Abstract
In 2008, a radio receiver that works in very low frequency (VLF; 20-60 kHz) and LF (150-300 kHz) bands was developed by an Italian factory. The receiver can monitor 10 frequencies distributed in these bands, with the measurement for each of them of the electric field intensity. Since 2009, to date, six of these radio receivers have been installed throughout Europe to establish a ‘European VLF/LF Network’. At present, two of these are into operation in Italy, and the remaining four are located in Greece, Turkey, Portugal and Romania. For the present study, the LF radio data collected over about two years were analysed. At first, the day-time data and the night-time data were separated for each radio signal. Taking into account that the LF signals are characterized by ground-wave and sky-wave propagation modes, the day-time data are related to the ground wave and the night-time data to the sky wave. In this framework, the effects of solar activity and storm activity were defined in the different trends. Then, the earthquakes with M ≥5.0 that occurred over the same period were selected, as those located in a 300-km radius around each receiver/transmitter and within the 5th Fresnel zone related to each transmitter-receiver path. Where possible, the wavelet analysis was applied on the time series of the radio signal intensity, and some anomalies related to previous earthquakes were revealed. Except for some doubt in one case, success appears to have been obtained in all of the cases related to the 300 km circles in for the ground waves and the sky waves. For the Fresnel cases, success in two cases and one failure were seen in analysing the sky waves. The failure occurred in August/September, and might be related to the disturbed conditions of the ionosphere in summer.
Highlights
Since 1980, studies of the interactions between seismic activity and disturbances in radio broadcasts have been carried out
On the basis of the results presented by Biagi et al [2006b], we can consider that the night-time data represent the sky wave, while the day-time data represent the ground wave
For the earthquakes that occurred in the Fourier zones, we looked for anomalies in the night-time data; the day-time data were examined only to identify any anomalies in the night-time data related to some local day/night cause or to meteorological disturbances
Summary
Since 1980, studies of the interactions between seismic activity and disturbances in radio broadcasts have been carried out. Effects of ionising radiation from gases (mainly radon) or aerosols, or electromagnetic emissions from the ground [Hayakawa and Sato 1994, Alperovitch 1997, Pulinets et al 1998, Biagi et al 2001b]; the second model assumes indirect effects of the production of gravity waves in the atmosphere/ionosphere [Hayakawa et al 1996, Molchanov and Hayakawa 1998] as a consequence of pre-seismic processes in the ground. According to the considerations made in the previous section, the sky wave might be affected mainly by the ionosphere conditions, and
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