Tide amplitudes.
Tidal forcing provide a signal that can be used to calibrate broadband seismographs. While the absolute local forcing and response are difficult to calculate, it is possible to monitor any changes in the response by making systematic measurements of the recorded tide.
In the figure below, we show the sine component of the recorded semidiurnal tide at the station NWAO-IU, a borehole station. The three traces are E-W, N-S and vertical at the bottom. Each sympol indicates a daily estimate of the tide, derived either from the LH or the VH channel.
The black line shows the best fit to these measurements, using the three dominant "beat" frequencies. Deviations from this periodic pattern (which are not seen for this station) can be used to identify a station problem.