Was the observed pre-seismic total electron content enhancement a true
precursor of the 2011 Tohoku-Oki Earthquake?
Abstract
Here we test the precursory enhancement in ionospheric total electron
content (TEC) measured by GNSS leading up to the 2011 Mw9.0 Tohoku-Oki
Earthquake. We verify the frequency of this TEC enhancement via analysis
of a two-month vertical TEC (VTEC) time series that included 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 just a
product of chance. Furthermore, we analyze the spatial distribution of
the pre-seismic TEC enhancement and co-seismic TEC depletion for the
Tohoku-Oki Earthquake. We observe significant post-seismic depletion
that lasted at least 2 h after the earthquake and extended at least 500
km from the center of the large-slip area. The spatial distribution of
this post-seismic depletion appears to be independent of the pre-seismic
enhancement. The enhancement reported by Heki [2011] before the
Tohoku-Oki Earthquake may therefore be an apparent phenomenon related to
the combined effects of a largescale traveling ionosphere disturbance
and co-seismic ionospheric disturbance.