Tidal modulation of seismic noise and volcanic tremor

We present seismic data from Fogo Volcano (Cape Verde Republic) that document a tidal control on the amplitude of seismic noise and volcanic tremor. The effect is detectable in the raw data by simple eye inspection. We identified the main periodicity in the noise modulation, which coincides with the tidal component M-2. The signal duration of 40 days, together with a sampling rate of 1440 samples per day (seismic r.m.s. time series) allowed the resolution of M-2 from the neighbor solar frequencies that are often related to environmental effects. Spectral analysis shows that the modulation affects selectively the frequency range 2.0 Hz-3.0 Hz, which we interpret as volcanic tremor. White noise from 1.5 Hz to 7 Hz is also modulated. Comparison with meteorological data reinforces the conclusion that the effect is not due to environmental parameters.