Stress and strain of a plate boundary - the Reykjanes Peninsula, SW Iceland

The Reykjanes Peninsula in southwest Iceland offers excellent opportunities to study the dynamics of an obliquely divergent plate boundary zone. Both leftlateral shear and extension are accommodated on the peninsula, resulting in a plate boundary zone characterised by high earthquake activity as well as recent volcanism. This thesis investigates crustal deformation and earthquakes along the plate boundary on the Reykjanes Peninsula, using a variety of geophysical methods. In the first paper, we use GPS velocities from 2000–2006 to derive a kinematic elastic half-space model of the plate boundary deformation on the Reykjanes Peninsula. The model predicts left-lateral motion of 18+4 −3 mm/yr and opening of 7+3 −2 mm/yr below a locking depth of 7+1 −2 km (95% confidence levels). The resulting deep motion, of 20+4 −3 mm/yr in the direction of N(100+8 −6)"E, agrees well with the predicted relative North America - Eurasia rate, showing that the observed surface deformation is consistent with the plate motion models. The GPS strain rate fields, however, reveal temporal and spatial variations within the plate boundary zone due to shallow sources related to earthquakes or geothermal activity. The second paper presents the first comprehensive analysis of the seismicity on the Reykjanes Peninsula, since early instrumental earthquake recordings in 1926. The seismicity on the peninsula shows a systematic change from primarily earthquake swarms in the west to mainshock-aftershock sequences in the east, reflecting the transition from seafloor spreading along the Reykjanes Ridge to transform motion in the South Iceland Seismic Zone. The state of stress during 1997–2006, as estimated from inversion of micro-earthquake focal mechanisms, is mainly strike-slip with a tendency toward a normal stress state. We find an excellent agreement between the directions of least compressive stress from inversion of earthquake data and the directions of greatest extensional strain rate derived from GPS data, indicating that the earthquakes are primarily driven by plate motion. Finally, the third paper presents a geodetic study of the crustal deformation on the Reykjanes Peninsula, using Interferometric Synthetic Aperture Radar (In- SAR) data from 1992–1999 and 2003–2008, as well as GPS data from 2000–2009. The geodetic data reveal deformation due to the plate spreading, anthropogenic subsidence due to geothermal fluid extraction in the Reykjanes, Svartsengi and Hellisheidi fields and, possibly, increasing pressure in the Krísuvík geothermal system. The installation of the Reykjanes geothermal power plant in 2006 results in subsidence of around 10 cm during the first two years of production. Short-lived swarms of micro-earthquakes as well as aseismic fault movement appear to be triggered by the stresses due to geothermal fluid extraction.

Nordic Volcanological Centre, Eimskip Fund of the University of Iceland