On the lithosphere-atmosphere coupling of seismo-electromagnetic signals

Employing borehole and terrestrial antennas, vertical electric field components of the naturally occurring very low frequency electromagnetic emissions at the frequency of 3 kHz have been monitored simultaneously at Agra (geographic latitude 27.8°N, longitude 78°E), India, from 15 March 1999 to 30 September 1999. This period of observation included a major seismic swarm activity in the months of March and April which occurred in the Chamoli hills of north India, the next two months of May and June being quiet with respect to local lightning and spheric activities, and the rest of the three months from July to September being highly disturbed due to local lightning and thunderstorm activities. The abnormal electric field changes occurred in the form of noise bursts of varying amplitude and duration and included three kinds of data: (1) noise bursts observed by borehole antenna only, (2) noise bursts observed by terrestrial antenna only, and (3) noise bursts observed by both the antennas. We find that the occurrence of the first kind of data is positively correlated with major seismic activities in the region over the period of observations. The third kind of data indicates coupling between the two antennas dominated overall by atmospheric emissions. The long distance propagation of the seismo-electromagnetic emissions through the middle layer crust working as waveguide or through seismic faults is found to be associated with large attenuation ≈13 dB/km. Hence, the observation of the emissions at Agra, about 400 km from Chamoli, is interpreted in terms of leakage to the atmosphere through “windows” of low conductivity in the skin layer near the epicenter, possibly produced by some geophysical formations as discussed by other workers and then propagation in the Earth-ionosphere waveguide.