Abstract: We study the sub-ionospheric VLF transmitter signals recorded by the Austrian Graz
station in the year 2020. Those radio signals are known to propagate in the Earth-ionosphere
waveguide between the ground and lower ionosphere. The Austrian Graz facility (geographic
coordinates: 15.46◦E, 47.03◦N) can receive such sub-ionospheric transmitter signals, particularly
those propagating above earthquake (EQ) regions in the southern part of Europe. We consider in this
work the transmitter amplitude variations recorded a few weeks before the occurrence of two EQs
in Croatia at a distance less than 200 km from Graz VLF facility. The selected EQs happened on
22 March 2020 and 29 December 2020, with magnitudes of Mw5.4 and Mw6.4, respectively, epicenters
localized close to Zagreb (16.02◦E, 45.87◦N; 16.21◦E, 45.42◦N), and with focuses of depth smaller than
10 km. In our study we emphasize the anomaly fluctuations before/after the sunrise times, sunset
times, and the cross-correlation of transmitter signals. We attempt to evaluate and to estimate the
latitudinal and the longitudinal expansions of the ionospheric disturbances related to the seismic
preparation areas
