GNGTS 2013 - Atti del 32° Convegno Nazionale

Germany, 77.5 kHz; Inskip, England, 81.0 kHz. Considering also transmitters in VLF band, monitoring reaches a total of 21 transmitters, from all directions around the station. Strong attenuations appeared in the carrier wave of a LF transmitters at the Chieti Station, whereas exaltations appeared in another carrier wave, no noise reduction was recorded for the rest of the bands at the same time. Additionally, no attenuation was observed for all of the other carrier wave transmitters at the same times. Depletions appeared in the signal from Mainflingen while exaltations from an unknown transmitter at 87 kHz, both occurring hours before the recorded time of shock, see Fig. 3 at the bottom. The sub-ionospheric channel between Chieti and Mainflingen exactly overhang the earthquake epicenter. Opposed amplitude phases between the two carrier waves also appeared. In this case, power spectral attenuations exceeded 20 dB up to 30 dB under the normal power intensity level. The other LF carrier waves from all three CIEN stations were stable in intensity. A slight depletion from the Maraton transmitter began and ended hours before the Ancona quake at the Fermo Station, see Fig. 3 at the top, the power loss was about 5 dB. Finally, the sub-ionospheric channel between Fermo and Maraton does not overhang the earthquake epicenter. Two kinds of spectrum perturbations were observed in the Chieti recordings hours before and after the Ancona earthquake, see Fig. 3 on the top. The most striking perturbation began near 23:00 LT on July 20, 2013, when a power spectra increased about 5 dB between 85 and 95 kHz, reaching about 10 dB above the normal noise level at two different times before the quake. About one hour after the shock, this perturbation disappeared. The LF increased power intensity distribution formed a spot similar to an electromagnetic noise, which presented a Fig. 3 – VLF and LF spectrograms recorded between 22:30 on July 20 and 04:30 on July 21, 2013, VLF at the Fermo Station at the top and LF at the Chieti Station at the bottom; the vertical black line indicates the earthquake time. 69 GNGTS 2013 S essione 2.1