GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 2.1 89 station, as water drops are charged and produced intense signals on spectrograms. These data were used to label ELF oscillations occurring during rainfalls. Finally, ELF data were analysed from April 2013 to April 2014 for a total of 9,480 hours of records, producing about 146 Gb of data. Spectrograms were prepared to better localise ELF anomalies, compared to electromagnetic noise. A scale of colours, corresponding to 5 dB variations in power spectral amplitudes, was sufficient enough to distinguish the oscillations. Being so, all anomalies > 5 dB above noise were collected. In order to verifywhenmeteorological phenomena could have produced the electric perturbations, rainfall data were superimposed on ELF data. Moreover, electrical oscillations occurring less then 30 minutes from rainfall events were excluded from the analysis. Strong electrical oscillations were identified from records taken a few hours or days before the moderate shocks. A summary of recordings from the Perugia station of both electrodes, N-S and E-W, over the last year is shown in Fig. 2. To verify an association between earthquakes electric perturbations, seismic activity in a radius of 30 km and 70 km around Perugia were plotted on the same graph and shown in red. Electrical oscillations at the time of Massa Martana-Colfiorito seismic swarm in 2014. An increase in the seismicity rate occurred in the Massa Martana area since de middle of March 2014 and on March 26, 2014, with a shock of M = 3 (Bina Observatory, 2014). The swarm epicentre was about 45 km south of the Perugia station that started to record data in October 2008. Likewise, the epicentre was about 10 km east of the Avigliano Umbro station, which started to record data in June 2013. Two shocks with a magnitude M = 3.1 struck the same area seven days later, on April 2. The seismic swarm migrated to the Colfiorito area on April 15, 2014, with a shock of M = 3.3 and other smaller seismic events (Bina Observatory, 2014). Fig. 2 – ELF oscillation intensities recorded from the beginning of April 2013 to the end of April 2014, are shown on the top panel; yellow areas indicate lost data; green bars under ELF oscillations indicate rainfall detected by the Perugia station; the black bars indicate the electrical oscillations occurred on E-W and S-N directions; the red bars indicate the seismic events of M > 2 occurred inside a 30 km radius from the station and finally the seismic events of M > 3 occurred inside a 70 km radius.