GPSによる地球自由振動検出の続報：スタッキング法の改善と時間―周波数解析

抄録

In a previous study, GPS (Global Positioning System) was presented as a new instrument for detecting Earth's free oscillation in three-direction displacement. We improve a method of the data stacking and perform two types of time-frequency analyses as additional tests, using the GEONET data after the 2011 Tohoku-oki earthquake analyzed by a kinematic PPP method. We find that a time-domain stacking notably reduces background noise in spectra, although the time-domain stacking seems improper at frequencies higher than 3.5 mHz. The STFT (short-time Fourier transform) time-frequency analysis and the CWT (continuous wavelet transform) time-frequency analysis reveal temporal changes of the spectral peaks for the Earth's free oscillation at frequencies higher than 1 mHz. In several fundamental modes of the Earth's free oscillation, the amplitudes of the spectral peaks tend to increase after their temporal decreases. The results of the analyses indicate that the stacked GPS data truly detected the low-frequency ground oscillation. But we additionally note that we might not be able to exclude effects of satellite clock error especially at very low frequencies (< 1 mHz).