Derivation of instantaneous frequencies of tectonic plates from gravimetric data

We present a mathematically rigorous procedure for obtaining spectrograms of tectonic plate eigenmodes at extremely low frequencies, corresponding to oscillation periods of 15 min to 8 h. The data is sourced from superconducting gravimeters of International Geodynamics and Earth Tide Service (IGETS) network. The motivation for this research stems from the proposal by Pavlov et.al. to use this spectral data in monitoring of the state of active zones located along plate boundaries.