Stochastic Evaluation of Pre-Earthquake TEC Enhancements

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 2-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. The frequency assessment by Heki and Enomoto (2015, ) using only one satellite underestimates the frequency of the TEC enhancement. In fact, the TEC enhancement is sufficiently frequent to explain all the reported enhancements prior to the great earthquakes as a product of chance. We also analyze the spatial distribution of the preseismic TEC enhancement and coseismic TEC depletion for the Tohoku-Oki earthquake with the data after reducing intertrace biases. We observe significant postseismic 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 & Heki 2016, ; 2017, ; 2018, ) is in danger of mistaking a large and long-lasting postseismic TEC depletion for a preseismic enhancement.