Páll Einarsson

Earthquakes and present-day tectonism in Iceland

Abstract The mid-Atlantic plate boundary in Iceland is expressed by a series of seismic and volcanic zones. The structure of the plate boundary is strongly influenced by the Iceland hot spot. The relative motion of the Mid-Atlantic Ridge with respect to the hot spot leads to ridge jumps, propagating rifts and other complexities. Most large earthquakes in Iceland occur within two transform zones that connect the presently active Northern and Eastern Volcanic Zones to the ridges offshore. In the south the South Iceland Seismic Zone is marked by a 10–15 km wide, E-trending epicentral belt. The large earthquakes occur by faulting on N-S striking right-lateral faults. The left-lateral transform motion along the zone thus appears to be taken up by slip on numerous parallel faults by counterclockwise rotation of the blocks between them (bookshelf tectonics). It is argued that the South Iceland Seismic Zone is a transient feature, migrating sideways in response to propagation of the Eastern Volcanic Zone. In Northern Iceland the transform motion is taken up along the Tjornes Fracture Zone. At least three parallel, NW-trending seismic belts have been identified within the zone. The seismicity of the volcanic zones is characterized by spatial clustering of epicenters. Most clusters coincide with central volcanoes. Rifting structures such as fissure swarms and normal faults are mostly aseismic except during episodes of rifting and magmatism such as the present events in Krafla. Earthquake recording has been used very successfully at Krrafla to monitor the level of inflation and deflation of the volcano, and to trace the path and speed of lateral magmatic intrusions into the associated fissure swarm. Seismic activity at the Bardarbunga volcano in Central Iceland correlates in time with the Krafla events, and it seems as if inflation of Krafla is followed by deflation of Bardarbunga. It is postulated that the pressure drop in the partially molten mantle beneath Krafla is transmitted to neighboring volcanoes, leading to magma withdrawal from their shallow reservoirs. Bursts of seismicity of the Katla volcano in South Iceland in 1967 and 1977 may similarly be the result of magma withdrawal in response to the 1964–1967 Surtsey and 1973 Heimaey eruptions. Annual periodicity seen in the Katla seismicity is explained as the result of the triggering effect of pore pressure in the crust beneath the glacier covering Katla. Several volcanoes exhibit persistent, low-magnitude seismicity. In the Hengill volcano in Southwest Iceland, many events involve a non-double-couple mechanism. The seismicity is interpreted as the result of extensional failure and heat extraction from a cooling magma chamber. Two classes of intraplate earthquakes have been identified in Iceland. One includes events in the lithospheric blockbetween the transform zones. These events are related to crustal extension above the hot spot. The other class includes events on the insular shelf off the east and southeast coasts which are possibly caused by a differential cooling rate in the crust across the shelf edge.

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.

S-wave shadows in the Krafla Caldera in NE-Iceland, evidence for a magma chamber in the crust

During the present tectonic activity in the volcanic rift zone in NE-Iceland it has become apparent that the attenuation of seismic waves is highly variable in the central region of the Krafla volcano. Earthquakes associated with the inflation of the volcano have been used to delineate two regions of high attenuation of S-waves within the caldera. These areas are located near the center of inflation have horizontal dimensions of 1–2 km and are interpreted as the expression of a magma chamber. The top of the chamber is constrained by hypocentral locations and ray paths to be at about 3 km depth. Small pockets of magma may exist at shallower levels. The bottom of the chamber is not well constrained, but appears to be above 7 km depth. Generally S-waves propagate without any anomalous aftenuation through laver 3 (vp=0.5 km sec−1) across the volcanic rift zone in NE-Iceland. The rift zone therefore does not appear to be underlain by an estensive magma chamber at crustal levels. The Krafla magma chamber is a localized feature of the Krafla central volcano.

Färoe‐Iceland Ridge Experiment 2. Crustal structure of the Krafla central volcano

The seismic velocity structure of the Krafla central volcano is characterized by large variations in compressional velocity. A 40 km wide high-velocity dome extends from the lower crust (11–14 km depth) beneath the volcano narrowing upward. A magma chamber sits at its top near 3 km depth. It is defined by both 0.2–0.3 s compressional wave delays and shear wave shadowing to be 2–3 km N-S, 8–10 km E-W, and 0.7–1.8 km thick. The near-surface structure (uppermost 2.5 km) of the Krafla caldera is approximately flat-lying, with only minor lateral heterogeneities. The crust beneath the magma chamber has low shear wave attenuation and anomalously high compressional and shear wave velocities. Shear waves, reflected from a 19 km deep Moho, are clearly visible for some paths through the crustal volume below the magma chamber, even though the more shallow diving S waves are severely attenuated. The midcrust beneath the shallow magma chamber cannot contain partial melt or even be at near-solidus temperatures. The Krafla central volcano plays a major role in crustal genesis along the plate boundary. The high-velocity dome, in our view, represents crust generated in and around the magma chamber, which has subsequently been advected to greater depths.

The 1991 eruption of Hekla, Iceland

The eruption that started in the Hekla volcano in South Iceland on 17 January 1991, and came to an end on 11 March, produced mainly andesitic lava. This lava covers 23 km2 and has an estimated volume of 0.15 km3. This is the third eruption in only 20 years, whereas the average repose period since 1104 is 55 years. Earthquakes, as well as a strain pulse recorded by borehole strainmeters, occurred less than half an hour before the start of the eruption. The initial plinian phase was very short-lived, producing a total of only 0.02 km3 of tephra. The eruption cloud attained 11.5 km in height in only 10 min, but it became detached from the volcano a few hours later. Several fissures were active during the first day of the eruption, including a part of the summit fissure. By the second day, however, the activity was already essentially limited to that segment of the principal fissure where the main crater subsequently formed. The average effusion rate during the first two days of the eruption was about 800 m3 s−1. After this peak, the effusion rate declined rapidly to 10–20 m3 s−1, then more slowly to 1 m3 s−1, and remained at 1–12 m3 s−1 until the end of the eruption. Site observations near the main crater suggest that the intensity of the volcanic tremor varied directly with the force of the eruption. A notable rise in the fluorine concentration of riverwater in the vicinity of the eruptive fissures occurred on the 5th day of the eruption, but it levelled off on the 6th day and then remained essentially constant. The volume and initial silica content of the lava and tephra, the explosivity and effusion rate during the earliest stage of the eruption, as well as the magnitude attained by the associated earthquakes, support earlier suggestions that these parameters are positively related to the length of the preceeding repose period. The chemical difference between the eruptive material of Hekla itself and the lavas erupted in its vicinity can be explained in terms of a density-stratified magma reservoir located at the bottom of the crust. We propose that the shape of this reservoir, its location at the west margin of a propagating rift, and its association with a crustal weakness, all contribute to the high eruption frequency of Hekla.

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 structure of the northern Reykjanes Ridge and Reykjanes Peninsula, southwest Iceland

Results from the Reykjanes-Iceland Seismic Experiment (RISE) show that the thickness of zero-age crust decreases from 21 km in southwest Iceland to 11 km at 62°40′N on the Reykjanes Ridge. This implies a decrease in mantle potential temperature of ∼130°C, with increasing distance from the center of the Iceland mantle plume, along this 250 km transect of the plate boundary. The crust thins off-axis at 63°N, from 12.7 km thick at 0 Ma to 9.8 km at 5 Ma, most likely due to a ∼40°C change in asthenospheric mantle temperature between these times. This provides evidence for the passage of a pulse of hotter asthenospheric mantle material beneath the present spreading center. A reflective body, the top of which lies at 9–11 km depth, is identified in the lower crust just west of the tip of the Reykjanes Peninsula. Synthetic seismogram modeling of the wide-angle reflections from this body suggests that it corresponds to a zone of high-velocity (≥7.5 km s−1), high-magnesium rocks in the lower crust. The P to S wave velocity ratio beneath the peninsula is 1.78, implying that crustal temperatures are below the solidus. Gravity modeling shows the RISE models to be consistent with the observed gravity field. Mantle densities are lower beneath the ridge axis than beneath older crust, consistent with lithospheric cooling with age.

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.

The Hekla eruption 1980–1981

The sixteenth eruption of Hekla since 1104 began on August 17th, 1980, after the shortest repose period on record, only ten years. The eruption started with a plinian phase and simultaneously lava issued at high rate from a fissure that runs along the Hekla volcanic ridge. The production rate declined rapidly after the first day and the eruption stopped on August 20th. A total of 120 million m3 of lava and about 60 million m3 of airborne tephra were produced during this phase of the activity. In the following seven months steam emissions were observed on the volcano. Activity was renewed on April 9th 1981, and during the following week additional 30 million m3 of lava flowed from a summit crater and crater rows on the north slope.The lavas and tephra are of uniform intermediate chemical composition similar to that of earlier Hekla lavas. Although the repose time was short the eruptions fit well into the behaviour pattern of earlier eruptions. Distance changes in a geodimeter network established after the eruptions are interpreted as due to inflation of magma reservoirs at 7–8 kilometers depth.

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.