Search

Acoustic-gravity waves in the neutral atmosphere induce disturbances in the distribution of electrons in the ionosphere that can be observed in the total electron content (TEC) of global navigation satellite system (GNSS) signals. Large GNSS networks, such as Japan's GEONET, provide spatially dense samples of TEC time series, which can be cross-correlated to detect the coherent structure of these disturbances. Identifying the atmospheric wave-induced perturbations from the short-term background fluctuations can help further our understanding of geophysical processes. In this article, we introduce a method based on wavelet coherence and cross-correlation for detecting and analyzing earthquake/tsunami-induced ionospheric disturbances. We apply this method to detect and isolate the potential disturbances from the data of large GPS networks in Japan, USA, and New Zealand for the 2011 Tohoku-Oki earthquake and the 2015 Illapel earthquake and their resulting tsunamis. We then filter the regions corresponding to each strong coherence structure in time–frequency space to extract the separately identified ionospheric disturbances. The speeds and directions of arrival of these disturbances are found to be compatible with the acoustic-gravity waves from main-shock and after-shock epicenters of each event, offshore tsunami propagation, and seismic Rayleigh waves. Furthermore, we introduce a method to determine the observing heights for the far-field observations.