The large motion below 0.1 Hz might be caused by a tilt-horizontal coupling. 

 

In low-frequency (below 0.1 Hz), seismometer has two noise coupling; temperature coupling and tilt-horizontal coupling. According to p. 29 in the user guide of Trillium 120QA (T1707554), we can distinguish the couplings by checking three output signals in seismometer. Temperature change the mechanical response of all the pendulums in the seismometer, so coherence exist in all three channels. On the other hand, tilt motion only couple to the horizontal motion because the vertical motion has few sensitivity to tilt. So, if there is a coherence only in horizontal axes, tilt-horizontal coupling contaminate the horizontal signal of seismometer.

 

I check some coherences. The coherence among the same axis but different seismometers (left figures in attached image) indicate that the motion below 0.1 Hz is a local noise in Y-end rather than global ground motion. Indeed, the coherence among the same seismometer but different axes (right figures) shows the high coherence in horizontal axes (red line in bottom right figure). So, I think the large motion might be caused by the tilt motion.  (I used diaggui files in /users/Miyo/dropbox/LOG/200304/check_eyv_seismometer.xml)

 

If my guess is correct, what is the noise source? Water flow in Y-end? I want to know drain activity near Mozumi entrance near the Y-end station.