Keep in mind that seismograms
and acoustograms are not true records of ground motion. Seismometers and microphones are machines
that (usually unintentionally) filter the ground
motion or pressure fluctuations so that certain frequencies are not truthfully
recorded.
Active filtering of
seismograms and acoustograms is often useful.
Often we look for ground motion or atmospheric waves in a
specific frequency band, either because this is indicative of some
important process, or because we wish to diminish “noise” that is unassociated
with a signal of interest.
The example of the next two
slides shows time series and associated frequency spectra
for seismic and acoustic data filtered into two different bandwidths. The frequency
spectra is an important diagnostic tool for classifying or interpreting
volcanic earthquakes.
For instance, we can use frequency spectra from broad-band seismic data
to classify events as long-period (LP or B-type), very-long
period (VLP), or volcano tectonic (VT or A-type)
earthquakes. The seismic figure for an
explosion earthquake at Santiaguito (next slide) shows a VLP
component in the upper trace and a short-period component in the
lower trace. This is data from the same
station filtered digitally in various bandwidths.