Crustal deformation due to fluid extraction and re-injection in the Hengill geothermal area in Southwest Iceland

Abstract:

Several geothermal energy production fields are currently harnessed in Iceland. One of these is located at the Hengill triple junction, where the oblique plate motion along the Reykjanes peninsula is partitioned between the E-W oriented transform along the South Iceland Seismic Zone (SISZ) and spreading across the Western Volcanic Zone in SW Iceland. The Hengill volcano has high temperature geothermal areas utilized by the Hellisheiði and Nesjavellir power plants. The region around the power plants is subject to motion and deformation of the Earth's surface due to several processes. These include the motion of the Earth's crust due to plate spreading, co- and post seismic deformation due to earthquakes in the South Iceland Seismic Zone and deformation due to water and steam extraction and wastewater re-injection near geothermal power plants. We measure surface displacement using both GPS and InSAR data. The former are obtained from four continuous and more than 15 campaign GPS stations, with time-series starting after two M6 earthquakes on 29 May 2008 in Ölfus - the western most part of the SISZ. The InSAR data consist of 10 images taken on track 41 of the TerraSar-X mission, starting October 2009. We can see a clear subsidence signal in the proximity of the power plants with a maximum of ~18 mm/yr in Line-of-Sight direction (LOS) at Hellisheiði. The subsidence is elongated in NNE-SSW direction and possibly related to the orientation of the Hengill fissure swarm. In addition to subsidence, we observe an uplift signal of ca. 10 mm/yr in LOS west of the Hellisheiði site, potentially due to wastewater re-injection in the area. The area of maximum uplift is located close to the epicenters of two M4 earthquakes that occurred in October 2011. Since the signal around the power plants is the most prominent, we start our investigation with trying to find an appropriate model for deformation due to fluid extraction and re-injection with the aim of simulating the data. By doing so, we hope to learn more about parameters that we don't have a priori knowledge about, e.g. hydraulic conductivity and elastic properties of the crust. Furthermore, we hope to gain a better understanding of what are the key mechanisms that drive crustal deformation in areas of geothermal energy production.