Quite often the displayed
seismogram from a volcano is a record of the vertical, short-period
velocity response of a point on the earth’s surface (i.e., where that
seismometer is buried).
Most seismometers record ground motion directly as a velocity trace,
often measured in micrometers/s, although with digital data this can
easily be converted to displacement or acceleration traces. Vertical component means that the
ground velocity are measured only in the vertical orientation. Short-period implies that the seismic
ground motions are sensitive to frequencies above about 1 Hz (or one cycle per
second).
Broad-band and three-component
(triaxial) seismometers are less often deployed at volcanoes
(mostly because of their higher price).
Broad-band implies a low frequency response
where signals as low as 120 s can be detected.
Three component means that the seismometer records
vertical, north-south, and east-west ground motions as separate
data traces (or seismograms).