Arnold T. Okamura

Surface deformation in volcanic rift zones

Abstract The principal conduits for magma transport within rift zones of basaltic volcanoes are steeply dipping dikes, some of which feed fissure eruptions. Elastic displacements accompanying a single dike emplacement elevate the flanks of the rift relative to a central depression. Concomitant normal faulting may transform the depression into a graben thus accentuating the topographic features of the rift. If eruption occurs the characteristic ridge-trough-ridge displacement profile changes to a single ridge, centered at the fissure, and the erupted lava alters the local topography. A well-developed rift zone owes its structure and topography to the integrated effects of many magmatic rifting events. To investigate this process we compute the elastic displacements and stresses in a homogeneous, two-dimensional half-space driven by a pressurized crack that may breach the surface. A derivative graphical method permits one to estimate the three geometric parameters of the dike (height, inclination, and depth-to-center) and the mechanical parameter (driving pressure/rock stiffness) from a smoothly varying displacement profile. Direct comparison of measured and theoretical profiles may be used to estimate these parameters even if inelastic deformation, notably normal faulting, creates discontinuities in the profile. Geological structures (open cracks, normal faults, buckles, and thrust faults) form because of stresses induced by dike emplacement and fissure eruption. Theoretical stress states associated with dilation of a pressurized crack are used to interpret the distribution and orientation of these structures and their role in rift formation.

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.

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.

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.

Geodetic analysis of dike intrusion and motion of the magma reservoir beneath the summit of Kilauea Volcano, Hawaii: 1970–1985

We use leveling and trilateration data collected on Kilauea volcano to constrain the location of deformation sources caused by magma accumulation, intrusion, and eruption. For the 13 inflationary epochs examined, combinations of an expanding point source and one or two opening rectangular dislocations mimic inflation of the summit reservoir and formation of dike(s), respectively. The combined model adequately accounts for the deformation data and is consistent with the seismicity observed during each epoch. For 10 deflationary epochs, however, the data require only a contracting point source. Confidence in these results is gained by noting that locations of the sources of both inflation and deflation are coincident, within the observed uncertainties of the data, the function of network geometry, and the inversion procedure. It appears, therefore, that magma accumulation at Kilauea volcano may be characterized by the growth of dikes during inflation of the summit reservoir. Drainage of the reservoir, on the other hand, is not accompanied by significant closure of dikes. In contrast to previous studies (e.g., Fiske and Kinoshita, 1969; Dvorak et al., 1983) that do not include the dislocation (or dike growth) component of summit magma accumulation and concluded that the source of inflation migrates over a 5 km2 area, we find that a single magmatic reservoir source accounts for data collected during all inflationary and deflationary epochs, results, which compare favorably with those obtained from the point ellipsoid model, can be used to estimate the distribution of stresses within the volcano in the near field of the source.

Variations in tilt rate and harmonic tremor amplitude during the January–August 1983 east rift eruptions of Kilauea Volcano, Hawaii

Abstract During January–August 1983, a network of telemetered tiltmeters and seismometers recorded detailed temporal changes associated with seven major eruptive phases along the east rift of Kilauea Volcano, Hawaii. Each eruptive phase was accompanied by subsidence of the summit region and followed by reinflation of the summit to approximately the same level before renewal of eruptive activity. The cyclic summit tilt pattern and the absence of measurable tilt changes near the eruptive site suggest that conditions in the summit region controlled the timing of the last six eruptive phases. The rate of summit subsidence progressively increased from one eruptive phase to the next during the last six phases; the amplitude of harmonic tremor increased during the last four phases. The increases in subsidence rate and in tremor amplitude suggest that frequent periods of magma movement have reduced the flow resistance of the conduit system between the summit and the rift zone.

Recent uplift and hydrothermal activity at Tangkuban Parahu volcano, west Java, Indonesia

Tangkuban Parahu is an active stratovolcano located 17 km north of the city of Bandung in the province west Java, Indonesia. All historical eruptive activity at this volcano has been confined to a complex of explosive summit craters. About a dozen eruptions-mostly phreatic events- and 15 other periods of unrest, indicated by earthquakes or increased thermal activity, have been noted since 1829. The last magmatic eruption occurred in 1910. In late 1983, several small phreatic explosions originated from one of the summit craters. More recently, increased hydrothermal and earthquake activity occurred from late 1985 through 1986. Tilt measurements, using a spirit-level technique, have been made every few months since February 1981 in the summit region and along the south and east flanks of the volcano. Measurements made in the summit region indicated uplift since the start of these measurements through at least 1986. From 1981 to 1983, the average tilt rate at the edges of the summit craters was 40–50 microradians per year. After the 1983 phreatic activity, the tilt rate decreased by about a factor of five. Trilateration surveys across the summit craters and on the east flank of the volcano were conducted in 1983 and 1986. Most line length changes measured during this three-year period did not exceed the expected uncertainty of the technique (4 ppm). The lack of measurable horizontal strain across the summit craters seems to contradict the several years of tilt measurements. Using a point source of dilation in an elastic half-space to model tilt measurements, the pressure center at Tangkuban Parahu is located about 1.5 km beneath the southern part of the summit craters. This is beneath the epicentral area of an earthquake swarm that occurred in late 1983. The average rate in the volume of uplift from 1981 to 1983 was 3 million m3 per year; from 1983 to 1986 it averaged about 0.4 million m3 per year. Possible causes for this uplift are increased pressure within a very shallow magma body or heating and expansion of a confined aquifier.