Toroidal free oscillations of the Earth observed by a ring laser system: a comparative study
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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.
KeywordsFree oscillations Rotational seismology Love waves
A special thank-you goes to those who contributed to this paper: Kostas Lentas for his help in bench-marking the synthetic study on the High Performance Computing Cluster supported by the Research Computing Service at the University of East Anglia. Walter Zürn and an anonymous reviewer for their constructive comments and helpful suggestions. Also we extend our heartfelt gratitude to Dr. John R. Evans for his great support editing the paper. We acknowledge support from the European Commission (Marie Curie Actions, ITN QUEST, www.quest-itn.org) and the German Research Foundation (Project Ig16/8).
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