Ring Laser Gyroscope
From The Flat Earth Wiki
The Earth Line
Strapdown Inertial Navigation Technology
By David Titterton, John L. Weston, John Weston
From Chapter 15.6 on p.497
“ The Fourier transform of the output from the laser cavity gives a spectral line associated with the rotation of the Earth often termed 'the Earth line'. Analysis of this spectral line is used to monitor the impact of seismic waves and other events. This ability to make high-precision measurements of rotation rates has led to applications in geodesy. ”
p.498
“ The effect of seismic events is to induce frequency-modulated side bands, in the 0.2-1 Hz region, around the 'Earth line', which indicate the presence of rotational components associated with seismic events. ”