Analog seismographs (known as helicorders)
have long been used to record seismic data from seismometers. Although the technology
is old, and modern seismic research relies almost exclusively on other digital recording
formats, helicorders provide a very simple, clear, and reliable way of immediately displaying,
assessing, and archiving seismicity on a daily basis. Data being recorded on New Mexico
Tech's Seismic Network seismometers are radio-telemetered from seismic
stations located at remote sites around New Mexico, where they are recorded digitally
and (for a few stations) onto helicorders.
To impose a time stamp onto the
helicorder signals, time tick marks are superimposed on the records every 10 seconds following the minute.
Drums make one complete revolution in 15 minutes and the pen base moves so that one sheet
of thermally-sensitive paper records 24 hours of ground motion.
These instruments are most sensitive to ground velocity at frequencies between
approximately 1 and 15 cycles per second. One millimeter (0.03 inch) of pen deflection
corresponds to about 1 micron (one millionth of a meter, or 0.00004 inch) per
second of ground motion at the seismometer site.
Seismometers are sensitive to all motions of the ground. Besides sensing earthquakes,
these instruments commonly record mining blasts, local wind gusts, thunder, trains,
sonic booms, and many other types of signals.
Reproduced below are several representative earthquakes and other types of seismic events
that are typical of what may be seen on a day's recording from our helicorders. Although
most of New Mexico Tech's seismic data is recorded digitally, these examples are
provided for historical and visual interest.
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