Ionospheric Disturbances Related to Earthquakes

Large earthquakes disturb the ionosphere in several different ways, and we can observe them with GNSS networks as changes in TEC. First, I discuss coseismic disturbances that appear ~10 minutes after earthquakes as sudden TEC oscillations propagating towardmagnetic equator with the acoustic wave speed (~1 km/s). Coseismic uplift/subsidence excite acoustic waves, and components with periods of 4–5 minutes propagate to the ionospheric F region and cause TEC oscillations. An empirical law is proposed to relate their amplitudes to moment magnitudes (Mw) of earthquakes using ~30 events Mw6.6–9.2. A very large earthquake is often followed by resonant atmospheric oscillation lasting for hours. Rayleigh surface waves also make acoustic waves and cause TEC oscillations thousands of kilometers away from epicenters. Secondly, I review current knowledge of TEC changes occurring immediately before large earthquakes. They start 10–80 minutes before earthquakes and make positive electron density anomalies above faults about to rupture. Comparison of 18 earthquakes Mw7.3–9.2 suggests positive correlations with Mw in their leading times, changes in TEC rates, and cumulative anomalies. The three-dimensional structure of electron density anomalies suggest they are formed by E× B drift caused by the penetration of electric fields, possibly made by surface charges, through the ionosphere.