Theoretically, one may claim that electromagnetic signals
should propagate in the Earth with the electromagnetic-wave
speed and hence they should be observed much earlier than
the arrival of seismic waves, nearly simultaneously with the
origin time of the main shock. We have clearly shown that
this is not the case, observationally. The pieces of evidence
shown in this letter impose firm constraints on the mechanism
of propagation of electromagnetic signals, including
attenuation of the high-frequency signals in the conducting
Earth (Honkura and Kuwata, 1993). In particular, this letter
stimulates us to a challenging theoretical work on the seismic
dynamo effect; electromagnetic induction in the conducting
Earth by a traveling electromotive source due to seismic
waves radiated from the earthquake source area.