Anthropogenic and natural ground deformation in the Hengill geothermal area, Iceland

Abstract

We investigate crustal deformation due to the extraction of water and steam from a high‐enthalpy geothermal reservoir; a common occurrence, yet not well understood. The cause of this deformation can be a change in pressure or in temperature in the reservoir, both of which can be caused by extraction or injection of geothermal fluids. Our study area, the Hengill mountains in SW Iceland, is an active volcanic center and a plate triple junction that hosts two power plants producing geothermal energy. This combination of natural and anthropogenic processes causes a complex displacement field at the surface. We analyze geodetic data—Global Navigation Satellite System and Interferometric Synthetic Aperture Radar—to obtain the surface velocity field, which we then simulate using an inverse modeling approach. We focus on the deformation around the geothermal power plants but need to model the regional tectonic and volcanic deformation as well, because the signals are overlapping. We find that plate motion and a deep contracting body can explain the broad scale signal in the area. Local deformation near the two power plants, Hellisheidi and Nesjavellir, can be explained by extraction of geothermal fluids. We estimate reservoirs extending from 0.6 to 3.0 km depth at Hellisheidi, and 1.0 to 3.0 km depth at Nesjavellir for observed pressure decrease rates of 0.25 MPa/yr and 0.1 MPa/yr, respectively. We find that the main cause for the subsidence in the geothermal area is the observed pressure drawdown.

A Preseismic Strain Anomaly Detected Before M6 Earthquakes in the South Iceland Seismic Zone From GPS Station Velocities

“…After subtracting the plate motion, the station velocities are primarily directed toward Hengill. This is consistent with a recent study by Juncu et al (), where the subsidence in the Hengill area observed by GPS and InSAR during 2012–2015 is modeled by pressure decrease in two shallow geothermal reservoirs due to exploitation and a deeper (∼7 km) contracting source. The rates of extraction are much lower at Hellisheidi for the time interval 2005–2007 (8 Mton/year) compared to 38 Mton/year during 2012–2015 used in the study of Juncu et al () but similar at Nesjavellir (13 Mton/year in 2005–2007 and 16 Mton/year for 2012–2015).…”

“…Annual campaign style GPS measurements have been conducted in the SISZ since June 2000 (Árnadóttir et al, ; Decriem et al, ; Juncu et al, ; Keiding et al, ). Figure shows the network of campaign benchmarks and continuously operating GPS stations in South Iceland.…”

“…HVER also has a subtle increase in northward motion prior to the 29 May main shocks. The time series from OLKE show an increase in subsidence starting after 2006, contemporaneous with the start of a new power plant at Hellisheidi, and increased production at Nesjavellir (Juncu et al, ). The westernmost station (VOGS) shows a clear increase in eastward velocity following the 2008 main shocks.…”

“…In 2006 a new larger plant started operation at Hellisheidi and the Nesjavellir plant was expanded. The extraction of geothermal fluids causes pressure decrease in the geothermal reservoirs and subsidence (Juncu et al, ). The subsidence observed by GPS and interferometric synthetic aperture radar (InSAR) during 2012–2015 has been modeled by a pressure decrease in three sources: two shallow ellipsoidal sources in the areas of fluid extraction and a deep spherical source at about 7‐km depth located under the eastern part of Hengill, about 2 km SW of the CGPS station OLKE (Juncu et al, ).…”

“…The extraction of geothermal fluids causes pressure decrease in the geothermal reservoirs and subsidence (Juncu et al, ). The subsidence observed by GPS and interferometric synthetic aperture radar (InSAR) during 2012–2015 has been modeled by a pressure decrease in three sources: two shallow ellipsoidal sources in the areas of fluid extraction and a deep spherical source at about 7‐km depth located under the eastern part of Hengill, about 2 km SW of the CGPS station OLKE (Juncu et al, ). Reinjection of geothermal wastewater at the Hellisheidi power plant started in September 2011, causing swarms of induced earthquakes with two M 4 earthquakes in October 2011 (Juncu et al, ).…”