Thóra Árnadóttir

Intrusion triggering of the 2010 Eyjafjallajökull explosive eruption

Gradual inflation of magma chambers often precedes eruptions at highly active volcanoes. During such eruptions, rapid deflation occurs as magma flows out and pressure is reduced. Less is known about the deformation style at moderately active volcanoes, such as Eyjafjallajökull, Iceland, where an explosive summit eruption of trachyandesite beginning on 14 April 2010 caused exceptional disruption to air traffic, closing airspace over much of Europe for days. This eruption was preceded by an effusive flank eruption of basalt from 20 March to 12 April 2010. The 2010 eruptions are the culmination of 18 years of intermittent volcanic unrest. Here we show that deformation associated with the eruptions was unusual because it did not relate to pressure changes within a single magma chamber. Deformation was rapid before the first eruption (>5 mm per day after 4 March), but negligible during it. Lack of distinct co-eruptive deflation indicates that the net volume of magma drained from shallow depth during this eruption was small; rather, magma flowed from considerable depth. Before the eruption, a ∼0.05 km3 magmatic intrusion grew over a period of three months, in a temporally and spatially complex manner, as revealed by GPS (Global Positioning System) geodetic measurements and interferometric analysis of satellite radar images. The second eruption occurred within the ice-capped caldera of the volcano, with explosivity amplified by magma–ice interaction. Gradual contraction of a source, distinct from the pre-eruptive inflation sources, is evident from geodetic data. Eyjafjallajökull’s behaviour can be attributed to its off-rift setting with a ‘cold’ subsurface structure and limited magma at shallow depth, as may be typical for moderately active volcanoes. Clear signs of volcanic unrest signals over years to weeks may indicate reawakening of such volcanoes, whereas immediate short-term eruption precursors may be subtle and difficult to detect.

The 1989 Loma Prieta earthquake imaged from inversion of geodetic data

We invert geodetic measurements of coseismic deformation from the 1989 MS7.1 Loma Prieta earthquake to determine the geometry of the fault and the distribution of slip on the fault plane. The data include electronic distance measurements, Global Positioning System and very long baseline interferometry vectors, and elevation changes derived from spirit leveling. The fault is modeled as a rectangular dislocation surface in a homogeneous, elastic half-space. First, we assume that the slip on the fault is uniform and estimate the position, orientation, and size of the fault plane using a nonlinear, quasi-Newton algorithm. The best fitting dislocation strikes N48°±4°W and dips 76°±9°SW, consistent with the trend of the aftershock zone and moment tensor solutions. Bootstrap resampling of the data is used to graphically illustrate the uncertainty in the location of the rupture plane. The 95% confidence envelope overlaps the aftershock zone, arguing that there is not a significant discrepancy between the geodetic data and the aftershock locations. Second, we estimate the slip distribution using the best fitting uniform slip fault orientation but increase the fault length to 40 km and the downdip width to 18 km. The fault is divided into 162 subfaults, 18 along strike and 9 along dip. Each subfault is allowed to have constant right-lateral and reverse components of slip. We then solve for the slip on each subfault that minimizes a linear combination of the norm of the weighted data residual and the roughness of the slip distribution. The smoothing parameter, which determines the relative weight put on fitting the data versus smoothing the slip distribution, is chosen by cross validation. Simulations indicate that cross-validation estimates of the smoothing parameter are nearly optimal. The preferred slip distribution is very heterogeneous, with maximum strike slip and dip slip of about 5 and 8 m, respectively, located roughly 10 km north of the hypocenter. There is insignificant dip slip in the southeastern most part of the fault, causing the rake to vary from nearly pure right-lateral in the southeast to oblique right-reverse in the northwest. The change in rake is consistent with a uniform stress field if the fault dip increases by about 10° toward the southeast, as indicated by the aftershock locations. There was little slip above 4 km depth, consistent with the observation that there was little, if any, surface rupture.

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.

Fault slip distribution of two June 2000 MW6.5 earthquakes in South Iceland estimated from joint inversion of InSAR and GPS measurements

Abstract We present the first detailed estimates of co-seismic slip distribution on faults in the South Iceland Seismic Zone (SISZ), an area of bookshelf tectonics. We have estimated source parameters for two M W 6.5 earthquakes in the SISZ on June 17 and 21, 2000 through a joint inversion of InSAR and GPS measurements. Our preferred model indicates two simple 15 km long, near vertical faults extending from the surface to approximately 10 km depth. The geometry is in good agreement with the aftershock distribution. The dislocations experienced pure right-lateral strike-slip, reaching maxima of 2.6 m and 2.9 m for the June 17 and 21 events, respectively. We find that the distribution of slip with depth may be correlated to crustal layering, with more than 80% of the total geometric moment release occurring in the uppermost 6 km. According to the distributed slip model the middle and upper crust appears to be more apt to generate large displacements than the lower crust. The geodetic estimates of seismic moments are 4.4×10 18 Nm ( M W 6.4) and 5.0×10 18 Nm ( M W 6.5). The total moment released by the two events equals that generated by several decades of plate motion in the area, but is only a fraction of the moment accumulated in the area since the last major earthquake in 1912.

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.

Icelandic rhythmics: Annual modulation of land elevation and plate spreading by snow load

] We find strong correlation between seasonal variationin CGPS time series and predicted response to annual snowload in Iceland. The load is modeled using Green’sfunctions for an elastic halfspace and a simple sinusoidalload history on Iceland’s four largest ice caps. We deriveE = 40 ± 15 GPa as a minimum value for the effectiveYoung’s modulus in Iceland, increasing with distance fromthe Eastern Volcanic Zone. We calculate the elastic responseover all of Iceland to maximum snow load at the ice capsusing E = 40 GPa. Predicted annual vertical displacementsare largest under the Vatnajo¨kull ice cap with a peak-to-peakseasonal displacement of 37 mm. CGPS stations closest tothe ice cap experience a peak-to-peak seasonal displacementof 16 mm, consistent with our model. East and north ofVatnajo¨kull we find the maximum of annual horizontaldisplacements of 6 mm resulting in apparent modulationof plate spreading rates in this area.

Glacio‐isostatic deformation around the Vatnajökull ice cap, Iceland, induced by recent climate warming: GPS observations and finite element modeling

[1] Glaciers in Iceland began retreating around 1890, and since then the Vatnajokull ice cap has lost over 400 km 3 of ice. The associated unloading of the crust induces a glacio-isostatic respo ...

A fault model for the 1989 Kilauea South Flank Earthquake from leveling and seismic data

The geometry of the fault that ruptured during the M6.1 south flank earthquake on Kilauea volcano in 1989 is determined from leveling data. The elastic dislocation, in a homogeneous elastic half-space, that best fits the data is found using a nonlinear inversion procedure. The best fitting model is a gently dipping thrust fault that lies at 4 km depth. This is significantly shallower than the 9 km hypocentral depth determined from the local seismic network. Two-dimensional finite-element calculations indicate that at least part of this discrepancy can be attributed to the focusing of the surface deformation by the upper few kilometers of compliant, low-density lavas. We conclude that it is important to include realistic elastic structure to estimate source geometry from geodetic data.

Crustal deformation measured by GPS in the South Iceland Seismic Zone due to two large earthquakes in June 2000

Two large earthquakes struck the South Iceland Seismic Zone in June 2000, the first on June 17 (MW =6.5) and the second on June 21 (MW =6.4). These are the largest earthquakes in the area in the past 88 years. A network of GPS stations was remeasured followingthe earthquakes. The whole network was last measured in 1995, and partly in 1999. We correct for the interseismic motion from 1995 to 2000, to obtain the coseismic displacements. The largest co- seismic motion we observe is about 0.55 m in the epicentral area of the June 17 event. We model the surface deforma- tion for the two earthquakes usingrectang ular dislocations in an elastic half space. Best fit uniform slip models indi- cate that the events occurred on two parallel, N-S vertical faults, with right-lateral strike slip motion. This is the same style of faultingbelieved to have occurred in larg e historical earthquake sequences in South Iceland.

Sources of crustal deformation associated with the Krafla, Iceland, eruption of September 1984

A decade-long plate-boundary rifting episode in northern Iceland ended with the September 1984 fissure eruption of Krafla volcano. We apply a nonlinear inversion method to geodetic data collected before and after the eruption to infer the location, geometry, and strengths of deformation sources associated with the eruption. The net outflow of magma from a 3-km-deep magma chamber beneath the Krafla caldera was 30−120× 106 m³. A similar volume of magma, 50−70×106 m³, was emplaced in a 1-meter-wide, ∼9-km-long dike extending from the surface to ∼7 km depth. Furthermore, at least 110×106 m³ of magma erupted. Accordingly, a surplus of magma must have been expelled from a second reservoir, the location of which, although uncertain, is likely to lie at depths greater than ∼5 km beneath Krafla volcano. It would be difficult to detect this deeper source because of the narrow aperture of the geodetic networks.