GNGTS 2014 - Atti del 33° Convegno Nazionale

90 GNGTS 2014 S essione 2.1 After July 21, 2013, when an earthquake of M = 5 struck Ancona, 100 km east of Perugia, the Perugia station recorded a particular electrical oscillation on a recently installed instrument which measures the vertical electric component. Similar detectors were installed in another six CIEN stations: Fermo, Città di Castello, Gubbio, Pozzuolo del Friuli, Camerino and Avigliano Umbro. Clear signals were detected at the Avigliano Umbro Station a few hours before the two strongest shocks occurring in Massa Martana on March 2014. The unusually clear electric disturbances recorded before the second moderate shock are shown in Fig. 3. This figure shows a 3 hour spectrogram of vertical component electric field variations recorded at the Avigliano Umbro station. The spectrogram frequency range is 1-450 Hz. The electrical oscillations ranges between 10 Hz and 100 Hz, following a continuous and irregular path. The total time of the phenomena was about 150 minutes, with maximum intensities 25 dB above the noise power. A similar phenomena was also observed before the first moderate shock. No similar oscillations were recorded on the vertical detector by other CIEN stations during the same period. The day before the moderate shock of M = 3.3 in the Colfiorito area, a signal of -55 dB was recorded at the Perugia station along the E-W electrode. This electrical oscillation lasted about one hour and had the characteristic shape of an arc, as described in previous publications (Fidani, 2011). This signal was very well defined in frequency but had a very irregular pattern, similar to past cases (Fidani, 2011). Conclusions. Data recorded from the Perugia CIEN station suggests that electric perturbations become slightly more intense and frequent during meteorological and seismically active periods, as recorded before and after several Pietralunga and Gubbio earthquakes. In fact, in these periods, the power spectra of the electrical oscillations became greater than other natural electromagnetic phenomena, such as the Schumann Resonances. Candidate pre-seismic oscillations in electric intensity were quantified and resulted being of the same order of the 2009 L’Aquila earthquakes (Fidani, 2011). Moreover, the moderate seismic activity in the Umbria Region has been associated to degassing activity, which are often related to the presence of over pressurized fluids and to diffusion processes (Noir et al. , 1997; Miller et al. , 2004; Antonioli et al. , 2005). CIEN observations in the atmosphere, near the Earth’s surface, have discovered that sources of electrical oscillations induced in the electrodes are localised near the stations. The presence of air ionisation has been suggested when seismic activity increases (Freund et al. , 2009). Rainfalls have also been known to be preceded by electric charges in the atmosphere (Takahashi, 1972). All of these observations have evidenced that electrical oscillatory phenomena occurred in the presence of air ions, ground gas emissions and/or meteorological instability. Due to the general difference in temperatures between the ground and the atmosphere, when gas escapes Fig. 3 – The ELF electric perturbations recorded at the Avigliano Umbro station on March 30, 2014, are shown; the oscillations lasted 150 minutes and followed an irregular pattern between 10 Hz and 100 Hz with a relative magnitude of up to -50 dB.