Mantle fault zone beneath Kilauea Volcano,Hawaii

Relocations and focal mechanism analyses of deep earthquakes (>/=13 kilometers) at Kilauea volcano demonstrate that seismicity is focused on an active fault zone at 30-kilometer depth, with seaward slip on a low-angle plane, and other smaller, distinct fault zones. The earthquakes we have analyzed predominantly reflect tectonic faulting in the brittle lithosphere rather than magma movement associated with volcanic activity. The tectonic earthquakes may be induced on preexisting faults by stresses of magmatic origin, although background stresses from volcano loading and lithospheric flexure may also contribute.     

High magma storage rates before the 1983 eruption of kilauea, hawaii

After a magnitude 7.2 earthquake in 1975 and before the start of the ongoing eruption in 1983, deformation of Kilauea volcano was the most rapid ever recorded. Three-dimensional numerical modeling shows that this deformation is consistent with the dilation of a dike within Kilauea's rift zones coupled with creep over a narrow area of a low-angle fault beneath the south flank. Magma supply is estimated to be 0.18 cubic kilometers per year, twice that of previous estimates. The 1983 eruption may be a direct consequence of the high rates of magma storage within the rift zone that followed the 1975 earthquake.     

Rifting in iceland: new geodetic data

Small but measurable lengthening of several survey lines within the eastern rift zone of Iceland occurred between 1967 and 1970. The changes can be interpreted as a widening of the rift by 6 to 7 centimeters, possibly during the 1970 eruption of Hekla volcano.     

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 Kilauea's 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 Kilauea's rift system, both at rates of at least 15 centimeters per year.     

Crustal Deformation from 1992 to 1995 at the Mid-Atlantic Ridge, Southwest Iceland, Mapped by Satellite Radar Interferometry

Satellite radar interferometry observations of the Reykjanes Peninsula oblique rift in southwest Iceland show that the Reykjanes central volcano subsided at an average rate of up to 13 millimeters per year from 1992 to 1995 in response to use of its geothermal field. Interferograms spanning up to 3.12 years also include signatures of plate spreading and indicate that the plate boundary is locked at a depth of about 5 kilometers. Below that depth, the plate movements are accommodated by continuous ductile deformation, which is not fully balanced by inflow of magma from depth, causing subsidence of the plate boundary of about 6.5 millimeters per year.     

Magma flow instability and cyclic activity at soufriere hills volcano, montserrat, british west indies

Dome growth at the Soufriere Hills volcano (1996 to 1998) was frequently accompanied by repetitive cycles of earthquakes, ground deformation, degassing, and explosive eruptions. The cycles reflected unsteady conduit flow of volatile-charged magma resulting from gas exsolution, rheological stiffening, and pressurization. The cycles, over hours to days, initiated when degassed stiff magma retarded flow in the upper conduit. Conduit pressure built with gas exsolution, causing shallow seismicity and edifice inflation. Magma and gas were then expelled and the edifice deflated. The repeat time-scale is controlled by magma ascent rates, degassing, and microlite crystallization kinetics. Cyclic behavior allows short-term forecasting of timing, and of eruption style related to explosivity potential.     

Kinematics of the asal rift (djibouti) determined from the deformation of fieale volcano

Because of its subaerial exposure the Asal rift segment provides an exceptional opportunity to quantify the deformation field of an active rift and assess the contribution of tectonics and volcanism to rifting processes. The present topography of the Asal rift results from the tectonic dismemberment during the last 100,000 years of a large central volcanic edifice that formed astride the rift zone 300,000 to 100,000 years ago. Three-dimensional deformation of this volcano has been quantified from the combined analysis of the topography and geology. The analysis indicates that spreading at 17 to 29 millimeters per year in a N40 degrees +/- 5 degrees E direction accounts for most of the separation between Arabia and Somalia. The small topographic subsidence relative to extension suggests that tectonic thinning of the crust has been balanced by injection and underplating of magmatic material of near crustal density. The methodology developed in this study could also be applied to quantify deformation in relatively inaccessible areas where the main available information is topography or bathymetry.     

"Helium spots": caused by a diapiric magma from the upper mantle

"Helium spots," where a significant amount of helium is present in the soil [up to 350 parts per million with a high (3)He to (4)He ratio of (8.90 +/- 0.31) x 10(-6)], have been found along the fault zone formed by the 1966 Matsushiro swarm earthquakes. The formation of the "helium spots" and the occurrence of the earthquakes are interpreted as the results of a diapiric uprise of a magma approximately 1 kilometer in diameter.     

Rapid intraplate strain accumulation in the new madrid seismic zone

Remeasurement of a triangulation network in the southern part of the New Madrid seismic zone with the Global Positioning System has revealed rapid crustal strain accumulation since the 1950s. This area experienced three large (moment magnitudes >8) earthquakes in 1811 to 1812. The orientation and sense of shear is consistent with right-lateral strike slip motion along a northeast-trending fault zone (as indicated by current seismicity). Detection of crustal strain accumulation may be a useful discriminant for identifying areas where potentially damaging intraplate earthquakes may occur despite the absence of large earthquakes during historic time.     

Seismic detection of the summit magma complex of kilauea volcano, hawaii

Application of simultaneous inversion of seismic P-wave arrival time data to the investigation of the crust beneath Kilauea Volcano yields a detailed picture of the volcano's heterogeneous structure. Zones of anomalously high seismic velocity are found associated with the volcano's rift zones. A low-velocity zone at shallow depth directly beneath the caldera coincides with an aseismic region interpreted as being the locus of Kilauea's summit magma complex.     

Seismic precursors to the mount st. Helens eruptions in 1981 and 1982

Six categories of seismic events are recognized on the seismograms from stations in the vicinity of Mount St. Helens. Two types of high-frequency earthquakes occur near the volcano and under the volcano at depths of more than 4 kilometers. Medium- and low-frequency earthquakes occur at shallow depths (less than 3 kilometers) within the volcano and increase in number and size before eruptions. Temporal changes in the energy release of the low-frequency earthquakes have been used in predicting all the eruptions since October 1980. During and after eruptions, two types of low-frequency emergent surface events occur, including rockfalls and steam or gas bursts from the lava dome.     

Mechanistic interpretation of rift valley formation

The stress distribution on the lithospheric plate due to excess magma pressure is obtained from an exact solution of the three-dimensional theory of elasticity. The analysis indicates that rift valley formation and associated structural and geophysical characteristics can be suitably explained by dike-like intrusions of magma or igneous mush into the lithospheric plate.     

Eruption of the nevado del ruiz volcano, Colombia, on 13 november 1985: gas flux and fluid geochemistry

The 13 November 1985 eruption of Nevado del Ruiz volcano, in Colombia, released a small volume of pyroclastic material and a disproportionately large volume of volcanic gas. Before the eruption, summit fumarole gases became less water-rich, and the sulfur/chlorine ratio increased. Remote measurements of sulfur dioxide flux after the eruption indicated active degassing at levels associated with eruptive or inter-eruptive stages of other volcanoes. Thermal water analyses revealed increases in magnesium, calcium, and potassium and an increase in the magnesium/chlorine ratio, suggesting that these elements may have been leached from new magma. Ash leachate data showed sulfate and chloride concentrations and ratios that would be expected for the late stages ofa major Plinian eruption. Water from the lahar contained high concentrations of sulfate and had a sulfur/chlorine ratio of 4.67, suggesting that water ejected from the crater lake and turbulent mixing of pyroclasts and glacial ice triggered the lahar. Microprobe analyses of pumice from this eruption and the most recent previous event showed similar mixed andesites. The uniform composition of the pumices and the unusually high ratio of gas to magma suggest that, although a new batch of magma triggered this eruption, the pumice that erupted may actually be old. Large volumes of new magma and glacial ice make the volcano dangerous and should stimulate development of an integrated long-term monitoring program to include Tolima volcano, 25 kilometers to the south.     

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 volcano's 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 volcano's 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. Kilauea's 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.     

A mechanical model for intraplate earthquakes: application to the new madrid seismic zone

We present a time-dependent model for the generation of repeated intraplate earthquakes that incorporates a weak lower crustal zone within an elastic lithosphere. Relaxation of this weak zone after tectonic perturbations transfers stress to the overlying crust, generating a sequence of earthquakes that continues until the zone fully relaxes. Simulations predict large (5 to 10 meters) slip events with recurrence intervals of 250 to 4000 years and cumulative offsets of about 100 meters, depending on material parameters and far-field stress magnitude. Most are consistent with earthquake magnitude, coseismic slip, recurrence intervals, cumulative offset, and surface deformation rates in the New Madrid Seismic Zone. Computed interseismic strain rates may not be detectable with available geodetic data, implying that low observed rates of strain accumulation cannot be used to rule out future damaging earthquakes.     

Linking petrology and seismology at an active volcano

Many active volcanoes exhibit changes in seismicity, ground deformation, and gas emissions, which in some instances arise from magma movement in the crust before eruption. An enduring challenge in volcano monitoring is interpreting signs of unrest in terms of the causal subterranean magmatic processes. We examined over 300 zoned orthopyroxene crystals from the 1980-1986 eruption of Mount St. Helens that record pulsatory intrusions of new magma and volatiles into an existing larger reservoir before the eruption occurred. Diffusion chronometry applied to orthopyroxene crystal rims shows that episodes of magma intrusion correlate temporally with recorded seismicity, providing evidence that some seismic events are related to magma intrusion. These time scales are commensurate with monitoring signals at restless volcanoes, thus improving our ability to forecast volcanic eruptions by using petrology.     

Magma ascent and the pressurization of Mount Etna's volcanic system

After a period of deflation during the 1991-1993 flank eruption, Mount Etna underwent a rapid inflation. Seismicity and ground deformation show that since 1994, a huge volume of magma intruded beneath the volcano, producing from 1998 onward a series of eruptions at the summit and on the flank of the volcano. The last of these, started on 27 October 2002, is still in progress and can be considered one of the most explosive eruptions of the volcano in recent times. Here we show how geodetic data and seismic deformation, between 1994 and 2001, indicate a radial compression around an axial intrusion, consistent with a repressurization of Mount Etna's plumbing system at a depth of 6 to 15 kilometers, which triggered most of the seismicity and provoked the dilatation of the volcano and the recent explosive eruptive activity.     

The role of magma overpressure in suppressing earthquakes and topography: worldwide examples

In an extending terrane basaltic magma supplied at a pressure greater than the least principal stress (overpressure) may be capable of suppressing normal faulting and the earthquakes and topographic relief that commonly accompany normal faulting. As vertical dikes intrude, they press against their walls in the direction opposite the least principal stress and increase its magnitude. The emplacement of tabular intrusions causes the internal magma pressure to act selectively in opposition to tectonic stresses. This process tends to equalize the stresses and thus diminishes the deviatoric stress (difference between maximum and minimum stresses) that creates faults and causes earthquakes. Observations of the pattern of seismicity and magmatism worldwide indicate that magmatism commonly supplants large earthquakes as the primary mechanism for accommodating tectonic extension. Recognizing the extent of magmatic stress accommodation is important in assessing seismic and volcanic risks.     

Mechanism of Phreatic Eruptions at Aso Volcano Inferred from Near-Field Broadband Seismic Observations

Broadband seismometers deployed at Aso volcano in Japan have detected a hydrothermal reservoir 1 to 1.5 kilometers beneath the crater that is continually resonating with periods as long as 15 seconds. When phreatic eruptions are observed, broadband seismograms elucidate a dynamic interplay between the reservoir and discharging flow along the conduit: gradual pressurization and long-period (approximately20 seconds) pulsations of the reservoir during the 100 to 200 seconds before the initiation of the discharge, followed by gradual deflation of the reservoir concurrent with the discharging flow. The hydrothermal reservoir, where water and heat from the deeper magma chamber probably interact, appears to help control the surface activity at Aso volcano.     

Deep earthquakes beneath mount st. Helens: evidence for magmatic gas transport?

Small-magnitude earthquakes began beneath Mount St. Helens 40 days before the eruption of 20 March 1982. Unlike earlier preeruption seismicity for this volcano, which had been limited to shallow events (less than 3 kilometers), many of these earthquakes were deep (between 5 and 11 kilometers). The location of these preeruptive events at such depth indicates that a larger volume of the volcanic system was affected prior to the 20 March eruption than prior to any of the earlier dome-building eruptions. The depth-time relation between the deep earthquakes and the explosive onset of the eruption is compatible with the upward migration of magmatic gas released from a separate deep reservoir.