Ionospheric Anomalies Associated with Large Earthquakes: Tomographic Approach

Many anomalous electromagnetic phenomena possibly associated with large earthquakes have been reported. TEC (Total Electron Contents) anomaly is one of the most promising phenomena preceding large earthquakes. We investigated statistically TEC anomalies before large earthquakes around Japan region during 1998-2010 and found that that positive anomalies significantly appear 1-5 days before M>=6.0 earthquakes in Japan area. In order to evaluate the previous statistical result, we investigate the 2011 off the Pacific coast of Tohoku Earthquake. GIM-TEC* anomalies exceeding +2 σ appear 3-4 days before the earthquake. The duration is more than 20 hours. This result satisfy the previous statistical results of positive anomalies for M>=6.0 earthquakes in Japan. To understand the mechanism, monitoring of 3D distributions of ionospheric electron density is considered to be effective. In this study, to investigate the three-dimensional structure of ionospheric electron density prior to large earthquake, the neural network based tomographic approach is adapted to GEONET and ionosonde data for the 2011 Off the Pacific Coast of Tohoku Earthquake (Mw9.0). As a result, the reconstructed distribution of electron density was enhanced in sub-ionosphere to over F-region in comparison with 15 days backward median distribution. Moreover the enhanced area seems to be developed to upper ionosphere from sub-ionosphere with time. The tomographic results suggest the existence of some energy influx from the surface associated with seismic activity. Then, in order to understand the relationship of detected phenomenon and earthquake, we performed the tomographic analysis for other earthquakes occurred in Japan. The details will be shown in the presentation.