Wave-like structures prior to very recent southeastern Mediterranean earthquakes as recorded by a VLF/LF radio receiver in Athens (Greece)

A VLF (10 – 47.5 kHz) radio receiver with call sign UWA has recently been installed at the University of West Attica in Athens (Greece) and is continuously monitoring the lower ionosphere by means of the receptions from many transmitters, in order to identify any possible pre-seismic signatures or other precursors associated with extreme geophysical and space phenomena. In this study, we examine the case of three very recent strong mainshocks with magnitude Mw ≥ 5.5 that happened in September and October of 2021 in the southeastern Mediterranean. The VLF data used in this work correspond to the recordings of one specific transmitter with the call sign “ISR” which is located in Negev (Israel). The borders of the 5th Fresnel zone of the corresponding sub-ionospheric propagation path (ISR-UWA) are close in distance with the epicenters of the two earthquakes (EQ), while the third one is located within the 5th Fresnel zone of the specific path. In this work, we computed the morlet wavelet scalogram of the nighttime amplitude signal in order to check for any embedded wave-like structures, which would indicate the existence of Atmospheric Gravity Waves (AGW) before each one of the examined EQs. In our investigation, we also checked for any other global extreme phenomena, such as geomagnetic storms and solar flares, which may have occurred close in time with the examined EQs and could have a contaminating impact on the obtained results. Our results revealed wave-like structures in the amplitude of the signal a few days before the occurrence of these three EQs.