Tilt measurements have been used to monitor volcanoes in the United States since the founding of the Hawaiian Volcano Observatory in 1912. When magma accumulates beneath the ground, causing the surface above to inflate, the slope of adjacent areas will usually tilt away from the center of uplift. Conversely, if the ground deflates as a consequence of magma draining from a subsurface reservoir, the slope of adjacent areas will tilt toward the center of subsidence.
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. Tilt is measured in microradians, which is a small fraction of a degree. One microradian (equivalent to 0.00006 degree!) is approximately the tilt caused by placing a dime under one end of a beam that is one half-mile long. The Volcano Hazards Program uses a variety of tiltmeter types, but instruments placed in shallow boreholes (1-6 meters or 5-20 feet deep) produce the best results because the sensors are insulated from noise that is common at Earth's surface—for example, changes in temperature and pressure.
At Kīlauea Volcano in Hawaii, borehole tiltmeters have detected cyclic deflation and inflation of the summit over periods lasting hours to days. These so-called DI (deflation-inflation) events are caused by changes in pressure within a magma reservoir beneath the surface. Analyzing tilt records from around Kīlauea's summit has helped scientists to locate the small magma reservoir that is the source of the pressure changes.
Strainmeters are highly sensitive instruments that measure extremely minute strain (change in shape) of the crust. These instruments are so sensitive that they can detect the loads on Earth's surface due to pressure changes caused by passing weather fronts.
Most strainmeters installed in the US are of two types, fluid filled or parallel plates, and both are buried in boreholes tens of meters (hundreds of feet) below the ground surface. As moving magma or earthquakes cause the ground to change shape, the borehole in which the strainmeter is installed also changes shape, squeezing, stretching, or shearing the instrument within. The fluid-filled types contain small reservoirs of silicon fluid and measure strain based on liquid volume changes between the reservoirs. The parallel-plate types track changes in the gaps between parallel plates caused by tensor strain, which are measured as extensions in 3 different horizontal directions.
Data from Yellowstone provide an example of how strain is used to better understand volcanoes. In 2013, researchers found that strainmeters located about 20 miles from Yellowstone Lake were detecting the normal sloshing of surface waves on the lake. Such a signal was not expected that far from the lake, but the presence of magma beneath the surface amplified the strain signal of the sloshing. Using this knowledge, scientists were able to better locate the body of magma beneath Yellowstone volcano.