Volcano Watch — Measuring how volcanoes move

The ground around an active volcano swells, deflates, or shifts as magma moves in and out of the volcano's underground plumbing system. The distribution and rate of ground deformation provides clues about processes occurring within the volcano.

Swelling at the summit of Mauna Loa Volcano, for example, occurs as magma moves into and expands the summit magma chamber. Such motions are generally subtle and can only be detected with precise surveying techniques or sensitive tilt or strain meters. Ground movements of several meters can occur during a large earthquake or when magma forces its way to the surface along a rift zone.

Scientists at the Hawaiian Volcano Observatory (HVO) monitor ground deformation around the volcanoes of Hawaii by periodically measuring the elevation or position of a large number of survey control "monuments." The monuments are inscribed brass tablets set in rock or concrete and are often called "bench marks."

You may have seen one of our bench marks along a road or on a hill top. We use a variety of land surveying techniques, including spirit leveling, electronic distance measurements, and Global Positioning System (GPS) to monitor ground deformation.

Each survey provides a snapshot of the positions of the bench marks that can be compared with that of previous surveys to determine accumulated ground deformation. In addition to the periodic surveys, we operate a network of continuously recording tiltmeters and GPS receivers to record the rapid changes that occur during deformation events.

We use spirit level surveys to measure elevation change. A spirit level is a telescope with its optical axis aligned to horizontal with a level vial. It functions like a long and very accurate carpenter's level. The spirit level is placed between two calibrated rods set vertically on the ground, and its height on each rod is read. The difference of these heights is the elevation difference of the ground on which the rods are set. To obtain the highest precision in our height measurements, we have to limit our sight lengths to about 50 meters. To carry the elevation difference between bench marks, we "leap frog" the rods. Level surveys provide the relative elevation of bench marks to a precision better than a few millimeters (there are 25.4 millimeters in an inch).

Spirit level surveys around the summit of Kīlauea Volcano show a broad region near Halemaumau Crater subsiding at an average rate of 0.1 m/yr. This subsidence has been occurring at more or less the same rate since the start of the Puu Oo eruption in 1983, and it indicates that the Kīlauea summit magma chamber is slowly deflating as the eruption continues.

Spirit level surveys on the summit of Mauna Loa show that the summit magma chamber has been slowly inflating since the 1984 eruption and has recovered about half of the deflation that occurred during the eruption. Surface flows from the 1984 eruption stopped about 4 miles from Hilo.

We are watching Mauna Loa very closely and expect that any impending eruption will be preceded by a sharp increase in the number of earthquakes near its summit. We detect no such increase at this time.

Distances increase between points near an inflating magma chamber, while they decrease near a deflating magma chamber. This can be simply demonstrated by inflating and deflating a balloon while examing the length change in a line or position change of points drawn on its surface. The actual deformation occurring on a volcano is more complex and involves many different sources.

We use electronic distance meter (EDM) and GPS surveys to measure position changes. The measurement principal for both systems is the same, that is, the time it takes for light (EDM) or radio waves (GPS) to travel between two points depends principally on the distance between the points. Position changes measured with electronic distance meter (EDM) surveys showed that a big piece of the south flank of Kīlauea Volcano was thrust seaward 3 meters during the 1975 Kalapana earthquake.

Over the last few years, we have converted most of our monitoring from EDM to GPS surveys. GPS surveys provide the most detailed and accurate measure of how volcanoes move. We will discuss how GPS works in our next article and summarize the results from annual island-wide surveys conducted between 1990 and 1995. These measurements show that the south flank of Kīlauea and the southeast flank of Mauna Loa are moving seaward at rates between 1 and 3 inches per year. Most of the rest of the island, however, is relatively stable.