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), 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.