Freysteinn Sigmundsson

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.

Ice–volcano interaction of the 1996 Gjálp subglacial eruption, Vatnajökull, Iceland

Volcanic eruptions under glaciers can cause dangerous floods and lahars and create hyaloclastite (fragmented glassy rock) mountains. But processes such as the rate of heat transfer between ice and magma, edifice formation, and the response of the surrounding glacier are poorly understood, because of the lack of data. Here we present observations from the fissure eruption at Vatnajökull ice cap, Iceland, in October 1996. In the 13 days of the eruption 3 km3 of ice were melted and the erupted magma fragmented into glass forming a hyaloclastite ridge 6–7 km long and 200–300 m high under 500–750 m of ice. Meltwater of temperatures of 15–20 °C flowed along a narrow channel at the glacier bed into the Grímsvötn subglacial lake for five weeks, before draining in a sudden flood, or jökulhlaup. Subsidence and crevassing of the ice cap occurred over the eruptive fissure and the meltwater path, whereas elsewhere the glacier surface remained intact, suggesting that subglacial eruptions do not trigger widespread basal sliding in warm-based glaciers.

Post‐glacial rebound and asthenosphere viscosity in Iceland

During the Weichselian glaciation Iceland was covered with an ice cap which caused downward flexure of the Earths surface. The post-glacial rebound in Iceland was very rapid, being completed in about 1,000 years. The length of this time interval constrains the maximum value of asthenosphere viscosity in Iceland to be 1 {times} 10{sup 19} Pa s or less. Further clarification of the ice retreat and uplift history may reveal lower viscosity. Current changes in the mass balance of Icelandic glaciers must lead to measurable elevation changes considering this low viscosity. Expected current elevation changes around the Vatnajoekull ice cap are of the order of 1 cm per year, due to mass balance change in this century.

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.

Rift‐transform kinematics in south Iceland: Deformation from Global Positioning System measurements, 1986 to 1992

Crustal deformation in the plate boundary regions in south Iceland is estimated from repeated Global Positioning System (GPS) geodetic measurements in the period 1986–1992. We compare coordinate solutions for the 1986 and 1989 surveys with the results from the most recent survey in 1992. Horizontal position uncertainty is about 2 cm for the 1986 and 1989 coordinates and about 4 mm for the 1992 coordinates. Little internal deformation is observed in the area west of the western volcanic zone (within the North American plate) and at the southern tip of the eastern volcanic zone (within the Eurasian plate). The observed relative velocity of these two areas is 2.1±0.4 cm/yr in direction N117±11°E (1σ uncertainties), compatible with the 1.94 cm/yr widening in direction N 104°E across south Iceland predicted from the NUVEL-1 global plate motion model. Left-lateral shear strain is developing across the intervening transform zone, the E-W trending south Iceland seismic zone (SISZ). Strain is concentrated within a 20- to 30-km-wide zone that correlates with seismic activity in the SISZ. About 85±15% of the relative plate motion is accommodated by this zone in such a way that the area south of the SISZ is moving toward the east with the Eurasian plate and the area north of the SISZ is moving toward the west with the North American plate. Accordingly, north of the SISZ the western rift zone can accommodate a maximum of 15±15% of the relative plate motion. Within the SISZ the shear strain results in an anticlockwise rotation of lineaments oriented north-south, at a rate of 0.5–1 μrad/yr. The shear strain accumulation can be accommodated by “bookshelf faulting” on mapped recent north-south faults in the SISZ. If the deformation is accommodated by an array of N-S faults spaced 1–5 km apart, an average slip rate of about 0.5–5 mm/yr is required on each fault. The rate of geometric moment release due to earthquakes averaged over centuries, m˙0, expected from such an array is about the same as expected from a simple transform fault, 2vAD, where 2v is the relative plate motion, A is the total length of the seismic zone, and D is the thickness of the brittle crust. If 2v = (0.85±0.15) × 1.94 cm/yr, D = 10–15 km, ana A = 75–85 km, then m˙0 = 1.0–2.5 × 107 m3/yr, comparable to the rate of geometric moment release in large historical earthquakes in the SISZ for the last several centuries. We conclude that historical seismicity in the SISZ can be attributed to left-lateral shear accumulation across the seismic zone at a similar rate throughout historical times as during the years 1986–1992.

Crustal deformation near Hengill volcano, Iceland 1993–1998: Coupling between magmatic activity and faulting inferred from elastic modeling of satellite radar interferograms

Tectonic activity in the Hengill volcanic area in southwestern Iceland accelerated in July 1994, when an unusually persistent swarm of moderate-sized earthquakes began. Although the largest events were magnitude 5, the pattern of upward crustal deformation at 2 cm/yr indicates that most of the activity is related to inflation of a magma chamber at depth. To monitor this activity, we analyze synthetic aperture radar (SAR) images acquired by the ERS-I and ERS-2 satellites between July 1993 and September 1998 using interferometry. Interferograms composed of images acquired during the snow-free summer months remain coherent on Holocene lava flows, even after 4 years. Some of the interferograms show a discontinuity in the fringe pattern, which we interpret as 8 mm of (aseismic) dip slip on a 3-km-long segment of a N5° striking normal fault, part of which had been mapped previously This slip must have occurred between July 31 and September 3, 1995 (inclusive), and has been confirmed by observations in the field. The predominant signature in all the interferograms spanning at least 1 year, however, is a concentric fringe pattern centered just south of the Hromundartindur volcanic center. This we interpret as mostly vertical uplift caused by increasing pressure in an underlying magma source. The volume source that best fits the observed interferograms lies at 7±1 km depth and remains in the same horizontal position to within 2 km. It produces 19±2 mm/yr of uplift. This deformation accumulates as elastic strain energy at a rate 2.8 times the rate of seismic moment release. Accumulated over 5 years, it increases the Coulomb failure stress by >0.6 bar in an area that includes some 84% of the earthquakes recorded between 1993 and 1998. Under our interpretation, magma is injected at 7 km depth, just below the seismogenic zone formed by colder, brittle rock. There the inflation induces stresses that exceed the Coulomb failure criterion, triggering earthquakes, possibly in a cyclical fashion.

Opening of an eruptive fissure and seaward displacement at Piton De La Fournaise Volcano measured by RADARSAT satellite radar interferometry

Interferometric analysis of Synthetic Aperture Radar images acquired by the RADARSAT-1 satellite of Piton de la Fournaise volcano, show that an eruptive fissure which opened on 9th of March 1998 caused asymmetric deformation, with displacement and bulging of the volcanic edifice on the seaward side of the fissure. Up to 50 cm ground-to-satellite range change occurred in a 3-km wide area, in response to a shallow (< 1-km-deep) inclined dike that opened by up to 60 cm. The erupted magma was transported from more than 7-km-depth below sea level, causing no observable volcano-wide co-eruptive deflation. A series of pre-eruption interferograms show also that no significant inflation occurred prior to the eruption. The dike injection slightly reduced the stability of the volcano seaward flank.

The 1994–1995 seismicity and deformation at the Hengill triple junction, Iceland: Triggering of earthquakes by minor magma injection in a zone of horizontal shear stress

Since July 1994 an unusually persistent swarm of earthquakes (M<4.0) has been in progress at the Hengill triple junction, SW Iceland. Activity is clustered around the center of the Hromundartindur volcanic system. Geodetic measurements indicate a few centimeters uplift and expansion of the area, consistent with a pressure source at 6.5±3 km depth beneath the center of the volcanic system. The system is within the stress field of the south Iceland transform zone, and the majority of the recorded earthquakes represent strike-slip faulting on subvertical planes. We show that the secondary effects of a pressure source, modeled as a point source in an elastic half-space, include horizontal shear that perturbs the regional stress. Near the surface, shear stress is enhanced in quadrants around the direction of maximum regional horizontal stress and diminished in quadrants around the direction of minimum regional stress. The recorded earthquakes show spatial correlation with areas of enhanced shear. The maximum amount of shear near the surface caused by the expanding pressure source exceeds 1 μstrain, sufficient to trigger earthquakes if the crust in the area was previously close to failure.

Readjustment of the Krafla Spreading Segment to crustal rifting measured by satellite radar interferometry

Readjustment of the Krafla spreading segment on the Mid-Atlantic Ridge in Iceland, after a rifting episode from 1975 to 1984, is detected by radar interferometry. Crustal deformation from 1992 to 1995 is dominated by ∼24 mm/year subsidence above a shallow magma chamber at Krafla, superimposed on ∼7 mm/year along-axis subsidence of the spreading segment relative to its flanks. The deformation is caused by cooling contraction at ∼3 km depth and ductile flow of material away from the spreading axis, at a rate decreasing with time.

Glacio‐isostatic crustal movements caused by historical volume change of the Vatnajökull Ice Cap, Iceland

Measurements of the lake level of Lake Langisjor at the SW edge of the Vatnajokull ice cap indicate a tilt of 0.26 ± 0.06 μrad/year away from the ice cap in the years 1959–1991. The tilt is too large to be explained as an elastic Earth response to ice retreat this century, or to be caused by change in the gravitational pull of the ice cap, but it can be explained by sub-lithospheric viscous adjustment. Regional subsidence in historical times in SE Iceland can similarly be attributed to viscous adjustment resulting from the increased load of Vatnajokull during the Little Ice Age. The inferred sub-lithospheric viscosity is 1 × 1018 − 5 × 1019 Pa s.