Seismometers can detect ground motion caused by different types of phenomena, such as wind, a herd of elk, a hovering helicopter, volcanic explosions, snow and rock avalanches, lahars, and earthquakes. Just as we have unique handwriting signatures, each type of ground-shaking event can generate a unique seismic "signature" that we can learn to recognize and identify as having been "written" by a specific event. To initially determine the signature of signals that occur on the ground or above ground (avalanche, helicopter, lahar, etc.) we often benefit from direct visual observations of the above ground phenomena near the station. Once identified, we can use that signature to determine the process that caused it in the future.
The different types of signals can often be distinguished by comparing the amplitude (height of waveform), frequency (width from peak to peak within waveform), and duration (length of waveform) of each signature. Comparing seismograms from multiple stations aids in accurately distinguishing these source types.