Mw dependence of the preseismic ionospheric electron enhancements

Ionospheric electron enhancement was reported to have occurred ~40 min before the 2011 Tohoku‐oki (Mw9.0) earthquake, Japan, by observing total electron content (TEC) with Global Navigation Satellite Systems receivers. Their reality has been repeatedly questioned due mainly to the ambiguity in the derivation of the reference TEC curves from which anomalies are defined. Here we propose a numerical approach, based on Akaike's information criterion, to detect positive breaks (sudden increase of TEC rate) in the vertical TEC time series without using reference curves. We demonstrate that such breaks are detected 25–80 min before the eight recent large earthquakes with moment magnitudes (Mw) of 8.2–9.2. The amounts of precursory rate changes were found to depend upon background TEC as well as Mw. The precursor times also showed Mw dependence, and the precursors of intraplate earthquakes tend to start earlier than interplate earthquakes. We also performed the same analyses during periods without earthquakes to evaluate the usefulness of TEC observations for short‐term earthquake prediction. Key Points Reference curves are not used anymore Larger earthquakes are preceded by larger precursors Precursors of larger earthquakes start earlier