Previously established GPS stations alerted the observatory's volcano-watchers that the Mauna Loa summit had begun inflating in April or May 2002 after nearly 10 years of deflation. That means the magma reservoir within the volcano is swelling, a possible precursor to an eruption.

Scientists say Mauna Loa, which rises 13,680 feet above sea level and covers half the Big Island, is sure to erupt again, but they have yet to detect telltale seismic activity such as that which preceded its most recent eruptions in 1975 and 1984.

Observatory geophysicist Asta Miklius said the newest GPS stations are in the northeast and southwest rift zones, on the north side of the summit caldera, and on the mountain's upper west flank and southeast flank.

The global positioning system uses a network of orbiting satellites to pinpoint locations on Earth. In the case of Mauna Loa, GPS data are used to detect changes in the surface shape of the volcano caused by earthquakes or the movement of magma. Electronic tiltmeters, which Miklius compared to a carpenter's level, also are used to detect changes in the mountain's shape.

"The GPS stations will give us some idea of the status of the magma and help provide the earliest possible alert of magma moving into the rift zones," Miklius said.

An eruption in the northeast rift zone could threaten Hilo; lava flowing from the southwest rift zone could affect Hawaiian Ocean View Estates.

Also this summer, observatory staff are repairing and upgrading a number of seismographic stations, which calculate the depth of earthquakes.

Miklius said a systematic shallowing of earthquake activity is another hint of an impending eruption, as underground rocks start breaking from the pressure of a magma buildup. She said that when this happens, she would expect an eruption within a year or two.

Seismicity at Mauna Loa has been at a somewhat elevated level this year, Miklius said, though still below the level of activity in the months leading up to the last two eruptions.

In the lead-up to a lava outbreak, the initial seismic activity will be followed by a higher rate of surface tilting and a sharp increase in the number of earthquakes beneath the summit, as magma rises from the reservoir.

"Once the stuff starts migrating to the surface, then it's a question of hours to days," Miklius said.

In addition to surface and seismic measurements, gas emissions can be used to monitor volcanic activity. Along with additional GPS stations, as money would allow, observatory scientists hope to establish emission monitoring stations that Miklius said can provide insight into changes in magma supply, composition and pathways.