Earthquakes, stress, and strain along an obliquely divergent plate boundary: Reykjanes Peninsula, southwest Iceland

We investigate the seismicity and the state of stress along the obliquely divergent Reykjanes Peninsula plate boundary and compare the directions of stress from inversion of earthquake focal mechanisms with the directions of strain rate from GPS data. The seismicity on the peninsula since early instrumental recordings in 1926 shows a systematic change from primarily earthquake swarms in the west to main shock-aftershock sequences in the east. The largest earthquakes on the Reykjanes Peninsula typically occur by right-lateral slip on N-S faults and reach magnitude 6 on the eastern part of the peninsula. During 1997-2006 most earthquakes on the Reykjanes Peninsula were located in two areas, Fagradalsfjall and Krísuvík on the central part of the peninsula, as recorded by the South Iceland Lowland (SIL) seismic network. The state of stress estimated by inversion of microearthquake focal mechanisms from the SIL catalogue is mainly oblique strike slip, with a tendency toward a normal stress state. Mapping the directions of the least compressive horizontal stress (S_hmin) shows an average S_hmin direction of N(120±6)°E and a remarkable agreement with the directions of greatest extensional strain rate (∈_Hmax) derived from GPS velocities during 2000-2006. The agreement between the directions of stress at depth and strain rate observed at the surface indicate that the earthquakes are primarily driven by plate motion.