Ground deformation measurements provide an important indicator about what is happening beneath a volcano. As magma accumulates in an underground reservoir before an eruption, the ground surface typically swells (named inflation). Likewise, as magma leaves the reservoir, potentially to erupt, the ground above the reservoir subsides (named deflation). Volcanoes also deform due to stresses that can result in movement on faults during earthquakes.
Hawaiian Volcano Observatory (HVO) scientists have developed and tested many techniques for measuring volcano deformation since HVO's founding in 1912. The resulting data have been used to better understand and forecast volcanic activity. GPS, tilt, and InSAR (satellite radar) are the primary methods used today to track ground movement.
Global Positioning System (GPS) technology was first tested by HVO in the late 1980s. Equipment is both deployed periodically to measure established benchmarks and installed permanently at fixed sites. There are hundreds of GPS benchmarks on the Island of Hawai‘i and dozens on the Island of Maui. More than 60 permanent GPS stations transmit data continuously to HVO and track motion of these sites to within less than a centimeter (less than half an inch). Permanent stations are located mostly on Kīlauea and Mauna Loa, but also Hualālai, Mauna Kea and Haleakalā volcanoes.
GPS measurements can be used to estimate the location and amount of magma accumulating beneath the surface. For example, Mauna Loa Volcano has experienced multiple episodes of inflation since its 1984 eruption, and it has been well documented since the mid-1990's. These data have helped HVO scientists to better understand magma movement and storage beneath the volcano. Additionally, today's GPS networks record data in real time and detect rapid changes associated with magma moving towards the surface in the hours to days before an eruption.
GPS instruments also track a wide range of activity on Hawaiian volcanoes, some of which would not otherwise be known. For example, it has long been known that the south flank of Kīlauea is moving seaward at a rate of several centimeters (a few inches) per year. This motion is continuous, but GPS monitoring has also detected discrete episodes of accelerated motion about every 2 years. Known as a slow earthquake, the motion takes place over 2-3 days and would be equivalent to a ~M5.5 earthquake if it were to occur all at once.
As early as 1913, HVO scientists recognized that ground tilt associated with volcanic activity could be measured, and they designed unique pendulum-type instruments to track long-term changes in the ground slope over time. Precise measurement of ground tilt began in 1956, when HVO scientists developed a network of water-tube tiltmeters. Although modern electronic tiltmeters are more precise and provide continuous data, HVO scientists still measure one water-tube tiltmeter at the summit of Kīlauea to continue the decades-old record—the longest deformation record for any volcano.
Today, ground tilt is continuously recorded with electronic tiltmeters installed in drill holes about 3–5 meters (10–16 feet) beneath the ground surface—a location that insulates the instruments from the effects of environmental (temperature and wind) and cultural noise. About 20 tiltmeters are installed on Kīlauea and Mauna Loa volcanoes, mostly near their summits and along their rift zones. Rapid changes in tilt are usually detected in the hours to days before an intrusion or eruption. Tilt has also been used to identify subtle deflation-inflation (DI) events at Kīlauea, which occur on a regular basis because of pressure changes in the magma storage region about 1.5 km (1 mile) beneath the east margin of Halema‘uma‘u crater.
Interferometric Aperture Radar (InSAR) uses radar images of the ground that are collected by airplanes or orbiting satellites to make maps of ground deformation. The Group on Earth Observations' "Supersite" initiative identified Hawai‘i as a critical site for regular monitoring, so more satellite InSAR data are available for Kīlauea and Mauna Loa volcanoes than for any other volcano on Earth. Because InSAR detects deformation over broad areas, it is an excellent tool for mapping both large- and small-scale changes.
On Mauna Loa, InSAR helps scientists detect subtle shifts in the deformation style of the volcano. Starting in mid-2014, inflation began at the volcano's summit and along its Southwest Rift Zone (SWRZ). In late 2015, however, InSAR data showed inflation occurring only beneath the upper SWRZ. This change was apparent in GPS data, but the details of the shift were better observed with the broad view provided by InSAR.
InSAR also helps track ground movement associated with sudden volcanic events. In March 2011, Kīlauea's normal activity was interrupted by the 4.5-day-long Kamoamoa fissure eruption west of Pu‘u ‘Ō‘ō on the East Rift Zone. InSAR data from several satellites captured deflation of the summit and expansion of the East Rift Zone, indicating that magma had drained from beneath the summit to feed the new eruptive fissure. Careful analysis of the deformation allowed scientists at NASA and HVO to develop a model for the magma pathway below ground.