EGU General Assembly 2020
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Non-eruptive Uplift and Subsidence episodes beneath the Hengill Triple Junction, SW Iceland

Cécile Ducrocq1, Halldór Geirsson1, Thóra Árnadóttir1, Daniel Juncu2, Bjarni R. Kristjánsson3, Gunnar Gunnarsson3, and Vincent Drouin4
Cécile Ducrocq et al.
  • 1Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Reykjavík, Iceland (
  • 2COMET, University of Leeds, Leeds, UK
  • 3Reykjavík Energy, Reykjavík, Iceland
  • 4Iceland GeoSurvey, Reykjavík, Iceland

Non-eruptive uplift and subsidence episodes at volcanic systems have been observed on volcanic systems around the world and understanding the complex source processes of the deformation is key to mitigate the hazard assessment or geothermal potential of the area. The Hengill area, an approximately 100 km² area in SW Iceland, located at the triple junction of the Eurasian plate, North-American plate and Hreppar Microplate, is one such example of a complex deforming volcanic system. The triple junction accommodates a total spreading and shear of 1.8 cm/yr through a systems of spreading ridges and “bookshelf-faulting” processes. The two active volcanoes of the area (Hrómundartindur and Hengill), last erupted ~2000 years ago. Beneath these adjacent volcanic systems, deep sources (5-7 km depth) caused successive episodes of broad-scale uplift (1993 – 1999; 2017 – 2018) and subsidence (2006 – 2017; 2018 – ongoing at the time of writing) in the area. These deep sources may be closely related as they have been located only 2-3 km from each other within the brittle-ductile transition zone of the area. More superficial sources (depth < 3 km) of deformation are also observed in the Hengill area, related to the extraction and injection of fluids in the Nesjavellir and Hellisheiði geothermal power plants.

Through the combination of GNSS, InSAR, analytical models and geophysical data sets from the area we investigate the spatial and temporal relation between these deep sources. The observed ground motions associated with these deep sources may be magmatic in nature (e.g. magma accumulation, degassing of older intrusions), however previous seismic tomography work (Tryggvason et al. 2002) in the area does not suggest a large partially melted magmatic body at those depths, hinting that other processes (e.g. hydrothermal) may be at the origin of some of these episodes. The correlation of geodetic measurements with geophysical and geothermal datasets may bring clues to constrain the nature of uplift and subsidence episodes in volcanic and high temperature geothermal areas such as the Hengill area.

How to cite: Ducrocq, C., Geirsson, H., Árnadóttir, T., Juncu, D., Kristjánsson, B. R., Gunnarsson, G., and Drouin, V.: Non-eruptive Uplift and Subsidence episodes beneath the Hengill Triple Junction, SW Iceland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18000,, 2020

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Presentation version 1 – uploaded on 08 May 2020
  • CC1: Questions and answers from the live chat during EGU2020, Michael Heap, 11 May 2020

    Q: Interesting temporal changes of deformation. You pose 3 different possible explanations for the uplift/subsidence periods, which do you think is the most likely?

    A: Hard to say - combination? !!

    Q: Interesting! Considering that the last time a volcano-tectonic event stroke the Hengill volcanic system was in 1789 AD (also, Strokkur geyser was first mentioned that year too) and that the Holuhraun 2014-2015 eruption occurred ~200 years after the last one in the same location in Bárðarbunga volcanic system, may this unrest at Hengill be a sign that the system can be “ripe” for something? ;-)

    A: Good point! Well, it seems like we had something similar in the 1950s and possibly earlier then that, so, sorry, I would not take this as an indicator of a major episopde 

    Q: Is Cécile currently trying to find evidence to help explain the uplift/subsidence episodes?

    A: Yes, understanding why we have these uplift/subsidence episode is really interesting, but not so much we can constrain in terms of data. We have also considered some borehole measurements in the area, but there are little changes there. And there were little changes in seismicity, which, I suppose is expected after a decade of subsidence in the same area

    Q: Have you carried out any gravity measurements to assess whether or not significant mass changes occurred?

    A: No we have not been redoing the gravity - there are some gravity measurements in the area, but they are probably largely affected by changes in groundwater levels between surveys. Also, for this mini-inflation episode, we would not be able to resolve that.

    Q: Is there any observation of opposite trends of ground deformation outside of the vicinity of where these uplift-subsidence patterns are occurring? Is it only constrained to one particular region?

    A: Interesting question. The answer is yes and no: there are some opposite trends, as seen to the west of the main inflation area, but we belive that stems from a different process, related to fluid incjections from the power plant.