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

Abstract

The Reykjanes Peninsula in southwest Iceland offers excellent opportunities tostudy the dynamics of an obliquely divergent plate boundary zone. Both leftlateralshear and extension are accommodated on the peninsula, resulting in aplate boundary zone characterised by high earthquake activity as well as recentvolcanism. This thesis investigates crustal deformation and earthquakes alongthe plate boundary on the Reykjanes Peninsula, using a variety of geophysicalmethods.In the first paper, we use GPS velocities from 2000–2006 to derive a kinematicelastic half-space model of the plate boundary deformation on the ReykjanesPeninsula. The model predicts left-lateral motion of 18+4−3 mm/yr and openingof 7+3−2 mm/yr below a locking depth of 7+1−2 km (95% confidence levels). Theresulting deep motion, of 20+4−3 mm/yr in the direction of N(100+8−6)"E, agreeswell with the predicted relative North America - Eurasia rate, showing that theobserved surface deformation is consistent with the plate motion models. TheGPS strain rate fields, however, reveal temporal and spatial variations within theplate boundary zone due to shallow sources related to earthquakes or geothermalactivity.The second paper presents the first comprehensive analysis of the seismicityon the Reykjanes Peninsula, since early instrumental earthquake recordings in1926. The seismicity on the peninsula shows a systematic change from primarilyearthquake swarms in the west to mainshock-aftershock sequences in the east,reflecting the transition from seafloor spreading along the Reykjanes Ridge totransform motion in the South Iceland Seismic Zone. The state of stress during1997–2006, 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 frominversion of earthquake data and the directions of greatest extensional strain ratederived from GPS data, indicating that the earthquakes are primarily driven byplate motion.Finally, the third paper presents a geodetic study of the crustal deformationon 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, anthropogenicsubsidence due to geothermal fluid extraction in the Reykjanes, Svartsengi andHellisheidi fields and, possibly, increasing pressure in the Krísuvík geothermal system.The installation of the Reykjanes geothermal power plant in 2006 results insubsidence of around 10 cm during the first two years of production. Short-livedswarms of micro-earthquakes as well as aseismic fault movement appear to betriggered by the stresses due to geothermal fluid extraction.