Oblique-Incidence Ionospheric Radio-Sounding: Seismo-Ionospheric Effects

The observation of disturbances in the ionosphere during moderate (M ≈ 5.9 - 6.6) earthquakes of July 7, 2018, September 5 2018, and April 11, 2019 in Japan are described. Analysis of the signal amplitude, of the Doppler spectra variations, and of the Doppler shift of frequency in the signals traveling along the main ray path on the reference days and on the day when the earthquakes occurred have revealed that they are different. The disturbance propagation exhibits two apparent speeds, of 3 km/s characteristic of seismic waves and of ~300 - 1000 m/s characteristic of acoustic and atmospheric gravity waves in the Earth's ionosphere. The amplitude of electron density disturbances caused by the infrasonic and atmospheric gravity waves has been estimated to be ~0.15 - 9% and 7 - 55%, respectively.