Kinematic models of plate boundary deformation in southwest Iceland derived from GPS observations

[ 1] We use data from GPS campaign and continuous measurements from 1992 to 2004 in SW Iceland to map the surface velocity field from the Reykjanes Peninsula to the Eastern Volcanic Zone. We divide the time series into preseismic ( July 1992 to June 2000) and postseismic ( June 2000 to May 2004) time intervals, and we estimate GPS station velocities for each interval as well as coseismic offsets due to the June 2000 earthquake sequence in the south Iceland seismic zone (SISZ). In addition to the plate spreading, the preseismic velocity field shows the effects of inflation at Hengill and Hekla volcanoes, whereas the postseismic velocities show deformation following the June 2000 earthquakes. We consider several kinematic models to explain the preseismic velocities. Our preferred model includes several dislocations and point sources in an elastic half-space, with left-lateral slip along the plate boundary on the Reykjanes Peninsula and below the SISZ, opening across the Reykjanes Peninsula, and the Western and Eastern rift zones. The optimal model has a locking depth of about 8 km in the central and eastern part of the Reykjanes Peninsula with a deep slip rate of about 17 mm/yr and an opening of about 9 mm/yr. This locking depth is in agreement with the thickness of the seismogenic crust on the Reykjanes Peninsula, which appears to vary between 7 and 9 km. For the SISZ, we obtain a deep slip rate of about 19 mm/yr below 16 km depth, which is considerably deeper than the earthquake hypocenter depths in the area.