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 MW ≥ 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.

[1]  F. Klopping,et al.  From Chandler wobble to free oscillations: comparison of cryogenic gravimeters and other instruments in a wide period range , 1995 .

[2]  J. Tromp,et al.  Normal-mode and free-Air gravity constraints on lateral variations in velocity and density of Earth's mantle , 1999, Science.

[3]  U. Schreiber,et al.  The Effects of Tilt on Interferometric Rotation Sensors , 2009 .

[4]  F. Gilbert,et al.  An application of normal mode theory to the retrieval of structural parameters and source mechanisms from seismic spectra , 1975, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[5]  Minoru Takeo,et al.  Earthquake source asymmetry, structural media and rotation effects , 2006 .

[6]  Heiner Igel,et al.  Can we estimate local Love wave dispersion properties from collocated amplitude measurements of translations and rotations? , 2010 .

[7]  Jeannot Trampert,et al.  Robust Normal Mode Constraints on Inner-Core Anisotropy from Model Space Search , 2003, Science.

[8]  R. Widmer-Schnidrig,et al.  Perspectives for Ring Laser Gyroscopes in Low-Frequency Seismology , 2009 .

[9]  Asher Flaws,et al.  Broad-band observations of earthquake-induced rotational ground motions , 2007 .

[10]  J. Woodhouse,et al.  Regional Variation of Inner Core Anisotropy from Seismic Normal Mode Observations , 2010, Science.

[11]  G. Masters,et al.  Limits on differential rotation of the inner core from an analysis of the Earth's free oscillations , 1999, Nature.

[12]  Heiner Igel,et al.  Short Note Rotational Motions of Seismic Surface Waves in a Laterally Heterogeneous Earth , 2009 .

[13]  A. Pancha,et al.  Ring laser detection of rotations from teleseismic waves , 2000 .

[14]  John H. Woodhouse,et al.  Determination of earthquake source parameters from waveform data for studies of global and regional seismicity , 1981 .

[15]  Asher Flaws,et al.  Ring Laser Gyroscopes as Rotation Sensors for Seismic Wave Studies , 2006 .

[16]  Frank L. Vernon,et al.  Ring Laser Measurements of Ground Rotations for Seismology , 2009 .

[17]  G. Masters,et al.  Observations of coupled spheroidal and toroidal modes , 1983 .

[18]  D. Giardini,et al.  Three-dimensional structure of the Earth from splitting in free-oscillation spectra , 1987, Nature.

[19]  K. Ulrich Schreiber,et al.  Observations of Earth's toroidal free oscillations with a rotation sensor: The 2011 magnitude 9.0 Tohoku‐Oki earthquake , 2011 .

[20]  D. L. Anderson,et al.  Preliminary reference earth model , 1981 .