LAUSR

Ionospheric anomalies possibly related to significant earthquakes (EQs) have been extensively investigated in recent years and could be considered promising for short-term EQ prediction. In this work, we study the subionospheric propagation data recorded during a time period of 3 years (1/1/2014–28/02/2017) by a network of 8 VLF/LF receivers deployed throughout Japan, to identify possible signatures of pre-EQ processes. Specifically, we analyzed the nighttime receiver amplitude data possibly related to any main EQ event with magnitude M W  ≥ 5.5 that occurred in a wider area of Japan during the above-mentioned time period (16 main EQ events) by applying the method of critical fluctuations which is able to identify critical dynamics. Our results identified intermittency-induced criticality in the lower ionosphere before all the main seismic events that occurred in Japan during the time period of interest. Criticality was also identified prior to other global or local extreme events such as magnetic storms, solar flares and volcano eruptions, that happened during the analyzed time period, indicating that more research efforts combining the results of more analysis methods, should be devoted in the future in order to discriminate between the subionospheric perturbations related to EQs and the ones related to other extreme geophysical events.