Toroidal free oscillations of the Earth observed by a ring laser system: a comparative study

In this study, we explore the potential of measuring systematically the Earth's free oscillations using ring laser gyro (RLG) vertical axis rotational records. The RLG that we use is the vertical axis G-ring laser system of the Geodetic Observatory Wettzell (Germany). In 2009, its signal-to-noise ratio was considerably improved over the broadband frequency range of seismic measurements. Since then, three large magnitude earthquakes have occurred (Samoa Islands 2009; Maule, Chile, 2010; and Tohoku, Japan, 2011), leading to the first direct observations of rotational ground motions induced by toroidal free oscillations of the Earth. We compare these G-ring laser observations with synthetic seismograms computed by summing normal modes. We also analyse amplitude spectra of real and synthetic data to aid in the interpretation of the observations. We show that several toroidal modes are detected by the G-ring laser for earthquakes with a moment magnitude M _W????8.0 and that our observations are in reasonable agreement with the synthetic spectra. We also report evidence for mode coupling in both translation and rotation spectra.