the distance from the source.
With commonly accepted values of σ between 10-2 and 10-5 S/m in rock, results show that, starting at 10 km, only very low frequency (VLF) waves (under 10 Hz) can reach the surface.
Physical processes likely to be the source of these signals can be sorted into two categories, depending on whether or not they include the presence of a fluid capable of travelling in a network of interconnected capilaries and pores within the rock matrix.
The two categories of physical processes are as follows:
Processes due to the presence of water in the rock
Geophysical measurements taken on the surface can only give the average resistivity of earth materials. In temperature and pressure conditions like the ones encountered in the first kilometres of the earth’s crust, cool and dry rock is very resistant (over 10,000Ω/m). In the case of porous rock gorged with water, the average measured resistivity (ρ) is proportionate to f-1ρfluid, with f-1 the volumetric percentage of fluid and ρfluid its electrical resistivity.
At equilibrium, pore diameter depends on the nature of the rock, hydrostatic pressure, and stress tensor affecting the rock. Change in the stress tensor will result in changing pore diameters, thus affecting the rock’s porosity. This in turn will affect f-1 and the environment’s average resistivity.
Two ion layers form where the rock matrix and the water come into contact. The first, usually with a negative charge, is strongly bound to the rock. The second layer has an opposite charge (usually positive), and contains ions which are free to travel within the liquid phase. This results in an electric potential difference between the solid rock and the liquid phase. When the pressure in the pores is different from the hydrostatic pressure (due for example to stress variations), the liquid phase moves; the movement of free ions in the liquid phase then create an electric current. This process shows the circulation of water within a network of interconnected pores and capillaries: this is called electrofiltration, or electrokinetic effect.
Piezoelectric and triboelectric effects
When applied to crystalline solids such as quartz, mechanical force induces movement of electric charges and thus creates a polarisation field within the crystal. This field has a linear relationship with the stress tensor through a third-order tensor called the piezoelectric tensor. It is at its highest when stress is applied along certain specific directions, the mechanical directions, which depend on the crystal structure.
When quartz-rich rock is fractured, the piezoelectric field can induce an electromagnetic wave. This happens when the creation of a new capillary doesn’t immediately modify the polarisation field. In other terms, this happens when a static capillary can retain its charge (against leakage currents) for a longer time than moving capillaries can impede its movement (against mechanical stress).
The triboelectric effect is the appearance of electricity on material surfaces due to friction. Electric charge density is generally unrelated to the applied pressure or to the relative speed of the two surfaces. The resulting electric field cannot go beyond the explosive field Em in this environment (30 kV/cm in the air at surface pressure); so in a conducting material, emitted charge surface density stays below σm=ε0Em.