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Tiltmeters and strainmeters measure subtle changes in ground slope and shape at volcanoes
Measuring the slope angle or ’tilt’ of the ground and the shape or ‘deformation’ of the earth’s crust are proven methods of monitoring the deformation of volcanoes caused by moving magma.
Tiltmeters continuously measure the tilt of the ground surface.
Tilt measurements are used to monitor volcanoes. When magma accumulates beneath the ground, causing the surface to swell, the slope of adjacent areas generally tilts away from the centre of the uplift. Conversely, if the ground deflates due to the flow of magma from an underground reservoir, the slope of adjacent areas will slope towards the centre of the subsidence.
Like a carpenter’s level, an electronic tiltmeter uses a small container filled with a conductive fluid and a “bubble” to measure a change in slope. The slope is measured in microradians, which is a small fraction of a degree.
Sensitive extensometers can detect ground deformations due to gravitational attractions
Extensometers are very sensitive instruments that measure extremely small deformations (changes in shape) of the Earth’s crust. These instruments are so sensitive that they can detect loads on the Earth’s surface due to pressure changes caused by the passage of weather fronts.
Most installed extensometers are of two types, fluid-filled or parallel-plate, and both are buried in boreholes tens of metres below the ground surface. When earthquakes caused by moving magma change the shape of the ground, the borehole in which the strain gauge is installed also changes shape, compressing, stretching or shearing the instrument inside. Fluid-filled models contain small reservoirs of silicone fluid and measure strain as the volume of fluid between the reservoirs changes. The parallel plate types track changes in the spaces between the parallel plates caused by tensor deformation, which are measured as extensions in 3 different horizontal directions.
Water-tube tiltmeter “pot” installed in 1956 in an underground vault near Kīlauea volcano summit. Pots are connected by tubing and filled with water that flows between pots as wall tilts.
Borehole tiltmeter being lowered into a 3 m (about 10 ft) deep hole on Mauna Loa’s Southwest Rift Zone, Hawaii.
Strainmeter installation at Mauna Kea volcano in Hawaii. After strainmeter lowered into a hole drilled to a suitable depth, expansive grout added to cement instrument to surrounding rock.
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