Earthquake Signatures in the Ionosphere Deduced from Ground and Space Based GPS Measurements

Atmospheric perturbations induced by weather fronts, nuclear explosions, volcano eruptions, and earthquakes can generate signatures in the ionospheric plasma density by atmospheric-ionospheric coupling processes. Because of their sensitivity to the ionospheric ionization, ground and space based GPS measurements offer a unique opportunity for detecting earthquake signatures in the ionosphere. Although numerous case studies and statistical analyzes were made, the GPS radio occultation measurements on CHAMP did not show a clear ionospheric response to earthquakes. On the other hand the retrieved total electron content (TEC) data along numerous ray paths between ground based receivers and GPS satellites has shown clear earthquake related signals for selected earthquakes of magnitudes larger than 6. By using the dense GPS network in North America, earthquake related structures have been found after the Denali earthquake on November 3, 2002 and during the California earthquake on December 22, 2003. Single station observations revealed also typical earthquake signatures after the Sumatra earthquake on December 26, 2004. It is assumed that these significant structures are generated by upward propagating atmospheric acoustic waves which are excited by seismic surface waves. Detection techniques and wave propagation features are discussed.