The RMS amplitude of the ground mot

The RMS amplitude of the ground motion displacement (integrated down to 0.1 Hz) is typically in the order of microns at the surface and submicrons for underground. As the main contribution to the RMS amplitude is concentrated in the low frequency band (1 Hz), the resonant peaks of the mechanical system in this frequency region must be suppressed. The microseismic peak between 0.2-0.5 Hz contributes most to the seismic RMS amplitude, therefore one tries to place the mechanical resonances away from this region. In underground locations, the microseismic motion itself is largely coherent across the entire interferometer, therefore it would be of little ill effect on the interferometer stability. The actual problem is that the resonances of the seismic isolation system can be excited with different and varying phases, thus resulting in large relative motions of the interferometer mirrors. Therefore the resonances of the seismic attenuation chains must be damped strongly.