VLF ELECTROMAGNETIC NOISE BURSTS RELATED TO MAJOR SEISMIC ACTIVITIES OBSERVED AT AGRA

Employing a borehole antenna, vertical electric field components of the seismo-electromagnetic emissions have been monitored at Agra, India, since February, 1998.Anomalous electric field changes in the form of noise bursts of different forms, amplitude, and duration have been observed.The data for the year 1998 has been analyzed by dividing it into two groups; one for the months in which sferic activities are prominent, and the other, for the months which include calm and disturbed months due to local lightning and thunderstorm activities.A comparison of the daily variations of occurrence numbers with the average data for respective group of months shows that the occurrence numbers are enhanced considerably on the days which are either followed or preceded by major earthquakes that occurred in those months in India and around.In 60% cases, the noise bursts occurred as precursors with periods ranging between 1 hour to 2.3 days and were caused by emissions which propagated along a seismic fault lying between north-west to north-east direction close to the observing station.