Jim, Fabrice:

To help understanding the Z to RZ coupling, we have setup a test in the staging building in which we drive an actuator not attached to the structure. The actuator is seating on the top of Stage 1 (a few inches above where it is normally installed).  No brackets connected, indeed, so the actuator can't drive any motion.

We do see some direct pickup, as shown in th attached plot. The coherence is, however, stronger in the vertical directions, unlike the coupling we are chasing in the units installed in the chambers.