Precursory TEC enhancement immediately before the 2011 Tohoku-Oki earthquake

Earthquakes are often preceded by electromagnetic precursors, and we have been searching for them in, e.g. electric currents in the ground and propagation anomalies of radio waves. Here I report that a positive anomaly of ionospheric Total Electron Content (TEC) appeared immediately before the 2011 Tohoku-Oki earthquake (Mw9.0). Microwave signals from GPS satellites experience delays inversely proportional to the square of the frequency as they propagate through the ionosphere. By monitoring the differences of the L1 and L2 carrier phases with the dense GPS array, we can infer 2-D distribution of TEC above Japan. Coseismic vertical crustal movements excite acoustic waves, and they arrive at the ionosphere in ~10 minutes and cause coseismic ionospheric disturbances (CID). Raw TEC time series on March 11 show such CID in addition to slow background variations due to the changes in elevation and diurnal variation of vertical TEC (VTEC). We modeled the VTEC over the studied time window of ~5 hours with a cubic polynomial of time, and defined the departure from the model as TEC anomalies. Positive anomaly was found to start ~40 minutes before the earthquake in data of four GPS satellites 9, 15, 26, 27. The anomaly was larger at points closer to the epicenter, and reached ~1/10 of the background TEC immediately before the earthquake. The anomaly disappeared after the passage of CID. TEC increases often occur irrespective of earthquakes. For example, enhanced UV flux due to solar flares causes sudden increase of TEC. This, however, occurs on the whole sun-lit hemisphere, and never localizes above the focal region. Large-scale traveling ionospheric disturbances (LSTID) excited in the auroral oval often propagate southward and reach Japan. This can be distinguished by carefully observing their movements. I analyzed the TEC time series of satellite 15 observed at 93009 over 120 days from January to April, 2011. The precursory anomaly on March 11 showed the largest departure from the model. The second (day 094) and the third (day 061) largest positive TEC anomalies both showed southward propagation, and are considered to be LSTID. If this was the precursor of the Tohoku-Oki earthquake, similar phenomena should have preceded other large earthquakes. I analyzed TEC before and after the 2010 Chile earthquake (Mw8.8) using over ten GPS stations in Chile and Argentine. I found that similar positive anomalies with amplitudes ~1/2 of the Tohoku-Oki case started ~50 minutes before the earthquake and lasted until CID arrivals. Preseismic TEC anomalies also possibly accompanied the 2004 Sumatra-Andaman (Mw9.2) and the 1994 Hokkaido-Toho-Oki (Mw8.3) earthquakes. However, they were not seen before smaller earthquakes, such as the 2006 Kuril (Mw8.2) and the 2003 Tokachi-Oki (Mw8.0) earthquakes although clear CIDs were seen. Here I present an objectively testable scientific hypothesis that M9 class earthquakes are immediately preceded by positive TEC anomalies lasting for an hour or so. Because the raw data files are available on the web, one can easily reproduce the results reported here and apply the method to other earthquakes. The physical mechanism of the preseismic TEC anomalies is not clear, but concentration of positively charged aerosols above the ground might have something to do.