Earth's free oscillation detected by dense GPS array: A case study of the 2011 Tohoku earthquake

We report signals of Earth's free oscillation following the 2011 Tohoku earthquake in a dense GPS network in Japan (GEONET). The GPS data are originally provided by GSI and analyzed by a kinematic precise point positioning method using the RTnet software. We stack 18-hour data about 300 stations near the source of the Tohoku earthquake to reduce atmospheric and ionospheric noise. The analyzed three-component data reveal clear spectral peaks for typical spheroidal oscillations (0S3-0S4, 0S7-0S32) and a toroidal oscillation (0T2). The signals appear both in horizontal and vertical directions, although levels of background noise are different. The amplitude of the signals in the Fourier domain correspond to that of steady cosine functions with submillimeter amplitude in the time domain. GPS truly succeeded in the detection of the submillimeter-scale deformation. Besides, in other 18 hour data for 1 day after the Tohoku earthquake, the signals of the free oscillations almost disappear but different spectral peaks appear around 3.3mHz, 6.6mHz, 9.9mHz, and 13.2mHz. They might reflect artificial clock corrections in the analysis of GPS data.