Today's seismometer diagnosis activities are still underway, this is just an update (since I did some reboots):

Problem 1: X and Z channels on both seismometers were flipped.  I unplugged an X cable (East/West on the cable labels) and the Z channel (vert) would go to floating ADC zero.  Rana suggested that the ADCs sometimes have random channel hopping, and that a reboot of the c0dcu1 computer which handles the PEM ADCU should fix this problem.  I keyed the c0dcu1 / c0daqawg crate, those computers came back just fine, and the channels were no longer flipped.  This is a good thing.  Although now it's actually the Z channels that were / are bad on both seismometers, not the X's.

While rebooting those computers, c1iovme, c1sosvme, c1susvme1 and c1susvme2 crashed.  I rebooted them, although for the first few power cycles c1susvme1&2 couldn't mount /cvs/cds/caltech.  Eventually they did, and life is good again.  Except that the seismometers are still funny.