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.

Smoke-drum seismograph
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.

Common seismic signatures

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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Maintained by John Watson
Updated 05.01.97