The TEC Enhancement Before Seismic Events Is an Artifact

Since 2011 there has been an ongoing debate about the possibility of short‐term earthquake prediction using total electron content (TEC) ionospheric monitoring by the Global Navigation Satellite System (GNSS). Heki (2011), https://doi.org/10.1029/2011gl047908 initiated this debate when he published results for the 2011 Tohoku event reporting a TEC enhancement 40 min before the earthquake; several later papers by Heki and coworkers have made similar claims for other earthquakes. If correct, Heki's methods might contribute to short‐term earthquake prediction. However, Heki's claims have been strongly criticized as being due to a decrease in the background TEC after earthquakes–the so called ionospheric hole–rather than an enhancement before. Depending on the choice of reference curve to be subtracted from the raw data to infer the “anomaly,” the data analysis can produce either a hole or an enhancement. We show that the choice of reference curve ‐calculated by Heki with a polynomial fit‐is strongly affected by the degree of the polynomial, as well as by the selection of the time window. We also show using synthetic examples that even if there is actually no signal before the event, Heki's methods can lead to spurious precursory signals (i.e., signals with non‐zero amplitude before the event) after the reference curve is subtracted. It thus appears likely that the reported TEC enhancements are artifacts. Plain Language Summary The present seismological consensus is that short‐term earthquake prediction is not possible. In 2011, Heki argued against this view, suggesting that it was possible to make short‐term earthquake predictions by measuring total electron content (TEC) in the atmosphere. He reported that data for the 2011 Tohoku earthquake showed a TEC “enhancement” 40 min before the event. This report has been strongly criticized: it has been suggested that the observed data instead were due to a decrease of the TEC after the earthquake, the so‐called ionospheric “hole.” The key point is the selection of the reference curve to be subtracted from the data to reveal either the hole or the enhancement. We show that the reference curve is highly sensitive to the degree of the polynomial and the selected time window. Thus, the claimed TEC enhancement could simply be an artifact that can be obtained by subjectively “tuning” the reference curve. We further support this conclusion by conducting synthetic tests that show the likely artificial nature of the enhance