Abstract
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.