  It has been suggested that fracture-induced MHz-kHz electromagnetic (EM)
emissions, which emerge from a few days up to a few hours before the main
seismic shock occurrence permit a real-time monitoring of the damage process
during the last stages of earthquake preparation, as it happens at the
laboratory scale. Despite fairly abundant evidence, EM precursors have not been
adequately accepted as credible physical phenomena. These negative views are
enhanced by the fact that certain 'puzzling features' are repetitively observed
in candidate fracture-induced pre-seismic EM emissions. More precisely, EM
silence in all frequency bands appears before the main seismic shock
occurrence, as well as during the aftershock period. Actually, the view that
'acceptance of 'precursive' EM signals without convincing co-seismic signals
should not be expected' seems to be reasonable. In this work we focus on this
point. We examine whether the aforementioned features of EM silence are really
puzzling ones or, instead, reflect well-documented characteristic features of
the fracture process, in terms of: universal structural patterns of the
fracture process, recent laboratory experiments, numerical and theoretical
studies of fracture dynamics, critical phenomena, percolation theory, and
micromechanics of granular materials. Our analysis shows that these features
should not be considered puzzling.
