Temporal Variations in Ground Deformation Caused by Geothermal Processes in the Hengill Area, SW Iceland, During 2009-2019

Fluid extractions and injections at shallow depth in geothermal areas cause deformation of the surrounding crust, which may be observable at the surface. The Hengill volcanic system, SW Iceland, is located at a tectonic triple junction and hosts several high-temperature geothermal systems. The high enthalpy geothermal fluids are harnessed in two geothermal plants, Nesjavellir and Hellisheiði, to provide hot water and electricity since 1990 and 2006, respectively. Both geothermal production fields are associated with local subsidence, up to ~25 mm/yr for Hellisheiði and up to ~20 mm/yr for Nesjavellir. In September 2011, injection of ~500 kg/s of wastewater started in the Húsmúli area, adjacent to the main production area of the Hellisheiði power plant, with the goal of improving the sustainability of the geothermal utilization and reducing surface waste water. During the first few months of injections, swarms of earthquakes were recorded near the injection area and up to 20 mm uplift was detected by InSAR and GPS during 2011 - 2012 measurements (Juncu et al., 2018). Seismicity has continued at lower levels up to present day. In late 2016, the Húsmúli area started to subside, with a total maximum displacement of -30 +/-10 mm within 1 km of the injection sites in the following 2 years despite the continuation of fluid injection. Changes in rates of extraction and injection of fluids, as well as changes in the location of production and injection wells affects the spatial and temporal deformation in the Hellisheiði and Nesjavellir geothermal fields since the respective start of their production. New wells have been drilled since 2016 in Hverahlíð, a few km south-east of the main Hellisheiði production field. This expansion of the production area, and plate spreading processes, further complicate the observed deformation pattern of the south-western area of Hengill. We present here the temporal and spatial variations in ground deformation near the extraction and production areas of the Nesjavellir and Hellisheiði geothermal power plants during 2009 to 2019. Geodetic data reveal alternating local ground uplift and subsidence motions between 2011 and 2019 in the Húsmúli injection field, as well as a gradual subsidence of the surface in the new Hverahlíð production field. InSAR and GNSS data combined with borehole measurements provide clues to the properties of geothermal systems (e.g. thermal contraction and pore-pressure changes) and the hydrothermal processes that may affect the geothermal systems in Hengill. This study shows the importance of surface deformation studies to observe and understand the long-term dynamic behavior of geothermal systems. significant ground motions associated the extraction and injection their capacities and main the subsidence to ~ 2.5 cm/yr Hellisheiði ~ 2 cm/yr pore-pressure changes in the geothermal reservoirs. the sustainability the geothermal exploitation Hellisheiði power plant, geothermal waste back into the beginning, injection took place in the area, SW of the power plant, 2011 injection has taken place in the Húsmúli area, NW of the Hellisheiði geothermal plant. A short-lived uplift (reaching ~2 cm/yr) and thousands of the start of injection of geothermal fluids into Húsmúli 2011 al. 2020). The capacity of the Hellisheiði power plant further 2016, with the start of production in the Hverahlíð locality. Our study show, using long-term geodetic data sets, the average long term deformation and potential shorter term motions of the area, demonstrating dynamic sources of anthropogenic 2009-2019.