Crustal deformation around the Húsmúli injection site, SW Iceland

Since fall 2011 frequent swarms of earthquakes have occurred in the Húsmúli area, Hengill geothermal field. The Hengill volcanic system, SW Iceland, sits at the junction of the Reykjanes Peninsula oblique rift zone, the Western Volcanic rift zone, and the South Iceland seismic zone. Two power plants are utilizing the heat of the Hengill high temperature geothermal field, the Nesjavellir power plant (from 1990) and the Hellisheidi power plant (from 2006). Injection of wastewater was initiated in the Húsmúli region, at the outer boundary of the Hellisheidi geothermal reservoir, in the beginning of September 2011. On 8 September 2011 a seismic swarm started in the region. Frequent swarms have occurred there since with magnitudes up to Ml 4.0. The earthquakes appear to liniate N-S trending structures consistent with strike-slip faulting in the adjacent South Iceland seismic zone and the Reykjanes Peninsula. Since the Hellisheidi power plant went into operation in 2006 increased subsidence has been measured in the region, observed at continuous GPS stations in the region and from the analysis of annual campaign measurements. In response to the Húsmúli seismic activity four GPS points close to the injection site were re measured in October 2011 and have been run in semi continuous manner since to monitor the situation and to evaluate the effect of reinjection on the region. Additional semi continuous GPS station was set up in November 2011 and TerraSAR-X images are being collected and analyzed. Preliminary data analysis shows a change in the horizontal deformation field around the injection site since fall 2011. The effect of injection on the change of style of deformation near the injection site could be an important clue when trying to understand the relationship between the seismicity and the injection.