A possible preseismic anomaly in the ground wave of a radio broadcasting (216 kHz) during July-August 1998 (Italy)
- 1Department of Physics, University of Bari, Via Amendola, 173, 70126 Bari, Italy
- 2Inter-Department Centre for the Evaluation and Mitigation of the Volcanic and Seismic Risk, Univ. of Bari, 70126 Bari, Italy
- 3Department of Engineering of Enterprise, University of Roma Tor Vergata, Via di Tor Vergata, 00133 Rome, Italy
- 4Department of Biomedical Sciences, University of Foggia, Via L. Pinto, 71100 Foggia, Italy
- 5United Institute of the Earth’s Physics, Russian Academy of Science, Bolshaya Gruzinskaya 10, 123995 Moscow, Russia
- 6Department of Electronic Engineering, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu City, Tokyo 182-8585, Japan
Abstract. On February 1996, a receiver able to measure the electric field strength of LF radio broadcastings, with a sampling frequency of ten minutes, was put into operation in a site (AS) located in central Italy. One of the broadcasting stations selected is MCO (f=216 kHz), located in southeast France, 518 km far from the receiver. The MCO data collected since February 1996 up to September 2004 were examined and, at first, the night time data and the day time data (in winter and summer) were separated. Then, the wavelet analysis on the night and day time data was applied. The main result of the analysis was the appearance of a very clear anomaly during summer (July–August) 1998, at day time and at night time. The anomaly is a strong exaltation of the signal components with period in the 25–40 days range. Theoretical calculations of electric field strength were made and the only way to justify this anomaly seems to be the occurrence of an increase of the ground wave propagation mode of the radio signal. Such an increase could have been produced by an increase of the ground conductivity and by modifications of some parameter of the troposphere, mainly the refractive index. On 15 August 1998 a seismic sequence started with 17 earthquakes (M=2.2–4.6) on the Reatini mountains, a seismogenic zone located 30 km far from the AS receiver along the path MCO-AS. In this paper, the possibility that the previous radio anomaly can be a precursor of this seismic sequence is proposed.