Figure 4.5 shows the predicted disp

Figure 4.5 shows the predicted displacement noise level of the suspended optics with the three types of KAGRA-SAS in comparison with the requirement. The seismic vibration curve assumes the high noise model of the Kamioka site as described in section 4.1. The graphs show the displacement noise spectra of the ground vibration, horizontal vibration of the suspended optics, contribution from the vertical vibration of the optics assuming 1% vertical-longitudinal couplings, and the requirements described in section 4.1. Since the systems have a smaller number of attenuation stages in the vertical direction, the couplings from vertical vibration dominate the displacement noise at high frequencies (2 Hz for type-A and type-B, and 10 Hz for type-Bp). The suspension systems meet the seismic attenuation requirement in the frequency region above 10 Hz, and even widen the detection band toward the few-Hz level. The type-A SAS has a safety margin of roughly 8 orders of magnitude at 10 Hz, and others have safety margins of 2 orders of magnitude. Taking into account that attenuation performance is degraded by internal resonances above 50 Hz, it is prudent to have large safety margins in that frequency region.