Ionospheric electron enhancement preceding the 2011 Tohoku-Oki earthquake

The 2011 March 11 Tohoku‐Oki earthquake (Mw9.0) caused vast damages to the country. Large events beneath dense observation networks could bring breakthroughs to seismology and geodynamics, and here I report one such finding. The Japanese dense network of Global Positioning System (GPS) detected clear precursory positive anomaly of ionospheric total electron content (TEC) around the focal region. It started ∼40 minutes before the earthquake and reached nearly ten percent of the background TEC. It lasted until atmospheric waves arrived at the ionosphere. Similar preseismic TEC anomalies, with amplitudes dependent on magnitudes, were seen in the 2010 Chile earthquake (Mw8.8), and possibly in the 2004 Sumatra‐Andaman (Mw9.2) and the 1994 Hokkaido‐Toho‐Oki (Mw8.3) earthquakes, but not in smaller earthquakes. Key Points Positive TEC anomaly appears before M9 class events The anomaly occurs above the epicenter and lasts ~1 hour M9 class earthquakes can be predicted