Spectrograms - Calibration Signals
Among the largest signals that you’ll see on seismograms and spectrograms are the almost daily calibration pulses on many stations. These are automatically-generated signals that report on certain station parameters such as battery voltage and amplification. Because calibration pulses have very strong amplitudes, they usually are very obvious on the recordings but could be mistaken for an earthquake by a careless observer. They occur only on one station at a time. A real earthquake of that amplitude would almost certainly hit all stations in a group.