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Hawaiian Volcano Observatory

Subsidence of Kilauea Volcano, 1983-2001

View of Halama`uma`u crater and Kilauea Volcano's summit caldera
Photograph by J. Kauahikaua on October 9, 1997
View is east toward Halema`uma`u Crater and across the summit caldera of Kilauea. The area of maximum subsidence between 1983 and 2001 is located about 2 km south (right) of Halema`uma`u (the crater is about 1 km in diameter). This area has subsided 1.5 m since 1983. The wide ground cracks at the lower right are part of the southwest rift zone.

1983-2001: Leveling surveys reveal pattern of subsidence

Deformation monitoring of the summit area of Kilauea Volcano has long shown brief episodes of inflation and deflation that correlate with the movement of magma into and out of the volcano's summit magma reservoir. Episodes of inflation occur as magma accumulates in the summit reservoir; episodes of deflation occur when magma erupts within the caldera (though some caldera eruptions have very little deflation) or moves into one of the rift zones, where it sometimes erupts.

Inflation and deflation of the magma reservoir result in changes in the ground surface (see summit tilt pattern during several episodes of the Pu`u `O`o-Kupaianaha eruption). For example, an increase in pressure within the summit reservoir owing to magma accumulation causes the ground surface above the reservoir to move outward and upward away from the center of inflation (illustration of inflation-deflation process). Several instruments and techniques measure these changes. One of the most accurate ways to measure elevation changes is by leveling. We conduct leveling surveys at the summit of Kilauea every 1-2 yr or more frequently if needed.

Leveling and other surveys show that the summit area has subsided as much as 1.5 m since the beginning of the Pu`u `O`o-Kupaianaha eruption in 1983, relative to a benchmark about 3 km northwest of the caldera. Between July 1999 and July 2001, the summit continued to subside at a maximum rate of 7 cm/yr. In the figures below, we present some of the results of leveling surveys since 1983.

Kilauea summit vertical changes: July 1999 to July 2001

Map of the summit area of Kilauea Volcano showing vertical changes at specific sites between 1983 and 1998

Map of benchmarks that form the leveling network around Kilauea's summit and across the upper rift zones. Numbers next to each station are elevation differences, in centimeters relative to benchmark HVO23, between the latest two surveys in July 1999 and July 2001. The area of greatest subsidence is in the southern part of the caldera (14 cm near station Rebar4). 

Note the location of benchmark Rebar4 along the south rim of the inner caldera. Elevation changes at Rebar4 between 1983 and 2001 are shown in the figure below.

Vertical changes at Rebar4, 1983 to 2001

Graph showing subsidence of benchmark REBAR4 between 1983 and 1998 Elevation of benchmark REBAR4 relative to HVO23 in meters. The rate of elevation change has been relatively steady since 1983, but some fluctuations in rate were associated with changes in eruptive activity, most notably when the eruption site moved from Pu`u `O`o to Kupaianaha in 1986 and then back to Pu`u `O`o in 1992 (see eruption chronology).

The small increase in elevation in early 1997 followed an episode of magma intrusion into the east rift zone and a small, short-lived fissure eruption at Napau Crater (see GPS monitoring results for information about this episode).

Vertical velocities across Kilauea summit

Graph showing subsidence of benchmark REBAR4 between 1983 and 1998 Vertical profiles of elevation changes (cm/yr) across the summit of Kilauea from Bird Park to the Hilina Pali Road from 1998 to 1999 (red) and 1999 to 2001 (blue). Error bars represent two standard deviations (95 percent confidence interval). See map above for location of profile A-A'.

 

What's going on? Withdrawal of magma or the movement of Kilauea's south flank or both?

The ongoing subsidence of the summit is thought to result mainly from magma movement out of the summit reservoir and into the east rift zone at a faster rate than it is being supplied from below (the hot spot beneath the Big Island). But other deformation surveys show that the subsidence has also been accompanied by the spreading or widening of the summit area and deformation of the rift zones. This broader region of deformation has led some scientists to suggest that another process--the movement of the volcano's south flank--may account for some of the subsidence.

Although the summit area of Kilauea has steadily subsided since the beginning of the Pu`u `O`o-Kupaianaha eruption, it has been subsiding and spreading (widening) at varying rates since a magnitude 7.2 earthquake occurred 9 to 10 km beneath the south flank of Kilauea on November 29, 1975. The earthquake probably originated as movement of the south flank along a broad slip surface defined by the boundary between the volcano and the underlying sea floor. Parts of the south flank moved suddenly seaward 3 to 8 m and subsided 1 to 3.5 m during the earthquake.

Scientists have long measured the seaward movement Kilauea's south flank and noticed correlations with intrusions of magma into the volcano's rift zones, eruptions, and large earthquakes. Three main hypotheses have been proposed to explain the fundamental cause of the south flank's mobility:

  1. repeated forceful intrusion of magma into Kilauea's rift zones, which forces the volcano apart
  2. gravitational settling of the entire south flank due to its enormous mass
  3. the presence of a deep magma reservoir system beneath the summit and rift zones
Thus, in addition to the withdrawal of magma from the summit reservoir since 1983, part of the subsidence and spreading at the summit may also be related to continued movement of the south flank since the 1975 earthquake.

Deformation of the summit, rift zones, and south flank of Kilauea continues to be a major focus of the monitoring program at the Hawaiian Volcano Observatory. Ongoing research into the underlying causes of the volcano's movement is sure to improve our understanding of how Kilauea responds to gravity and its magma reservoir system.

 

Further Reading

Clague, D.A., and Denlinger, R.P., 1994, Role of olivine cumulates in destabilizing the flanks of Hawaiian volcanoes: Bulletin of Volcanology, v. 56, p. 425-434.

Delaney, P.T., Fiske, R.S., Miklius, A., Okamura, A.T., and Sako, M.K., 1990, Deep magma body beneath the summit and rift zones of Kilauea Volcano, Hawaii: Science, v. 247, p. 1311-1316.

Delaney, P.T., Miklius, A., Arnadottir, T., Okamura, A.T, and Sako, M.K., 1993, Motion of Kilauea volcano during sustained eruption from the Puu Oo and Kupaianaha vents, 1983-1991: Journal of Geophysical Research, v. 98, p. 17,801-17,820.

Swanson, D.A., Duffield, W.A., and Fiske, R.S., 1976, Displacement of the south flank of Kilauea Volcano: the result of forceful intrusion of magma into the rift zones: U.S. Geological Survey Professional Paper 963, 39 p.

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Updated: 17 October 2001 (SRB)