Lower-ionosphere anomalies prior to strong earthquakes that occurred in north-central mainland Greece on March 2021 as revealed by multi-method analysis of VLF sub-ionospheric propagation data

In this work we present the multi-method analysis of very low frequency (VLF) data, acquired by the radio receiver with call name UWA, located in Athens (Greece), in the University of West Attica, focusing on two strong ( ) earthquakes (EQs) that occurred in north-central mainland Greece sequentially, on 3 and 4 March 2021, with very close epicenters. Specifically, we used the data acquired from seven VLF transmitters located in Europe, North/North-West to UWA, and their propagation paths include the specific EQs epicenters. We analyzed these data using multiple analysis methods in order to investigate for possible EQ-related anomalies, taking also into account all the other possibly ionosphere-influencing extreme events that occurred during the studied period. Especially, we applied the “nighttime fluctuation method” (NFM), as well as, the “terminator time method” (TTM) in order to reveal any statistical anomaly in the nighttime amplitude recordings of VLF sub-ionospheric propagation data within 15 days before each one examined EQs. Also, we calculated the scalogram (wavelet power spectrum over time) using Morlet mother wavelet of the same nighttime data searching for possible imprints of wave-like structures during the same time period. In terms of criticality analysis, first we applied the “natural time” (NT) analysis method to the daily-valued NFM VLF propagation quantities, and subsequently applied the “method of critical fluctuations” (MCF) to the raw nighttime amplitude VLF recordings, to check for any criticality signatures up to two weeks before the examined EQs. Taking into account all the above-mentioned analysis results, we conclude that there are multiple indications that the lower ionosphere was indeed disturbed due to the preparation processes of the above-mentioned EQs, offering different types of seismogenic indications.