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 approximately 100 km 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.