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What Measures a Volcano's Vibrations?

Volcanic activity generates a series of earthquakes prior to an eruption. These are caused by molten magma and gases moving through the interior of the Earth and are felt as vibrations on the surface. The magma can squeeze through narrow cracks and fissures and exert enough pressure on rocks to make them fracture and produce stronger vibrations. Mostly, these tremors are of low magnitude -- between 2 and 3 on the Richter scale -- and originate 10 kilometers or shallower below the ground.

Electronic Tiltmeters
- The huge volumes of gas and magna that push upward inside the crust prior to a volcanic eruption deform the Earth̵7;s surface. Tiltmeters measure small changes in the tilt -- the slope angle ̵1; on the ground̵7;s surface around the volcano. They work on the principle of a carpenter̵7;s level using a fluid and bubble to register the change in slope. Electrodes measure the bubble̵7;s movement and register a voltage output proportional to the change. Between June 1981 and August 1982, tiltmeters installed around the crater floor of Mount St. Helens registered ground deformation for several weeks before its eruptions.
Global Positioning System
- A GPS user receives digital signals at any time of the day from between five and eight satellites of a 24-satellite constellation orbiting the Earth. The only condition is that the line of sight to the sky should be clear. GPS receivers are installed in the ground around a volcano and can monitor the ground deformation continuously over a matter of months. They, in turn, transmit real time data to a control center for analysis.
Radar Images
- Interferometric Synthetic Aperture Radar is the use of radar images from Earth-orbiting satellites to study variations in the shape and size of the Earth. Images of the Earth̵7;s surface are recorded by satellite and combined to show the ground movements. Unlike tiltmeters and GPS, which relay information from a limited number of points, InSAR can produce a complete map of the ground deformation over a large area to centimeter accuracy.
Seismometer Networks
- Seismometers are the mainstay of ground vibration measurement. At their simplest, they consist of a frame, a spring with a suspended pendulum, and a recording device. As the ground shakes after a rock fracture, the frame moves while the pendulum̵7;s inertia keeps it in the same position. The relative movement between the two is recorded either on paper or electronically as a wavy signal. Modern broadband seismometers can register low-frequency vibrations caused by flowing magma, not just the rock fracture. These use sophisticated electronics to record this movement and transmit it to a control center. Scientists use the data to track magma movement in real time.

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