Threshold magnitude for Ionospheric TEC response to earthquakes

• TEC disturbances after earthquakes with magnitudes Mw 4.1–8.8 were analyzed.
• We used data of GPS receivers to calculate TEC variations.
• A threshold magnitude Mw~6.5 was revealed.
• There are no pronounced earthquake-induced TEC disturbances after quakes with Mw<6.5.

We have analyzed ionospheric response to earthquakes with magnitudes of 4.1–8.8 which occurred under quiet geomagnetic conditions in different regions of the world (the Baikal region, Kuril Islands, Japan, Greece, Indonesia, China, New Zealand, Salvador, and Chile). This investigation relied on measurements of total electron content (TEC) variations made by ground-based dual-frequency GPS receivers. To perform the analysis, we selected earthquakes with permanent GPS stations installed close by. Data processing has revealed that after 4.1–6.3-magnitude earthquakes wave disturbances in TEC variations are undetectable. We have thoroughly analyzed publications over the period of 1965–2013 which reported on registration of wave TIDs after earthquakes. This analysis demonstrated that the magnitude of the earthquakes having a wave response in the ionosphere was no less than 6.5. Based on our results and on the data from other researchers, we can conclude that there is a threshold magnitude (near 6.5) below which there are no pronounced earthquake-induced wave TEC disturbances. The probability of detection of post-earthquake TIDs with a magnitude close to the threshold depends strongly on geophysical conditions. In addition, reliable identification of the source of such TIDs generally requires many GPS stations in an earthquake zone. At low magnitudes, seismic energy is likely to be insufficient to generate waves in the neutral atmosphere which are able to induce TEC disturbances observable at the level of background fluctuations.