ALBERT

Description: Crust at many divergent plate boundaries forms primarily by the injection of vertical sheet-like dykes, some tens of kilometres long. Previous models of rifting events indicate either lateral dyke growth away from a feeding source, with propagation rates decreasing as the dyke lengthens, or magma flowing vertically into dykes from an underlying source, with the role of topography on the evolution of lateral dykes not clear. Here we show how a recent segmented dyke intrusion in the Baretharbunga volcanic system grew laterally for more than 45 kilometres at a variable rate, with topography influencing the direction of propagation. Barriers at the ends of each segment were overcome by the build-up of pressure in the dyke end; then a new segment formed and dyke lengthening temporarily peaked. The dyke evolution, which occurred primarily over 14 days, was revealed by propagating seismicity, ground deformation mapped by Global Positioning System (GPS), interferometric analysis of satellite radar images (InSAR), and graben formation. The strike of the dyke segments varies from an initially radial direction away from the Baretharbunga caldera, towards alignment with that expected from regional stress at the distal end. A model minimizing the combined strain and gravitational potential energy explains the propagation path. Dyke opening and seismicity focused at the most distal segment at any given time, and were simultaneous with magma source deflation and slow collapse at the Baretharbunga caldera, accompanied by a series of magnitude M 〉 5 earthquakes. Dyke growth was slowed down by an effusive fissure eruption near the end of the dyke. Lateral dyke growth with segment barrier breaking by pressure build-up in the dyke distal end explains how focused upwelling of magma under central volcanoes is effectively redistributed over long distances to create new upper crust at divergent plate boundaries.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Sigmundsson, Freysteinn -- Hooper, Andrew -- Hreinsdottir, Sigrun -- Vogfjord, Kristin S -- Ofeigsson, Benedikt G -- Heimisson, Elias Rafn -- Dumont, Stephanie -- Parks, Michelle -- Spaans, Karsten -- Gudmundsson, Gunnar B -- Drouin, Vincent -- Arnadottir, Thora -- Jonsdottir, Kristin -- Gudmundsson, Magnus T -- Hognadottir, Thordis -- Fridriksdottir, Hildur Maria -- Hensch, Martin -- Einarsson, Pall -- Magnusson, Eyjolfur -- Samsonov, Sergey -- Brandsdottir, Bryndis -- White, Robert S -- Agustsdottir, Thorbjorg -- Greenfield, Tim -- Green, Robert G -- Hjartardottir, Asta Rut -- Pedersen, Rikke -- Bennett, Richard A -- Geirsson, Halldor -- La Femina, Peter C -- Bjornsson, Helgi -- Palsson, Finnur -- Sturkell, Erik -- Bean, Christopher J -- Mollhoff, Martin -- Braiden, Aoife K -- Eibl, Eva P S -- England -- Nature. 2015 Jan 8;517(7533):191-5. doi: 10.1038/nature14111. Epub 2014 Dec 15.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, IS-101 Reykjavik, Iceland. ; Centre for the Observation and Modelling of Earthquakes and Tectonics (COMET), School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. ; GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand. ; Icelandic Meteorological Office, IS-150 Reykjavik, Iceland. ; 1] Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, IS-101 Reykjavik, Iceland [2] Icelandic Meteorological Office, IS-150 Reykjavik, Iceland. ; Canada Centre for Mapping and Earth Observation, Natural Resources Canada, 560 Rochester Street, Ottawa, Ontario K1A 0E4, Canada. ; Department of Earth Sciences, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK. ; Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. ; Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802, USA. ; Department of Earth Sciences, University of Gothenburg, SE-405 30 Gothenburg, Sweden. ; Seismology Laboratory, School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/25517098" target="_blank"〉PubMed〈/a〉

Description: Two types of signals are clearly visible in continuous GPS (cGPS) time-series in Iceland, in particular in the vertical component. The first one is a yearly seasonal cycle, usually sinusoid-like with a minimum in the spring and a maximum in the fall. The second one is a trend of uplift, with higher values the closer the cGPS stations are to the centre of Iceland and ice caps. Here, we study the seasonal cycle signal by deriving its average at 71 GPS sites in Iceland. We estimate the annual and semi-annual components of the cycle in their horizontal and vertical components using a least-squares adjustment. The peak-to-peak amplitude of the cycle of the vertical component at the studied sites ranges from 4 mm near the coastline up to 27 mm at the centre of the Vatnajökull, the largest ice cap in Iceland. The minimum of the seasonal cycle occurs earlier in low lying areas than in the central part of Iceland, consistent with snow load having a large influence on seasonal deformation. Modelling shows that the seasonal cycle is well explained by accounting for elastically induced surface displacements due to snow, atmosphere, reservoir lake and ocean variations. Model displacement fields are derived considering surface loads on a multilayered isotropic spherical Earth. Through forward and inverse modelling, we were able to reproduce a priori information on the average seasonal cycle of known loads (atmosphere, snow in non-glaciated areas and lake reservoir) and get an estimation of other loads (glacier mass balance and ocean). The seasonal glacier mass balance cycle in glaciated areas and snow load in non-glaciated areas are the main contributions to the seasonal deformation. For these loads, induced seasonal vertical displacements range from a few millimetres far from the loads in Iceland, to more than 20 mm at their centres. Lake reservoir load also has to be taken into account on local scale as it can generate up to 20 mm of vertical deformation. Atmosphere load and ocean load are observable and generate vertical displacements in the order of a few millimetres. Inversion results also shows that the Iceland crust is less rigid than the world average. Interannual deviation from the GPS seasonal cycle can occur and are caused by unusual weather conditions over extended period of time.

Description: 〈span〉〈div〉SUMMARY〈/div〉The Reykjanes geothermal system is a high-temperature seawater system situated in SW-Iceland. Interferometric analysis of the Sentinel-1 satellite synthetic aperture radar (InSAR) data has been used to determine a time series of ground deformation induced by geothermal utilization between April 2015 and October 2017. Surface displacements have been estimated at coherent pixels, indicating a steady and linear subsidence within a sub-circular bowl centered on the well field at a maximum near-vertical rate of about 25 mm/yr, together with horizontal contraction. The average line-of-sight (LOS) displacement rates from ascending and descending tracks are inverted to determine the characteristics of the deformation source at depth, modeling the geothermal reservoir as a body of simple geometry within an elastic half space. The results indicate a deformation source at about 1 km depth contracting at a rate of (0.7–0.9) × 10〈sup〉5〈/sup〉 m〈sup〉3〈/sup〉/yr during the 2015–2017 period. Using pressure and temperature monitoring data at 900 m depth as well as an analysis of the reservoir structure and rock properties, we infer that the recent estimated volume change can be attributed to processes in the steam cap situated in the topmost part of the geothermal reservoir, in the 800–1200 m depth range. Processes involve a combination of compaction under pressure decrease and/or thermal contraction due to cooling of the rocks within or near the steam cap. The steam cap expanded in response to a sudden pressure drop resulting from the increase in extraction of geothermal fluids for a new power plant in 2006.〈/span〉

Description: The Theistareykir geothermal field is located in North Iceland on the Mid-Atlantic ridge. A power plant produces 90 MWe using two 45 MWe turbines in operation since autumn 2017 and spring 2018, respectively. We performed hybrid microgravity measurements from 2017 to 2019 to monitor the short-term mass redistribution induced by geothermal production. Time-lapse microgravity surveys conducted each summer with a Scintrex CG5 gravimeter reveal the spatial gravity variations with respect to a reference, where the temporal gravity changes are monitored by absolute gravity measurements done with FG5#206 from Micro-g Solutions. In parallel, continuous gravity changes are recorded by a network of several GWR Instruments iGrav superconducting gravimeters and spring gravimeter, located in the injection and production areas. A height correction is applied to the gravity data using InSAR and GNSS measurements. We notice a regular residual gravity decrease in the production area versus a stable behaviour in the injection area. Time-lapse gravity measurements reveal a minimum residual decrease of − 38 ± 10 µGal (1 µGal = 10–8 m s−2) in 2019 with respect to 2017. Simplistic forward modelling of the produced geothermal fluid using a multiple Mogi sphere model can partly explain the residual gravity decrease. This suggest that a significant part of the injected geothermal fluid flows away, maybe drained by the Tjarnarás fault to the South where an increase of the water table level is observed. However, further modelling work is needed to confirm this.

Description: Understanding and constraining the source of geodetic deformation in volcanic areas is an important component of hazard assessment. Here, we analyse deformation and seismicity for one year before the March 2021 Fagradalsfjall eruption in Iceland. We generate a high-resolution catalogue of 39,500 earthquakes using optical cable recordings and develop a poroelastic model to describe three pre-eruptional uplift and subsidence cycles at the Svartsengi geothermal field, 8 km west of the eruption site. We find the observed deformation is best explained by cyclic intrusions into a permeable aquifer by a fluid injected at 4 km depth below the geothermal field, with a total volume of 0.11 ± 0.05 km3 and a density of 850 ± 350 kg m–3. We therefore suggest that ingression of magmatic CO2 can explain the geodetic, gravity and seismic data, although some contribution of magma cannot be excluded.