GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 2.1 - POSTER GNGTS 2023 29/11/2022 23:46 38.134 20.1138 19.1 4.6 Mar Ionio Meridionale (MARE) 30/11/2022 15:17 26.8453 55.1793 17.2 5.8 Southern Iran [Land: Iran] 02/12/2022 20:05 43.0918 18.1082 17.6 4.5 Bosnia and Herz, [Land] 04/12/2022 07:12 38.354 14.859 3.1 4.5 Isole Eolie (Messina) From the Mefite station, the monitoring of the carrier waves of some very far transmitters is performed (see Fig.2) by monitoring and recording singularly (with a sample every minute) their maximum spectral power inside fixed frequency intervals. The results of this monitoring consist of spectrograms, created with a colored pixel every 4 seconds and saved after 80 minutes (so covering nearly 1,000 pixels for each image). Each spectrogram is saved as jpeg file to have the possibility of recognizing some signals at the posterior. Moreover, ordered in columns of a txt file, the date, time and the corresponding power amplitudes, given in dB, are stored. Therefore, the signal amplitudes can be plotted or available for near real-time analysis and comparison with the earthquakes that occurred along the paths. During the first month of observations, several seismic events of moderate intensity occurred in the Mediterranean basin (see the epicentres positioned in Fig. 2). The crosses between sub-ionospheric channels and earthquake epicentres indicates which station's power spectrum is to be analysed: this is the case, for example, of the seismic event occurred on November 29, 2022, at 10 km of depth, M = 5.0, whose epicenter is crossed by the VLF path of the Maraton (Greece) transmitter. Fig. 3 shows the Maraton carrier wave amplitude from the beginning of the monitoring up to the end of November: a periodic oscillation of the amplitude is observed over a period of one day. It is produced by the ionospheric illumination of the Sun, which modulates the ionospheric reflectivity. Consequently, the power amplitude of the transmitter at Mefite appeared to oscillate between -40 dB and -50 dB, while on three occasions the power were down to -70 dB for a few hours, on November 15, 22, and 29, at the noise level. Days before the seismic event in Greece, the night-time recording appeared irregularly attenuated. The behaviour of this transmitter, together with other transmitters, are investigated to improve the identification of the sub-ionospheric channel anomalies.