Variation in natural short-period ionospheric noise, and acoustic and gravity waves revealed by the amplitude analysis of a VLF radio signal on the occasion of the Kraljevo earthquake (Mw = 5.4)

We analyse the lower ionosphere disturbances in the time period around the Mw 5.4 Kraljevo earthquake (EQ), which occurred on 3 November 2010 in Serbia. The results presented herein are based on analysis of the amplitudes of three VLF signals emitted in Italy, UK, and Germany and recorded in Serbia whose variations primarily result from changes in the electrical properties of the lower ionosphere at a distance more than 120 km from the epicentre of the EQ. The primary goals of this study are to reveal specific variations as possible EQ precursors as well as disturbances following the EQ event recorded by the observational equipment, and to investigate whether better time resolution data can affect the analysis of the lower ionosphere disturbances possibly connected to the EQ. We process two sets of data with sampling periods of 1 min and 0.1 s. As the first analysis indicates the absence of long-term disturbances, which can clearly be connected to the Kraljevo EQ, our attention is focused on the study of short-period noise amplitude and the excitation and attenuation of acoustic and gravity waves in the lower ionosphere. Processing of the amplitudes of three VLF signals during the nights of the four EQs with magnitude greater than 4 that occurred in Serbia, as well as EQs of all magnitudes during the three days of 3, 4, and 9 November, indicates that the detected ICV radio signal noise amplitude reduction starting before the Kaljevo EQ is also observed for 13 additional EQ events near the signal propagation path, and occurred during all three days (for all EQs with magnitude greater than 4 and several less intensive events). Excitation and attenuation of acoustic waves are also found for all these EQ events with a magnitude greater than 4.