Electric Fields that “Arrive” before the Time Derivative of the Magnetic Field prior to Major Earthquakes

The low frequency electric signals (emitted from the focal area when the stress reaches a critical value) that precede major earthquakes, are recorded at distances $\approx 100\mathrm{k}\mathrm{m}$ being accompanied by magnetic field variations. The electric field “arrives” 1 to 2 s before the time derivative of the horizontal magnetic field. An explanation, which is still awaiting, should consider, beyond criticality, the large spatial scale as well as that the transmission of the electromagnetic fields (through an inhomogeneous weakly conductive medium like the Earth) obeys diffusion type equations.

Figure 1

The SES activity recorded at the station IOA on April 19, 1995 (a): Electric field, (b): Magnetic field variations; for the scale of $V_m$, 20 mV correspond [7] to a constantly increasing magnetic variation with $dB/dt=0.1\mathrm{n}\mathrm{T}/\sec $.

Figure 2

(a) The cr values between one $E$ component and one $B$ component versus $\Delta t$, for the SES activity on April 19, 1995, after prewhitening [8]. The broken horizontal lines correspond to the 99% confidence level, indicating that the correlations at $\Delta t=1\sim 2\sec $ are statistically significant. (b) The cr values between the time series of either the positive Dt (first row) or the negative Dt (second row) of one $E$-field component and one $B$-field component. (c) the cr values after prewhitening, using the measurements in boreholes.

Figure 3

A 50 s excerpt of the SES activity recorded at the station IOA on April 19, 1995. Measurements in horizontal and in borehole electric dipoles (cf. the latter is multiplied by 10) depicted along with those of the magnetic field.

Figure 4

A conductive cylinder embedded in a more resistive medium. A point current dipole source is located 80 km away and coincides with cylinder ($z$) axis. $E_z(\rho =1\mathrm{k}\mathrm{m})$ lies between $B_{\varphi }(\rho =1\mathrm{k}\mathrm{m})$ and $E_z(\rho =0)$ and hence not drawn. The thick line depicts the emitted pulse.