Maurice K. Sako

Deep magma body beneath the summit and rift zones of Kilauea Volcano, Hawaii

A magnitude 7.2 earthquake in 1975 caused the south flank of Kilauea Volcano, Hawaii, to move seaward in response to slippage along a deep fault. Since then, a large part of the volcanos edifice has been adjusting to this perturbation. The summit of Kilauea extended at a rate of 0.26 meter per year until 1983, the south flank uplifted more than 0.5 meter, and the axes of both the volcanos rift zones extended and subsided; the summit continues to subside. These ground-surface motions have been remarkably steady and much more widespread than those caused by either recurrent inflation and deflation of the summit magma chamber or the episodic propagation of dikes into the rift zones. Kilaueas magmatic system is, therefore, probably deeper and more extensive than previously thought; the summit and both rift zones may be underlain by a thick, near vertical dike-like magma system at a depth of 3 to 9 kilometers.

Rapid deformation of Kilauea Volcano: Global Positioning System measurements between 1990 and 1996

Campaign Global Positioning System (GPS) measurements from 1990 to 1996 are used to calculate surface displacement rates on Kilauea Volcano, Hawaii. The GPS data show that the south flank of the volcano, which has generated several large earthquakes in the past 3 decades, is displacing at up to ;8 cm/yr to the south-southeast. The summit and rift zones are subsiding, with maximum subsidence rates of ;8 cm/yr observed a few kilometers south of the summit caldera. Elastic dislocation modeling of the GPS data suggests that the active sources of deformation include deep rift opening along the upper east and east rift zone, fault slip along a subhorizontal fault near the base of the volcano, and deflation near the summit caldera. A nonlinear optimization algorithm was used to explore the parameter space and to find the best fitting source geometry. There is a broad range of model geometries that fit the data reasonably well. However, certain models can be ruled out, including those that have shallow rift opening or shallow fault slip. Some offshore, aseismic slip on a fault plane that dips between 258 north- northwest and 88 south-southeast is required. Best fitting slip and rift opening rates are 23-28 cm/yr, although rates as low as 10 cm/yr are permitted by the data.

Rapid deformation of the south flank of Kilauea Volcano, Hawaii

The south flank of Kilauea volcano has experienced two large [magnitude (M) 7.2 and M 6.1] earthquakes in the past two decades. Global Positioning System measurements conducted between 1990 and 1993 reveal seaward displacements of Kilaueas central south flank at rates of up to about 10 centimeters per year. In contrast, the northern side of the volcano and the distal ends of the south flank did not displace significantly. The observations can be explained by slip on a low-angle fault beneath the south flank combined with dilation deep within Kilaueas rift system, both at rates of at least 15 centimeters per year.

Volcanic spreading at Kilauea, 1976–1996

The rift system traversing about 80 km of the subaerial surface of Kilauea volcano has extended continuously since the M 7.2 flank earthquake of November 1975. Widening across the summit has amounted to more than 250 cm, decelerating after 1975 from about 25 to 4 cm yr−1 since 1983. Concurrently, the summit has subsided more than 200 cm, even as the adjacent south flank has risen more than 50 cm. The axes of the upper zones, about 10 km from the summit, subsided before 1983 at average rates of 9 and 4 cm yr−1, respectively, and at rates of 4 and 3 cm yr−1 since. The middle southwest rift zone is also subsiding and, at the other end of Kilaueas subaerial rift system, subsidence along the lower east rift zone has averaged 1–2 cm yr−1. Deformation of Kilaueas south flank has been continuous, although subject as well to displacements caused by major rift zone seismic swarms. Whereas horizontal strains across the subaerial south flank seem to have been generally compressive after 1975, they have been extensional since about 1980 or 1981, interrupted only by the east rift zone dike intrusion of 1983. Because the magnitudes of these contractions and extensions are much less than the extension across the rift system, the subaerial south flank is apparently sliding seaward on its basal decollement more than it is accumulating horizontal strains within the overlying volcanic pile. Kilauea suffers from gravitational spreading made even more unstable by accumulation of magma along the rift system at depths in excess of about 4–5 km in the presence hot rock incapable of withstanding deviatoric stresses. This seismicly quiescent zone decouples the south flank from the rest of Hawaiis volcanic edifice; the rift zones at lesser depths exhibit a more brittle and, therefore, sporadic extensional behavior. Judging from the modern extension record of the summit, which both predates the M 7.2 earthquake of 1975 and has outlived its 10-year period of aftershocks, Kilauea will continue to spread along its rift system as its south flank slips seaward to accommodate the accretion of magma and its relatively dense olivine-rich differentiate.

Geophysical observations of Kilauea Volcano, Hawaii, 2. Constraints on the magma supply during November 1975–September 1977

Abstract Following a 22-month hiatus in eruptive activity, Kilauea volcano extruded roughly 35 × 10 6 m 3 of tholeiitic basalt from vents along its middle east rift zone during 13 September–1 October, 1977. The lengthy prelude to this eruption began with a magnitude 7.2 earthquake on 29 November, 1975, and included rapid summit deflation episodes in June, July, and August 1976 and February 1977. Synthesis of seismic, geodetic, gravimetric, and electrical self-potential observations suggests the following model for this atypical Kilauea eruptive cycle. Rapid summit deflation initiated by the November 1975 earthquake reflected substantial migration of magma from beneath the summit region of Kilauea into the east and southwest rift zones. Simultaneous leveling and microgravity observations suggest that 40–90 × 10 6 m 3 of void space was created within the summit magma chamber as a result of the earthquake. If this volume was filled by magma from depth before the east rift zone intrusive event of June 1976, the average rate of supply was 6–13 × 10 6 m 3 /month, a rate that is consistent with the value of 9 × 10 6 m 3 /month suggested from observations of long-duration Kilauea eruptions. Essentially zero net vertical change was recorded at the summit during the 15-month period beginning with the June 1976 intrusion and ending with the September 1977 eruption. This fact suggests that most magma supplied from depth during this interval was eventually delivered to the east rift zone, at least in part during four rapid summit deflation episodes. Microearthquake epicenters migrated downrift to the middle east rift zone for the first time during the later stages of the February 1977 intrusion, an occurrence presumably reflecting movement of magma into the eventual eruptive zone. This observation was confirmed by tilt surveys in May 1977 that revealed a major inflation center roughly 30 km east of the summit in an area of anomalous steaming and forest kill first noted in March 1976.

Mechanical response of the south flank of kilauea volcano, hawaii, to intrusive events along the rift systems

Abstract Increased earthquake activity and compression of the south flank of Kilauea volcano, Hawaii, have been recognized by previous investigators to accompany rift intrusions. We further detail the temporal and spatial changes in earthquake rates and ground strain along the south flank induced by six major rift intrusions which occurred between December 1971 and January 1981. The seismic response of the south flank to individual rift intrusions is immediate; the increased rate of earthquake activity lasts from 1 to 4 weeks. Horizontal strain measurements indicate that compression of the south flank usually accompanies rift intrusions and eruptions. Emplacement of an intrusion at a depth greater than about 4 km, such as the June 1982 southwest rift intrusion, however, results in a slight extension of the subaerial portion of the south flank. Horizontal strain measurements along the south flank are used to locate the January 1983 east-rift intrusion, which resulted in eruptive activity. The intrusion is modeled as a vertical rectangular sheet with constant displacement perpendicular to the plane of the sheet. This model suggests that the intrusive body that compressed the south flank in January 1983 extended from the surface to about 2.4 km depth, and was aligned along a strike of N66°E. The intrusion is approximately 11 km in length, extended beyond the January 1983 eruptive fissures, which are 8 km in length and is contained within the 14-km-long region of shallow rift earthquakes.

Motion of Kilauea Volcano during sustained eruption from the Puu Oo and Kupaianaha Vents, 1983–1991

Kilauea erupted almost continuously from January 1983 through 1991. Although the summit began subsiding during the rift zone dike intrusion that initiated this eruption, remarkably steady ground surface motions began in late 1983 after a magnitude 6.6 earthquake beneath the slopes of nearby Mauna Loa volcano and continued until the onset of brief upper east rift zone earthquake swarms in late 1990. During these 7 years the summit and upper rift zones subsided up to 10–11 and 4–8 cm yr−1, respectively, and summit baselines contracted up to 6 cm yr−1. Baselines directed northward from the summit to stations on Mauna Loa extended at rates up to 7 cm yr−1, and a baseline from south of the summit to Mauna Loa extended 4 cm yr−1. Much of this extension is inconsistent with deformation caused solely by summit magma reservoir collapse and more likely reflects rifting as the south flank of the volcano moved seaward from the summit and rift zones. Farther from the summit, baselines crossing the south flank extended up to 2 cm yr−1, and a south flank tide gauge rose 2 cm yr−1; the lower east rift zone, 40–50 km from the summit, subsided about 2 cm yr−1. Motion on Kilauea, then, is broadly consistent with slip along low-angle south flank faults, generating subsidence that is focused at the summit and along the rift system behind the faulting and uplift along the coastal south flank ahead of it. Dislocation models that combine these elements show that much of Kilaueas edifice migrated seaward, producing ground surface motions along the south flank of up to about 6 cm yr−1. The magnitude 6.1 earthquake of 1989 punctuated these motions along the eastern south flank, producing more than 25 cm of seaward displacement and, 15 km east of the epicenter, up to 24 cm of subsidence south of the lower east rift zone. Unlike the magnitude 7.2 south flank earthquake of 1975, the 1989 event was preceded neither by summit magma reservoir inflation nor by rift zone dike intrusions and accompanying compression of the south flank. Deformation was probably caused by the weight of the volcanic overburden and by ongoing dilation and slip within the rift system.