Here we test the precursory enhancement in ionospheric total electron content (TEC) which has been reported by Heki (2011) and numerous Global Navigation Satellite System (GNSS) TEC observational studies before the 2011 Mw9.0 Tohoku-Oki and many great earthquakes. We verify the frequency of this TEC enhancement via analysis of a two-month vertical TEC (VTEC) time series that includes the Tohoku-Oki Earthquake using the procedure, based on Akaike’s information criterion, and threshold of Heki and Enomoto (2015). The averaged occurrence rate of the TEC enhancement is much larger than that reported by Heki and Enomoto (2015) when all of the visible GPS satellites at a given station are taken into account. We cannot rule out the possibility that the pre-seismic VTEC changes before the great earthquakes that were reported by Heki and Enomoto (2015) are not precursors but just a product of chance. We also analyze the spatial distribution of the pre-seismic TEC enhancement and co-seismic TEC depletion for the Tohoku-Oki Earthquake with the data after reducing inter-trace biases. We observe significant post-seismic depletion that lasted at least 2 h after the earthquake and extended at least 500 km around the center of the large-slip area. This means that evaluation of the enhancements using reference curves which was adopted by Heki 2011 and even by the recent papers (e.g. He and Heki 2016, 2017, 2018) is in danger of mistaking a large and long-lasting post-seismic TEC depletion for a pre-seismic enhancement.

We investigate the veracity of the reports by Iwata & Umeno (2016, doi:10.1002/2016JA023036) and Iwata & Umeno (2017, doi:10.1002/2017JA023921), both of which claimed that the observed perturbations in GNSS-based ionospheric total electron content (TEC) could serve as a “precursor” of large earthquakes based on correlation analysis. Iwata & Umeno (2016) defined a spatial correlation of residuals between the observed and predicted TEC time series. They reported that the correlation value is significantly larger before large earthquakes than those observed during non-earthquake periods. Iwata & Umeno (2017), who applied the same method to other large earthquake, claimed that the preseismic ionospheric disturbances can be distinguished from other non-earthquake phenomena based on the small percentage of area where the correlation value exceeds the criterion. They also claimed that the low propagation velocity of the correlation peaks is also a pre-seismic characteristic. Here we tested their claims using a larger dataset. As a result, these three characteristics they claimed to have captured as evidence of earthquake precursors are not significant being frequently observed during normal (non-earthquake) days. In addition to that, the criteria of Iwata & Umeno (2017) cannot be applied to the large earthquake discussed by Iwata & Umeno (2016), and vice versa. Therefore, we can find no basis for claiming that they detected precursors to the earthquakes. The calculation procedure of the correlation function shows that the value is more of an indicator that amplifies small variations synchronized between nearby stations, like medium-scale traveling ionospheric disturbances rather than earthquake precursors.