Scientists at the Hawaiian Volcano Observatory (HVO) use a variety of techniques and instruments to measure ground deformation on volcanoes in Hawai`i, including GPS. Continuously recording GPS receivers are operating at 16 sites on Kilauea, Mauna Loa, and Mauna Kea volcanoes; data are sampled every 30 seconds and downloaded once a day to calculate one-day average positions. In addition, more than 100 other sites are surveyed intermittently using GPS receivers; each survey or data point is compared with previously sampled data to determine accumulated ground deformation and to calculate strain rates or velocities.

During a brief eruptive episode of Kilauea Volcano that began on January 30, 1997, continuously recording GPS receivers measured significant ground deformation near the eruption site along part of the volcano's east rift zone. For an overview of Kilauea's 1983-present eruption, visit HVO's Kilauea eruption summary.

Immediately after lava began erupting from several small new fissures on Kilauea Volcano on January 30, 1997, scientists from the USGS Hawaiian Volcano Observatory resurveyed several benchmarks near Pu`u `O`o (steaming cone in background) using GPS receivers in order to determine deformation caused by the eruption across Kilauea's east rift zone. This survey and the results from continuously operating GPS stations clearly show that the relatively small eruption (about 300,000 m²)was accompanied by a much larger volume of magma that intruded into the east rift zone and remained below ground. Continuous GPS receivers operated by scientists of the USGS Hawaiian Volcano Observatory and Stanford University about 3 km uprift from Napau Crater recorded a widening of the rift zone by 36 cm.

Map showing the location of several GPS receivers on the Island of Hawai`i. The two nearest continuous GPS receivers - NUPM and KTPM - span the east rift zone of Kilauea about 6 km from Pu`u `O`o. MLPM is located on the southeast flank of Mauna Loa and MKPM is located on the summit of Mauna Kea.

Graph showing the increase in distance between NUPM and KTPM during the intrusion of magma into the east rift zone and eruption of lava from Napau Crater on January 30, 1997. As magma forced its way into the rift beneath Napau Crater, these two GPS stations moved apart as indicated by the increase in the length between them. The stations moved apart about 36 cm during the activity and continued to move apart at a slower rate after the activity stopped on January 31. More than half of the extension between the GPS sites actually occurred during a period of volcanic tremor that began about 8 hours before the eruption started. This suggests that the magma took about 8 hours to reach the surface once it started to move.