Abstract.Horizontal and vertical rotational components of teleseismic surface and body waves are detected by large ring laser gyroscopes. This is illustrated with records from magnitudes 7.0 and 7.3 events at distances of 31 deg. and 42.6 deg. respectively. Phase comparisons with synchronous linear seismometer records confirm the gyroscopic coupling.
The second-order autoregressive AR(2) model is used to analyze rotational data for seismic events captured by a large ring laser gyroscope. Both the Sagnac frequency and linewidth estimates obtained from this model sense the rotational components of seismic waves. An event of magnitude M L = 6.5 at a distance of D = 5.4° from a large ring laser gyroscope operating at its quantum limit is used to compare the AR(2) model with the previous analytical phase angle method of analysis. The frequency, linewidth and analytic phase angle data each satisfactorily estimate the rotation magnitude. The direct detection of rotational motion in the P wave coda is observed, demonstrating the conversion to transverse S wave polarizations by the local geology.
Coupling mechanisms and detection thresholds are discussed for ring laser gyroscope measurement of seismic rotation, and simultaneous records from a ring laser and a standard EARSS seismograph 230 km from an ML 5.3 seismic event are compared. Rotation dominates tilt and strain in modulating the Sagnac frequency, and microseisms are not significant. Power spectral densities for the ring laser and for the seismograph signals are enhanced in the 0.2- to 10-Hz range by up to 18 and 60 dB, respectively, over the noise floors (− 160 dB, or order nanoradians per second, and − 130 dB, respectively). A seismic sideband of the Earth rotation spectrum is found. These ring laser signals have magnitudes consistent with the amplitude of the standard seismograph record.
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