Abstract
We performed the frequency analysis of time-series data of TEC, and examined the variations of TEC associated with earthquakes. Using Fast Fourier Transform processing, the spectral densities of coseismic TEC variations were calculated. The coseismic variations of TEC were detected in 16 events out of 26 events where the earthquakes larger than M6.4 occurred around Japan since 2000. In most events, the variations at the frequencies of 4.17, 5.21 mHz were observed. The spectral densities reached their maximums about 11 minutes after the earthquake. Since this delay corresponds to the propagation time of acoustic wave from the ground to the ionosphere, it is confirmed that the coseismic variations are due to the acoustic wave generated by the ground/sea surface perturbations. The enhancement of the signal density increases with the magnitude of earthquake and the height of tsunami. Using the enhancement of the TEC variation, the magnitude of wave source can be estimated.
