LF radio anomalies revealed in Italy by the wavelet analysis: Possible preseismic effects during 1997–1998

Since 1996, the electric field strength of the broadcasting station CZE (Czech Republic, f = 270 kHz) has been sampled each ten minutes, by a receiver (AS) located in central Italy, 818 km far from the transmitter. Here, we present the results obtained by a detailed analysis applied on the data recorded from February 1996 up to September 2004. At first, we separated the day time data and the night time data in the radio signals; then, in the day time data we separated the data collected in winter from the data collected in summer. Finally, we applied the wavelet analysis on the previous trends. The first result was the appearance of a very clear anomaly during February–March 1998, at winter day time and at night time. This result confirms an anomaly revealed previously in the same data but analysed with a different approach. The anomaly was related to a strong (M = 5.1–6.0) seismic sequence occurred in a zone (Slovenia) lying in the middle of the transmitter–receiver path. The present result reinforces the hypothesis of the occurrence of some disturbances in the ionosphere during the preparatory phase of the Slovenia seismic sequence. The second result came from the wavelet analysis applied to the summer day time data and it was the appearance of a very clear anomaly during August–September 1997. On September 26 the Umbria–Marche (central Italy) seismic sequence started with two earthquakes with magnitude M = 5.6 and M = 5.9 and the seismic activity lasted for more than six months. We consider the August–September 1997 radio anomaly as a precursor of the previous earthquakes and a possible explanation model is proposed.