There are certain structures where the modes in one direction are very high. For example, a thin flat plate will have low frequency modes in the direction perpendicular to the plate, but only very high frequencies in the plane of the plate. Even a cantilever beam will have low bending frequencies, but very high axial frequencies. So it's natural for models to have a low REM in one direction and that doesn't invalidate the PSD analysis, particularly if the direction of the PSD is not in the direction producing the low REM.
You are navigating away from the AIS Discovery experience