  On April 15, 2016, the Kumamoto earthquake (Mw 7.3) occurred in Japan with no
warning signals. Global Navigation Satellite System (GNSS) receivers provide
useful information on disturbances in ionosphere by calculating the changes in
Total Electron Content (TEC), which is the number of electrons in ionosphere.
Here we show our recently proposed correlation analysis of TEC data which can
detect the pre-seismic ionospheric anomalies from the public GNSS data. Our
method detected the ionospheric anomaly several tens of minutes before the 2016
Kumamoto earthquake near its epicenter. Furthermore, we gave an indicator to
distinguish between the pre-seismic TEC anomalies and the medium scale
traveling ionospheric disturbances (MSTIDs) by calculating the anomalous area
rates. These results support the hypothesis for existence of the preceding
phenomena before large earthquakes.
