Earth’s free oscillations excited by the 2011 Tohoku earthquake recorded in multiple GPS networks

Abstract We search in the continuous GPS 3-D displacement data for signals of normal modes Earth’s free oscillation that were excited by 2011 Mw 9.0 Tohoku earthquake. A previous study has reported such a detection; we here conduct more comprehensive and detailed study. use from three separate networks: (i) about 1000 stations Japan GEONET; (ii) 600 western USA PBO; (iii) 140 global IGS, solve form records 21 h length at 30-s sampling rate. various multiple-record stacking methods: frequency-domain power spectrum reduces variance noises, time-domain stackings boost SNR target while suppressing non-target modes. find method optimal sequence estimation (OSE) to be most effective, which show clearly high sensitivity detectability spectrum. For near-field GEONET where all have anti-nodes, spheroidal fundamental 0 S 9 – 43 below 5 MHz some lower-degree overtones as well low-degree toroidal up prominent spectral peaks against PREM model eigenfrequencies. The PBO sees less strong (being far-field generally off-antinodes), but still identifiable IGS network detect barely handful these because its sparsity small numbers stations. thus demonstrate does actually record tiny seismic can revealed means methods, potentially useful studying earthquake source mechanisms exciting