You know what is strange Steve, is that the YMR did not record the S wave at all. The other recorders all had the signature to some degree recorded quite well?
Godzilla, what is your take on that?
http://www.quake.utah.edu/helicorder/yell_webi.htm
This is why I leave this to trained seismologists. There are many factors that could have obscured an S wave signature from physical (wave propagation, materials etc) to installation deficiency. Why not contact them and ask.