Exalting in atmospheric tides as earthquake precursor

Abstract. During February–March 1998, we observed a significant increase (6–8 dB) in the electric field of the CZE (f = 270 kHz, Czech Republic) broadcasting station at a site named AS and located in central Italy. On 13 March 1998 an earthquake (M = 5.2) occurred in Slovenia, starting a strong seismic crisis (M = 6.0 on 12 April, M = 5.1 on 6 May). The distances of the epicentres from the radio receiver were over 400 km, but the epicentres are located in a zone that is in the middle of the CZE-AS path. Previously, we advanced the hypothesis that the increase of radio-signal electric field detected could be a precursor of the Slovenia seismic crisis. At the purpose to precise the connection between this radio anomaly and the preparatory phase of the earthquakes, we analysed in detail the data collected. At first we carried out a FFT on the data and clear harmonic components with period 0.5d, 1d, 14d, 28d and 365d were revealed. These periods are characteristic of gravity tides and therefore, it seems reasonable to consider the previous harmonics in the electric field of the radio signal as components related to tides of the atmosphere, assuming as tides of the atmosphere only the linear action of gravity tides on atmosphere. Then, we carried out band pass filters on our data and we discovered that the February–March 1998 increase is mainly related to an exalting of the harmonics with period 0.5d, 1d, 14d and 28d, i.e. of the (semi)monthly and (semi)diurnal atmospheric tidal components in the radio signal. Supposing that this exalting is produced by a corresponding exalting of the atmospheric tides we propose a model able to explain the production of such an effect during the preparatory phase of the Slovenia earthquakes.