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Attached plots compares the prediction of upconversion noise from DE window (see 24130 ) in the case when the seism below 10Hz is low and when it is high. Figures 1 (seism) and 2 (projection) uses the two times of entry Studies on the noise around 100 Hz, in this case the seismic excess is concentrated around 2Hz and it is due to road traffic. Figures 3 (seism) and 4 (projection) similarly show the case when the seismic excess is below 1Hz and associated to rough sea.
The effect is indeed similar. The increased seism at low frequency causes a broadening of the 100Hz projected peak, which seems to correspond well to the "breathing" observed in the dark fringe peak.

This seems to further support the hypothesis that 100Hz peak is diffused light from DE window. It would remain to demonstrate that the 100Hz signal measured by SETODE is REAL seism. Might is be a transformer (see Federico, Edwige comments to Studies on the noise around 100 Hz)?