GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 2.1 317 Past studies have reported that electric oscillations become slightly more intense and frequent during meteorological and seismically active periods. Magnetic data recorded at the Chieti, Città di Castello, Avigliano and Norcia CIEN Stations suggested that electric oscillations were characterisedbymagnetic intensities sosmall that, probably under the natural and anthropic noise levels, they were undetectable (Fidani and Marcelli, 2017). The proposed model for electric oscillations (Fidani and Martinelli, 2015) seemed to explain these small values, due to the symmetric charge distributions that were supposed to be the sources of the signals. This charged cloud model was also able to explain the disappearance of electric field oscillations far from the sources, thus providing an explanation for the observed absence of correlations among signals recorded by different CIEN stations (Fidani and Marcelli, 2017), as evidenced in past studies. The hardware architecture for magnetic measurements at the Chieti Station was composed of three main functional blocks: amplifier, low pass filter and signal discriminator (Orsini, 2011). Notice that the lowpass filter and the voltage discriminator are two virtual instruments realised on a Labview platform; this platform analyses the digital signal coming from the audio sound card. The magnetic receiver is designed to receive audio signals below 3 kHz using a pre-amplified small loop antenna, see Fig. 1. The pre-amplified signal is connected to two sound cards of two different PCs. In one of two PCs a software saves the spectrum of the received signals in ascii format for 24 hours only when they go beyond a fixed voltage threshold. The direction of the antenna had been previously fixed to exclude any magnetic noises coming from electric power lines. This acquisition process archives the time stamp of the events only when the frequency of the signal is lower than 30 Hz and greater than a voltage threshold of 110 mV. In another PC, the spectrogram of the magnetic loop signal is saved between 1 and 450 Hz in a logarithmic scale. Spectrograms of magnetic components are recorded every 80 minutes at the Chieti Station, and they evidence regular shapes that are interrupted a few times every day by vertical lines, which denote pulses. Daily pulse numbers do not increase also during strong meteorological perturbations and thunderstorms. Daily pulse numbers increased suddenly on October 26, when two moderate earthquakes, M = 5.4 and M = 5.9, struck central Italy about 100 km from the Chieti Station, see Fig. 1 - The loop for the magnetic detector installed at Chieti Station. Fig. 2 - Acceleration of the magnetic pulse rate, indicated by vertical signatures, recorded on October 26, 2016, from 13:45 to 15:25 LT, about four hours before the first magnitude five Castelsantangelo sul Nera shock. Many spots, like those evidenced within the black circle, appeared during the same period, and were between 20 and 40 Hz.