| 1. |
- Islam, Md. Tariqul, 1980, et al.
(författare)
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Continuous subsidence in the Thingvellir rift graben, Iceland: Geodetic observations since 1967 compared to rheological models of plate spreading
- 2016
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Ingår i: Journal of Geophysical Research - Solid Earth. - 0148-0227 .- 2156-2202. ; 121:1, s. 321-338
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Tidskriftsartikel (refereegranskat)abstract
- North America-Eurasia relative plate motion across the Mid-Atlantic Ridge in south Iceland is partitioned between overlapping ridge segments, the Western Volcanic Zone (WVZ) and the Eastern Volcanic Zone. The Thingvellir graben, a 4.7km wide graben, lies along the central axis of the WVZ and has subsided >35m during the Holocene. An ~8km long leveling profile across the graben indicates a subsidence rate of ~1mmyr−1 from 1990 to 2007, relative to the first (westernmost) benchmark. Modeled GPS velocities from 1994 to 2003 estimate a spreading rate of 6.7±0.5mmyr−1 or 35% of the full plate motion rate and up to 6.0mmyr−1 subsidence. The combined geodetic observations show that the deformation zone is 10 times wider than the graben width. We utilize these geodetic observations to test the effects of ridge thermal structure on the kinematics across divergent boundaries. We apply a nonlinear rheology, thermomechanical model implemented in a finite element model. A 700°C isotherm is applied for the brittle to ductile transition in the crust, representing a dry olivine rheology. We adjust the depth of this isotherm to solve for the best fit model. The best fit model indicates that the 700°C isotherm is at 8km depth below the ridge axis, which results in an average thermal gradient of 87.5°Ckm−1 in the upper crust. The thermomechanical model predicts a subsidence rate of 4mmyr−1, comparable to our geodetic observations.
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| 2. |
- Sturkell, Erik, 1962, et al.
(författare)
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Katla and Eyjafjallajökull Volcanoes
- 2010
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Ingår i: Developments in Quaternary Sciences, Vol. 13: The Mýrdalsjökull Ice Cap, Iceland. Glacial processes, sediments and landforms on an active volcano (Edited by Anders Schomacker, Johannes Krüger and Kurt H. Kjær). - : Elsevier Ltd. - 9780444530455 ; , s. 5-21
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The Katlavolcano is covered by the Mýrdalsjökull ice cap and is currently one of the most active volcanoes in Iceland. It has erupted twenty times the past 1,100 years. The neighbouring volcanoEyjafjallajökull has erupted twice, simultaneously with Katla. As glaciers cover both volcanoes, their eruptions are phreato-magmatic by nature. The volcanoes are located directly south of where surface expressions of the rift cease. Seismically, Katla is one of the most active volcanoes in Iceland, showing an annual cycle in activity, observed from at least 1960 and less pronounced since 2004. From 1999 to late 2004, GPS measurements revealed steady inflation of the volcano, showing uplift and outward horizontal displacement. Until 1990s, Eyjafjallajökull had been seismically quiet for several decades. Seismic activity there was high in 1994 and again in 1999, related to the emplacement of two intrusions.
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| 3. |
- Árnadóttir, Thóra, et al.
(författare)
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Glacial rebound and plate spreading : results from the first countrywide GPS observations in Iceland
- 2009
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 177:2, s. 691-716
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Forskningsöversikt (refereegranskat)abstract
- Iceland is one of the few places on Earth where a divergent plate boundary can be observed on land. Direct observations of crustal deformation for the whole country are available for the first time from nationwide Global Positioning System (GPS) campaigns in 1993 and 2004. The plate spreading across the island is imaged by the horizontal velocity field and high uplift rates (>= 10 mm yr(-1)) are observed over a large part of central and southeastern Iceland. Several earthquakes, volcanic intrusions and eruptions occurred during the time spanned by the measurements, causing local disturbances of the deformation field. After correcting for the largest earthquakes during the observation period, we calculate the strain rate field and find that the main feature of the field is the extension across the rift zones, subparallel to the direction of plate motion. Kinematic models of the horizontal plate spreading signal indicate a slightly elevated rate of spreading in the Northern Volcanic Zone (NVZ) (23 +/- 2 mm yr(-1)), while the rates at the other plate boundary segments agree fairly well with the predicted rate of plate spreading (similar to 20 mm yr(-1)) across Iceland. The horizontal ISNET velocities across north Iceland therefore indicate that the excessive spreading rate (>30 mm yr(-1)) observed by GPS in 1987-1992 following the 1975-1984 Krafla rifting episode was significantly slower during 1993-2004. We model the vertical velocities using glacial isostatic adjustment (GIA) due to the recent thinning of the largest glaciers in Iceland. A layered earth model with a 10-km thick elastic layer, underlain by a 30-km thick viscoelastic layer with viscosity 1 x 10(20) Pa s, over a half-space with viscosity similar to 1 x 10(19) Pa s can explain the broad area of uplift in central and southeastern Iceland. A wide area of significant residual uplift ( up to 8 mm yr(-1)) is evident in north Iceland after we subtract the rebound signal from the observed rates, whereas the Reykjanes Peninsula and the Western Volcanic Zone (WVZ) appear to be subsiding at a rate of 4-8 mm yr(-1). We observe a coherent pattern of small but significant residual horizontal motion (up to 3 mm yr(-1)) away from Vatnajokull and the smaller glaciers that is most likely caused by glacial rebound. Our study demonstrates that the velocity field over a large part of Iceland is affected by deglaciation and that this effect needs to be considered when interpreting deformation data to monitor subglacial volcanoes in Iceland.
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| 4. |
- Geirsson, Halldor, et al.
(författare)
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Volcano deformation at active plate boundaries : Deep magma accumulation at Hekla volcano and plate boundary deformation in south Iceland
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117:B11409
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Tidskriftsartikel (refereegranskat)abstract
- Most magmatic systems on Earth are located at actively deforming plate boundaries. In these systems, the magmatic and plate boundary deformation signals are intertwined and must be deconvolved to properly estimate magma flux and source characteristics of the magma plumbing system. We investigate the inter-rifting and inter-seismic deformation signals at the Eastern Volcanic Zone (EVZ) - South Iceland Seismic Zone (SISZ) ridge - transform intersection and estimate the location, depth, and volume rate for magmatic sources at Hekla and Torfajokull volcanoes, which are located at the intersection. We solve simultaneously for the source parameters of the tectonic and volcanic deformation signals using a new ten-year velocity field derived from a dense network of episodic and continuous GPS stations in south Iceland. We find the intersection of the axes of the EVZ and the SISZ is located within the Torfajokull caldera, which itself is subsiding. Deformation at Hekla is statistically best described in terms of a horizontal ellipsoidal magma chamber at 24(2)(+4) km depth aligned with the volcanic system and increasing in volume by 0.017(-0.002)(+0.007) km(3) per year. A spherical magma chamber centered at 24(-2)(+5) km depth with a volume rate of 0.019(-0.002)(+0.011) km(3) per year, or a vertical pipe-shaped magma chamber between 10(-1)(+3) km and 21(-4)(+7) km with a volume rate of 0.008(-0.001)(+0.003) km(3) per year are also plausible models explaining the deformation at Hekla. All three models indicate magma accumulation in the lower crust or near the Moho under Hekla.
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| 5. |
- Greiner, Sonja H. M., et al.
(författare)
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Interaction between propagating basaltic dikes and pre-existing fractures : A case study in hyaloclastite from Dyrfjoll, Iceland
- 2023
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier. - 0377-0273 .- 1872-6097. ; 442
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Tidskriftsartikel (refereegranskat)abstract
- Magma in the Earth's crust is commonly transported through dikes. Fractures and faults, which are common in the shallow crust, form structural weaknesses that can act as energy-efficient propagation pathways. Although examples of this are known from active and extinct volcanoes in varying host rocks, the conditions and mechanisms of how and when dikes are influenced by these structures are not yet fully understood. This study investigates how basaltic dikes propagating through hyaloclastite in the shallow crust interact with pre-existing fractures. Using virtual 3D-models from drone-based photogrammetry, we mapped basaltic dikes exposed in a caldera-filling hyaloclastite in the extinct Dyrfjoll volcano, NE-Iceland, to measure the orientations of fractures and dikes, and quantify their interactions. We observe 39 changes in strike among 45 dikes and found a strong control of the governing stress field on orientations and interactions. Three types of dike-fracture interaction were identified: (1) Dikes propagating along pre-existing fractures. This is most frequently observed for dikes following the tectonic stress field. (2) Dikes with an abrupt change in strike occurring near or at a crosscutting fracture, but without magma flow into the fracture. (3) Dikes arrested at a crosscutting fracture. Such dikes may develop offshoots near the dike tip, which may approach the fracture at different angles and be able to cut across. Understanding how dikes interact with pre-existing fractures in moderately fractured host rock such as hyalo-clastite is relevant for hazard assessment and monitoring of volcanically active areas.
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| 6. |
- Hobe, Alex, et al.
(författare)
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Seismicity Patterns Due to Magma Intrusions Underneath Geothermal Power Plants
- 2021
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Ingår i: World Geothermal Congress 2020+1.
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Konferensbidrag (refereegranskat)abstract
- The Reykjanes Peninsula, Iceland, could soon see a once-in-a-millennium eruption, based on its current unrest and its historical record. This period of volcano-tectonic events threatens nearby airfields, geothermal power plants, and the capital, Reykjavik, where two thirds of Iceland’s population reside. The main sources of risk are lava, volcanic ash, and M > 6 earthquakes on large strike-slip faults. Six such known faults lie between 15-35 km from the capital. Here, we investigate whether such large earthquakes and/or an eruption are likely, based on the seismicity seen so far. To do so, we look for specific seismicity patterns indicative of magmatically-induced deformation, and compare seismicity overviews of the current unrest, previous decades, and swarm activity in the 1970s. We identify several cascades of boundary movements in both the current unrest and the 1970s activity, that have neighboring segments activate from east to west along the peninsula. This direction is reversed during the current unrest in a slower cascade, which ended with a ~30 km long boundary segment moving. Based on this and other observations, we suggest that magma intrusion has accelerated boundary deformation significantly. We identify another pattern, where seismicity and surface deformation in Svartsengi, the most seismically active region during this period, is greatly reduced when this activity swaps to the neighboring Reykjanes system. We suggest magma intrusion is halted in one volcanic system, in favor of intrusion in another, and describe several possible mechanisms.We further describe possible scenarios, and their likelihood, for the evolution of the current unrest, which range from a rapid return to quiescence, to full-scale eruption. Whichever scenario occurs, M > 6 earthquakes on known faults near Reykjavik are likely.
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| 7. |
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| 8. |
- Hooper, Andrew, et al.
(författare)
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Increased capture of magma in the crust promoted by ice cap retreat in Iceland
- 2011
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Ingår i: Nature Geoscience. - 1752-0894 .- 1752-0908. ; 4:11, s. 783-786
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Tidskriftsartikel (refereegranskat)abstract
- Climate warming at the end of the last glaciation caused ice caps on Icelandic volcanoes to retreat. Removal of surface ice load is thought to have decreased pressures in the underlying mantle, triggering decompression melting, enhanced magma generation and increased volcanic activity(1-3). Present-day climate change could have the same effect, although there may be a time lag of hundreds of years between magma generation and eruption(4,5). However, in addition to increased magma generation, pressure changes associated with ice retreat should also alter the capacity for storing magma within the crust. Here we use a numerical model to evaluate the effect of the current decrease in ice load on magma storage in the crust at the Kverkfjoll volcanic system, located partially beneath Iceland's largest ice cap. We compare the model results with radar and global positioning system measurements of surface displacement and changes in crustal stress between 2007 and 2008, during the intrusion of a deep dyke at Upptyppingar. We find that although the main component of stress recorded during dyke intrusion relates to plate extension, another component of stress is consistent with the stress field caused by the retreating ice cap. We conclude that the retreating ice cap led to enhanced capture of magma within the crust. We suggest that ice-cap retreat can promote magma storage, rather than eruption, at least in the short term.
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| 9. |
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| 10. |
- Juncu, Daniel, et al.
(författare)
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Injection-induced surface deformation and seismicity at the Hellisheidi geothermal field, Iceland
- 2020
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 391
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Tidskriftsartikel (refereegranskat)abstract
- Induced seismicity is often associated with fluid injection but only rarely linked to surface deformation. At the Hellisheidi geothermal power plant in south-west Iceland we observe up to 2 cm of surface displacements during 2011–2012, indicating expansion of the crust. The displacements occurred at the same time as a strong increase in seismicity was detected and coincide with the initial phase of geothermal wastewater reinjection at Hellisheidi. Reinjection started on September 1, 2011 with a flow rate of around 500 kg/s. Micro-seismicity increased immediately in the area north of the injection sites, with the largest seismic events in the sequence being two M4 earthquakes on October 15, 2011. Semi-continuous GPS sites installed on October 15 and 17, and on November 2, 2011 reveal a transient signal which indicates that most of the deformation occurred in the first months after the start of the injection. The surface deformation is evident in ascending TerraSAR-X data covering June 2011 to May 2012 as well. We use an inverse modeling approach and simulate both the InSAR and GPS data to find the most plausible cause of the deformation signal, investigating how surface deformation, seismicity and fluid injection may be connected to each other. We argue that fluid injection caused an increase in pore pressure which resulted in increased seismicity and fault slip. Both pore pressure increase and fault slip contribute to the surface deformation.
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