Monitoring Volcano Ground Deformation with Tiltmeters
Scientists install a new tiltmeter site north of the west arm of the Crater Glacier. View is looking south with the snout of the west arm of the Crater Glacier the background. Measuring tiny changes in the slope angle or “tilt” of the ground at a volcano is one of the oldest methods for monitoring deformation caused by moving magma. When magma forces the ground up, the slope of adjacent areas will usually tilt away from the center of uplift by only a fraction of a degree. Conversely, if the ground subsides as a consequence of magma moving below, the slope of adjacent areas will tilt toward the center of subsidence. We use electronic tiltmeters for continuously recording such ground tilts on volcanoes, and they have become the most widely used instrument for measuring volcano ground deformation in real time.
What is an electronic tiltmeter?
Like a carpenter's level, an electronic tiltmeter uses a small container filled with a conducting fluid and a “bubble” to measure a change in slope. Electrodes placed in the fluid and into the bubble determine the bubble's position--as the bubble moves, voltage output from the electrode changes in a way that correlates to the amount of tilt that caused the bubble to move.
Close-up view of a tiltmeter being installed at Soufriere Hills Volcano on the Caribbean Island of Montserrat, 1995.
How sensitive are tiltmeters?
Tiltmeters measure the amount of tilt in microradians, which is the angle turned by raising one end of a beam one kilometer long the width of a dime (equivalent to 0.00006 degree!). Originally designed as part of the guidance and control system for military missiles, a variety of electronic tiltmeters are now available for volcano monitoring, each with different resolutions and ranges. For example, we use tiltmeters with ranges of between 100 and 10,000 microradians depending on the volcano and expected degree of tilt.
Selected case histories of tilt at volcanoes
- A Remarkable Pattern of Tilt at Kilauea Volcano, Hawai`i
- Eruption prediction aided by tiltmeters at Mount St. Helens
D. Dzurisin, 1992, Electronic tiltmeters for volcano monitoring: lessons from Mount St. Helens, in Ewert, J.W., and Swanson, D.A. (eds), Monitoring volcanoes: techniques and strategies used by the staff of the Cascades Volcano Observatory, 1980-90, U.S. Geological Survey Bulletin 1966, p. 125-134.
Murray, T.L., Ewert, J.W., Lockhart, A.B., and LaHusen, R.G., 1996, The integrated mobile volcano-monitoring system used by the Volcano Disaster Assistance Program (VDAP), in Scarpa, R. and Tilling, R.I. (eds), Monitoring and mitigation of volcano hazards, Springer-Verlag Berlin, p. 315-362.