A Six-Year (2014–2020) Statistical Correlation Study of VLF Terminator Time Shift with Earthquakes in Japan

In this study, we present a six-year (2014–2020) statistical analysis of VLF subionospheric propagation data at 19 VLF receivers from the VLF transmitter with call name JJI and frequency 22.2 kHz, all located in Japan. Moderate and strong earthquakes (EQs) (ML≥4.5 and depth ≤ 50 km) that occurred in the wider area around Japan during the same time period with the available VLF data are investigated. The terminator times’ (TT) shift in VLF amplitude data as a possible precursor of an EQ are statistically examined, focusing on the correlation with seismic activity. The concept of the effective EQ magnitude (Meff) is used in order to define the total EQ energy possibly affecting the midpoint of each path for each day. It is important to note that dates when geomagnetic storms or solar flares occurred as well as dates corresponding to the already known winter effect on TT statistics in the north–south direction were excluded. The cross-correlation between TT statistical anomalies and seismic activity, represented by Meff, was extracted. Maximum cross-correlation values were found for most of the cases prior to the subsequent seismic activity, indicating a link between the ionospheric anomalies and the subsequent seismicity. Finally, the wide temporal range of the cross-correlation maxima temporal locations is justified by the inhomogeneity of the lower ionosphere, coupled with the anisotropy of the preseismic effect of the impending seismicity, highlighting the complexity of the EQ preparation processes.