||The mainstay of volcano monitoring is the continuous recording of seismic
activity. Virtually all Hawaiian eruptions are preceded and accompanied by an
increase in the number of shallow earthquakes. As magma moves into the reservoir
during inflation, it must make room for itself by rupturing or crowding aside the
solidified lava that surrounds the reservoir. Such underground ruptures produce
seismic waves that travel through the volcano and are recorded by a network of
seis- mometers placed on the volcano's surface. Ground motions sensed by the
seismometer are converted into electronic signals, which are transmitted by radio
and are recorded on seismographs located at the volcano observatory. The seismic
data are analyzed to determine the time, location, depth, and magnitude of the
earthquakes. Mapping the earthquake activity allows HVO scientists to track the
subsurface movement of magma.
|All Hawaiian eruptions are accompanied by harmonic tremor (also called volcanic
tremor). Quite distinct from the discrete seismic shocks associated with
rupture-caused earthquakes, harmonic tremor is a continuous vibration of the
ground caused by magma movement. Harmonic tremor generally is detectable and
recorded only by seismic instrumentation; however, if especially vigorous, tremor
can be felt by people as far as 5 miles from the eruption site.
Above left: A smoke-drum seismograph.
A sharp-needle pen "writes" the seismic signature by scratching recording paper coated
with carbon black (soot). (Photograph by Robert W. Decker.) For more precise monitoring, however, HVO
records and analyzes seismic activity by use of photographic-film and
computerized magnetic-tape recording systems. Above right: Examples of common seismic
signatures typically recorded before and during eruptions.
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