As I reported in the ISC call today, the coherence of REFL_A_LF with DARM decreased in the band from 20 - 100 Hz after the earthquake, indicating that some other noise started contaminating DARM.

The attached shows the spectra and coherences before (4/7/2017 13:30 UTC) and after (19/7/2017 9:00 UTC) the earthquake, together with some other intensity monitors. The intensity monitors (POP_A_LF, 2nd loop inner and outer RINs) showed almost the same spectra except for some peaks in 6-9 Hz which are now gone. As for the spectrum of REFL_A_LF, its noise curve between 8 and 100 Hz of REFL_A_LF seems to go up and down as a function of time by 3 dB or so. However, the coherence with DARM is consistently lower than before regardless of the noise floor of REFL is elevated or not.

I conclude that intensity noise is innocent.