July 26, 1996

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Measuring the Mountains: Ground Deformation of Hawaii's Volcanoes

The ground's surface around the active Hawaiian volcanoes Kilauea and Mauna Loa is constantly changing. Lava flows laminate their sides during active eruptions. Less obvious, but more widespread, are the subtle movements that occur in response to the movement of magma within the volcano. The distribution and rate of these movements provide clues about processes occurring within the volcano and help us forecast impending eruptions, large earthquakes, or landslides.

Scientists at the U.S. Geological Survey's Hawaiian Volcano Observatory (HVO) monitor ground deformation around the volcanoes of Hawaii by periodically surveying the positions of a large number of bench marks. You may have seen one of our bench marks along a road or on a hill top; they are inscribed metal tablets set in rock or concrete. The accumulated ground movement between surveys is simply the observed change in position of the bench mark.

We recently completed our annual Global Positioning System (GPS) survey of the Big Island. Our surveying equipment and technique allow us to measure position changes to a fraction of an inch. The arrows on the accompanying figure show the average rate and direction that our bench marks moved (horizontally) between 1993 and 1996.

We observe Kilauea's south flank moving seaward at up to three inches per year. This area experienced a magnitude 7.2 earthquake in 1975; it is also where the most spectacular palis (the Hawaiian word for cliffs) are found. The southeast flank of Mauna Loa is also moving seaward, but at a slower rate. This region of Mauna Loa experienced a magnitude 6.7 earthquake in 1983. Although these motions are a small fraction of those that occurred during the earthquakes, they indicate that the forces that produced the earthquakes and created the palis are still active.

Results of our vertical measurements show continuing inflation of Mauna Loa's summit region. About half of the deflation that occurred during the 1984 eruption has been recovered. We are watching Mauna Loa closely and expect that any impending eruption will be preceded by a recognizable increase in the number of earthquakes near its summit.

The vertical changes also indicate subsidence of Kilauea's summit region. The deflation of Kilauea's summit is probably due to more lava being erupted during the ongoing Pu`u O`o eruption than magma is being supplied to the volcano.

Eruption Update

The Kilauea eruption continues unabated, and flows enter the ocean in the Lae`apuki region. The level of the lava pond within Pu`u `O`o fluctuates between 275 and 325 feet below the lowest part of the rim. At night, the fluctuating pond level often causes a bright glow to reflect off the fume cloud over the cone.

Earthquakes at Lo`ihi