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Volcano Watch — Monitoring Kīlauea and Mauna Loa with the Global Positioning System

June 28, 2001

A common sight this time of year, particularly in Puna and the National Park, is a bright yellow tripod topped with a white disk. Usually seen standing by the side of the road, these instruments are Global Positioning System (GPS) antennas, which scientists at the Hawaiian Volcano Observatory use to measure small ground motions. 

These measurements are used both to predict future eruptions and to better understand how the volcano works.

The GPS navigation system is well known to hikers, hunters, and boaters; small handheld GPS receivers permit the outdoor adventurer to rapidly establish his or her position anywhere on earth to within about 7 m (25 feet). But, this accuracy is not good enough for measuring the very small ground motions on volcanoes, which are usually only a few centimeters per year. To achieve this level of accuracy, scientists must use very precise GPS instruments, which are much too bulky for the amateur navigator, and the instruments must take many measurements over periods as long as 12 hours.

Measuring volcanic activity on Kīlauea and Mauna Loa volcanoes with GPS first started in the late 1980s. Since then HVO, in collaboration with Stanford University, has conducted annual GPS 'campaigns' that involve precisely measuring the positions of hundreds of benchmarks (small brass disks embedded into the ground) scattered throughout the island. By comparing today's position of a benchmark to last year's position, scientists can determine how far the benchmark has moved and how fast. Some of the benchmarks on the south coast of Kīlauea are moving at almost 10 centimeters (3 inches) per year. That doesn't sound like much, but it's more than twice as fast as the motions measured along the San Andreas Fault in California.

Beginning in 1995, HVO and Stanford, along with the University of Hawai'i, began to install permanent GPS stations that make continuous measurements. These instruments are solar powered and radio their data back to the Observatory every hour. Located mostly in remote regions of the National Park, there are now more than 20 instruments continuously monitoring Kīlauea and Mauna Loa. The advantage of continuous GPS is that sudden changes in volcanic activity can be instantly measured. The disadvantage is that continuous instruments stay in one place, so they cannot easily detect distant activity. But, by combining annual 'campaign' style measurements with the continuous GPS network, the scientists at HVO have struck a balance between high sensitivity to sudden changes and the ability to detect changes in activity almost anywhere on the island.

GPS measurements on Kīlauea have led to several exciting scientific discoveries. Data collected by the continuous GPS network before and during the 1997 Napau Crater eruption have greatly improved our understanding of how magma moves beneath the earth and why eruptions occur where and when they do. More recently, the GPS network detected a sudden seaward movement of Kīlauea's south flank that lasted about two days and which may have been related to the downpours of early November, 2000. At about 2 cm (1 inch) per day, this movement was extremely fast by geological standards, but not fast enough to cause ground shaking.

If you happen to see a GPS instrument deployed in your neighborhood, the HVO scientists ask: please do not get too close. The signals from the satellite will actually reflect off your body before hitting the antenna and this will lead to inaccurate measurements.

Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Lavamoves away from the vent toward the ocean in a network of tubes and descends Pulama pali in two separate areas. Active surface flows, mainly pahoehoe toes, were observed near the base of the pali in the eastern section of the coastal flats. Lava continued to enter the ocean in the area east of Kupapa`u throughout the week.

There were no earthquakes reported felt during the week ending on June 28.