Mark,
A horizontal seis is sensitive the the predominantly transverse waves
propagating from the source. So if you want Japan/western Pacific data,
orient it N-S. It will still pick up S. America events, but not as well.
In the network I operated in the Aleutians, every station had a vertical
and only an E-W horizontal, since most of the quakes were in the top of
the subduction zone to the south of the backark island chain.
Regards,
Sean-Thomas
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