An HVO scientist wearing a gas mask collects GPS data using a roving instrument on the west flank of Puu Oo. (Photo courtesy of USGS)

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Over the past four weeks, we have discussed how scientists monitor volcanoes using geophysics, geology, gas geochemistry, and seismology. As we reach the end of January and the second annual Volcano Awareness Month, we conclude our discussion of volcano monitoring by exploring deformation measurements.

As magma accumulates in subsurface reservoirs, it makes space for itself by expanding the reservoir walls. This motion causes the surface above the reservoir to rise. Likewise, when magma drains from a reservoir beneath the ground, the surface subsides. The pattern of surface deformation caused by magma accumulation and withdrawal is similar to that of a balloon being inflated and deflated.

By measuring surface deformation — inflation and deflation — volcanologists can assess the volume of magma that is accumulating or draining and can also estimate the depth of the magma reservoir. Any changes in the deformation pattern of a volcano indicate that magma is moving beneath the surface and can aid in assessing the likelihood of eruption.

Surface displacements at volcanoes tend to be small by human standards. Typical changes might amount to only a few centimeters or millimeters (inches or fractions of an inch).

Yet over the broad distances spanned by Kilauea and Mauna Loa, these small displacements reflect large amounts of magma beneath the surface. But how is it possible to measure surface deformation with such precision?

The current workhorse of volcano deformation monitoring is something that is familiar to most people—the Global Positioning System (GPS). HVO uses very sensitive GPS receivers, different from those you might use in your car or while hiking, to measure positions to within a few millimeters (fractions of an inch).

GPS provides both vertical and horizontal deformation and has been instrumental in mapping the locations of, and volume changes within, subsurface magma reservoirs at Kilauea and Mauna Loa over the last 20 years.

By installing a permanent GPS station in one location and running it continuously, changes in that position can be tracked over time. HVO collects data from more than 60 continuous GPS stations on the Island of Hawaii.

In addition, annual or biannual GPS surveys are carried out to supplement the continuous data, and mobile GPS rovers are used in highly active places, such as the Puu Oo eruptive vent, to measure localized ground displacements.

HVO also has 20 continuously operating tiltmeters on Kilauea and Mauna Loa. Tiltmeters, which are instruments that measure very small changes in the slope of the Earth’s surface over time, are installed in boreholes, about 2-3 m (6-9 feet) beneath the surface. Borehole installations insulate the tiltmeters against surface noise caused by temperature changes and other environmental effects.

Alert readers who follow Kilauea’s daily eruption update (http://volcanoes.usgs.gov/hvo/activity/kilaueastatus.php) may have noticed that Kilauea experiences cycles of deflation and inflation—so-called “DI events”—every few days or weeks. These events were first detected and are best measured using tiltmeters.

In addition to ground-based instruments, HVO scientists also utilize space-based sensors to measure ground deformation. Interferometric synthetic aperture radar, or InSAR, measurements use radar images of the surface acquired at different times to calculate changes in the distance between the satellite and the ground between the times that the images were taken.

As we conclude our month-long look at volcano monitoring, it is important to remember that no single monitoring type is the “magic bullet” that answers all questions about volcanic activity. Geophysics, geology, gas geochemistry, seismology, and deformation must be used in combination to best assess the level of volcanic activity and the potential for future eruption.

The HVO staff includes experts in all these fields, making the Observatory a one-stop shop for volcano monitoring!

We hope you have enjoyed this series of articles on how HVO scientists monitor the volcanoes of Hawaii, as well as other events associated with Volcano Awareness Month. The final talk of the month, about the history and current status of Mauna Loa, is Jan. 31 at the Lyman Museum.

Best wishes for happy and safe 2011 from the staff of the Hawaiian Volcano Observatory!