On May 26th, an earthquake cause all 4 IMs to trip, and they were free-swinging for more than an hour (which is rare), which has allowed me to measure some of their resonant frequencies.

The YAW peak between 0.7 and 0.72 is so unusual in IM3, that I'm posting the power spectra now, before collecting sll of the numbers for all the peaks. 

Plot spans 0.64Hz to 0.8Hz, and the important features to note are:

1. the IM3 YAW peak is huge in it's LENGTH, PITCH, and YAW signals, and is at 0.715Hz
2. the clear absence of the this feature in IM1 and IM4 LENGTH signals
3. IM2 has a strong YAW peak in PITCH, but still significantly smaller than the IM3 YAW peak in PITCH
4. IM3's YAW peak in YAW is significantly larger than all other YAW peaks
5. IM1 and IM4 also have their YAW peak showing up in LENGTH, PITCH, and YAW signals, but to a much lessor degree, however,...
6. IM4 in YAW seems to have the expected peak, AND a smaller peak that is at the same frequency as the IM3 YAW peak

I don't yet know what is causing the exaggerated YAW peak in IM3, but it's so unusual, I felt it needed it's own alog.