Gokhberg et al. (1982) were the first to observe the emission of an electromagnetic wave prior to an earthquake with a narrowband receiver (centred around 81 kHz) and a broadband receiver with a maximum frequency of 8 kHz. The 81 kHz emission typically rises gradually during the 1 or 2 final hours before the event, and falls abruptly at the rupture (see figure below). The 0 to 8 kHz broadband receiver, which was initially designed to study high-frequency sound propagation, detected impulsive noise of about 1.5 kHz 30 minutes before the event.
Rise in natural noise around 81 kHz observed during a seismic event
Observations made by American radio telescopes over several weeks prior to the 1960 earthquake in Chile were reviewed by Warwick et al. (1982). They found a 17.4 MHz signal occurring 6 days prior to and likely associated with the earthquake.
Following these initial findings, Parrot et al. (1985) studied the data available to them in France. Very Low Frequency (VLF) electromagnetic emissions ranging from 500 to 3,600 Hz were recorded by a TAAF geophysics station in the Kerguelen Islands. These emissions appeared to be linked to the second and third shocks in a series of earthquakes ranging from 4.6 to 4.9 in magnitude. These events took place about 100 km away from the station on 24th and 25th April 1980.
- The first emission was recorded during narrowband noise observation phase. It slowly appeared starting 1 hour prior to the second shock and ended 1 hour after the event.
- The second one was recorded during a series of very impulsive noises. It began abruptly about 14 minutes prior to the third shock and ended 25 minutes after the event.
For the entire duration of the event, ionospheric sounding data shows ionospheric disturbances, including rises in the f0ES critical frequency at the time of each shock.
During the 17th October 1989 Loma Prieta earthquake in California, Fraser-Smith et al. (1990) found an intensity variation in the ultra low frequency (ULF) signal. Recordings from a measurement system located 7 km from the epicentre show a significant rise in intensity on the 0.01 – 0.5 Hz range about 3 hours prior to the event.