USGS Uses Tiltmeters to Measure Tiny Volcanic Changes
by Scientists with the US Geological Survey
The USGS Hawaiian Volcano Observatory (HVO) uses a diverse set of instruments to monitor active volcanoes in Hawai‘i. These include seismometers, gas sensors, Global Positioning System (GPS) stations, and webcams. Each provides a unique type of data critical to understanding volcanic systems.
However, electronic tiltmeters are the instruments that are often the first to alert us to changes in a volcano that could lead to an eruption. This is because they are exceptionally sensitive, capable of measuring very small ground deformations that suggest the movement of magma into shallow parts of volcanoes.
While tiltmeters respond to many subsurface processes, they are particularly effective for tracking inflation and deflation of subsurface magma reservoirs, like the shallow Halemaʻumaʻu source at Kīlauea’s summit. As magma moves into a subsurface reservoir, the reservoir expands to accommodate additional magma. This causes the ground above the reservoir to bulge, depending on how shallow it is.
As it bulges upward, the slope of the ground surface changes in certain places and in a specific pattern. This change in slope is what a tiltmeter measures, much like a carpenter’s level.
Tiltmeters commanded the spotlight at Kīlauea during the events of May–August 2018. Large changes in tilt just ahead of the collapse of Puʻu ʻŌʻō on April 30, 2018, first heralded the major events about to happen. Tiltmeters and seismometers located along the volcano’s East Rift Zone (ERZ) were key for tracking magma intruding into the lower ERZ beneath Leilani Estates. At Kīlauea’s summit, sudden changes in the direction of surface tilting were primary indicators that repeated collapse events, and not just earthquakes, had occurred.
The unit used when measuring tilt at a volcano is usually the microradian. This is an angular unit, just like “degrees.” A full circle is 360 degrees, equivalent to 6.28 radians. One microradian is, therefore, about 50 millionths of a degree—a very small change in ground slope. If you put a tiltmeter on a rigid plank that is one mile long, and then put a quarter under one end of the plank, the measured change would be about one microradian of tilt.
The tiltmeters that HVO use can resolve even smaller tilt changes—as small 5 nanoradians. However, this great sensitivity comes at a cost. That’s because the tiltmeter records all changes in the ground tilt, whether they are due to changes in the volcano or another reason.
One non-volcanic source of tilt is the heating of the ground that happens on sunny days. Most of HVO’s tiltmeters are installed in boreholes about 3 to 4 m (10 to 13 ft) below the ground and are surrounded by rock. This rock expands as it warms up during the day, and any unevenness in its expansion will produce an easily measurable ground tilt. This is “diurnal noise,” which can be easily identified because it happens regularly during the day.
Rainfall can also cause small amounts of tilt. Rock contains small air pockets, called pore spaces, which can become filled with water during a storm, causing the rock to swell up like a sponge. Small differences in pore spaces on either side of a tiltmeter will cause a measurable change in ground tilt. Remember, these are tilts that are much smaller than can be discerned by simply looking at a patch of ground with your eyes.
Another important source of noise at a tiltmeter is settling of the instrument in its borehole. This can last months, if not years, in some cases. It is a major contributor to tiltmeter “drift,” and, on a tilt record, can look like a long-term tilt change or trend.
Because of tiltmeter drift, we mainly trust tiltmeters for short time-scale changes. For changes over several months or years, we must look to other instruments, such as GPS, for deformation data. It also means that the usefulness of a tilt record is how the tilt changes within a week or a month, and not necessarily the absolute value of tilt produced by the instrument.
Despite these effects, electronic tiltmeters offer one of our best views into subsurface changes at Kīlauea and Mauna Loa volcanoes, and are an important part of our monitoring toolbox. You can see current tilt data from HVO’s network on our website. Go to https://volcanoes.usgs.gov/observatories/hvo/, then click on “Deformation.”