Summary: Recent CS seismic levels above 10 Hz are about twice iLIGO levels (probably because of HVAC flows), and a few times power-out levels. With power out, the CS has higher vertical motion and lower horizontal motion than the vault, suggesting that Love waves are scattered from the CS slab and reflected from a subsurface layer. Thus the CS is likely to have different Newtonian noise levels than other stations.
During the power outage in August, I used battery power to obtain a Corner Station seismic spectrum with little self-inflicted noise. Figure 1, the X- and Y-axes, shows that the current level is, between 20 and 70 Hz, about twice the iLIGO level and about 4 times the power-out level. There is more self inflicted noise in the Y-direction than in the X-direction. This is consistent with the location of the broad-band sources, turbulent flows of air and mainly water, associated with the HVAC, roughly in the –Y direction from the seismometer.
Figure 2, the Z-axis, shows that, between 10 and 70 Hz, we are about a factor of 2 worse than iLIGO and a factor of more than 5 above the power-out level. The difference between recent and iLIGO spectra probably indicates that we haven’t yet completely returned HVAC air and water flows to iLIGO levels.
The Corner Station has always had more vertical and less horizontal seismic motion than other buildings (Figure 3). Figure 4 shows that the vertical/horizontal motion ratio at the corner station is considerably greater than one above 10 Hz, while the ratio is near one out away from buildings at the vault and at all other stations. Figure 4 also shows that this difference between locations is not associated with equipment in the LVEA but holds even when the power is out. With the power out, the CS horizontal motion in the 10 to 30 Hz band is slightly less than at the vault and the vertical slightly more (Figures 1 and 2). Assuming that the vault represents the building-free ambient background, it is as if the CS were converting horizontal into vertical motion.
These data are consistent with a hypothesis that the CS slab is large enough to scatter Love waves (horizontally polarized surface waves), and that the scattered waves increase vertical motion by reflecting off of higher velocity layers tens of meters below the CS. Measured propagation velocities suggest that 15 Hz Love waves would have a wavelength of about 20 m. The outlying stations would mostly fit within circles of 12 m radius, while the corner station requires one with a 50m radius, consistent with a significant scattering difference between stations. If the broad peak at 15 Hz in the power-out trace of Figure 2 represents a resonance between the reflecting layer and the CS slab, than the reflecting layer would be about 15m below the surface. Love waves do not alter density distributions and so do not couple gravitationally. Thus conversion of Love waves into waves that couple, would increase Newtonian noise. This difference between corner and end stations may result in different requirements for an optimal Newtonian feed forward system.
