Sudden modulation in the UHF wireless signals probably caused by the activation of pre‑earthquake processes. Case studies for the Balkans (SE Europe)

We study the atmospheric variations of broadband wireless signal propagation intensity correlated with pre‑earthquake processes. We have maintained ground observations in the VHF range of 1.8 and 3.5 GHz in Bulgaria (Southeast Europe), close to the border with Greece and Northern Macedonia, since 2012. The signal source is 1.8 GHz –LTE broadcasting cellular communication signal and the receivers are digital HF to SHF antennae collecting the data via mobile internet. Our observations revealed phenomena associated with a natural enhancement of the intensity of the signals (no change in the transmitting level) days/hours before the seismic events, even far from the observation region. We are presenting the results for four significant earthquakes in the area: (1) M5.6 on May 22, 2012, in Bulgaria, (2) M6.9 on May 24, 2014, in the Aegean Sea, Greece, (3) M6.5 on Nov 17, 2015, Lefkada, Greece, and (4) M6.3 of May 12, 2017, in Western Türkiye. Some changes in the atmospheric boundary layer (ABL), triggered by an intensification of radon and other released gases, could lead to a change in lower atmospheric conductivity. Although the intensity modulation was observed far (> 200 km) from the epicenter areas, the anomalies were always inside the estimates of the Dobrovolsky‑Bowman area of preparation. We examined the possible correlation between magnitude and the spatial size of the earthquake preparation zone in the Lithosphere‑Atmosphere-Ionosphere coupling (LAIC) framework.