Attenuation of ULF-VLF seismo-electromagnetic signals and their propagation to long distances

Abstract

Using a two-layer crust model between the source region and earth surface, the attenuation of seismo-electromagnetic signals is calculated. Results show that attenuation of Ultra Low Frequency (ULF) signals is very low and they can penetrate the crustal region to enter the atmosphere and ionosphere over the epicentral area of the earthquakes. However, the attenuation increases steeply in the c rust for the ELF and VLF ranges of signals. These results are utilized to discuss the propagation mechanism of VLF signals (<i>f </i>= 3 kHz), which are recorded at Agra by employing a borehole antenna during the periods of Chamoli earthquakes in India and large magnitude earthquakes in Afghanistan. It is suggested that these VLF signals propagate upward and find windows of very low conductivity in the skin layer through which they get transmitted to the atmosphere. They the n propagate to the observing station at Agra through earth -ionosphere waveguide.