HIGH-FREQUENCY SEISMO-ELECTROMAGNETIC EFFECTS

Electromagnetic perturbations caused by natural geophysical activity, such as earthquakes and volcanic eruptions, have been studied since the great Alaskan earthquake in 1964. In recent years, interest has been increasing because, when a precursor event occurs, it normally does so less than a few hours before the shock. This paper presents a review of the high-frequency seismo-electromagnetic phenomena already observed, and occurring before, as well as after the shock. The term 'high frequency' is to be taken in the seismic sense, i.e. larger than a few Hertz. Mechanisms that might produce these seismo-electromagnetic emissions will be described. It is shown that, at the moment, a causal relationship between electromagnetic signals and seismic activity still remains to be demonstrated. Theoretical as well as intensive experimental studies are necessary to understand the physical processes responsible for seismo or volcano-electric variations, electrical resistivity variations, electromagnetic effects associated with dilatance, as well as electromagnetic fields generated during laboratory experiments, or ionospheric disturbances.