Crustal deformation at the oblique spreading Reykjanes Peninsula, SW Iceland: GPS measurements from 1993 to 1998

In 1993 and 1998 a 38-point GPS network was surveyed on the Reykjanes Penin sula, SW Iceland. According to the NUVEL-1A plate motion model the spreadin g rate of the North American and Eurasian plates in SW Iceland is 18.9 +/- 0.5 mm/yr toward N102.7 degrees +/- 1.1 degreesE, highly oblique to the pla te boundary. Instead of oblique spreading, the measurements indicate left-l ateral shear strain accumulation parallel to the Reykjanes Peninsula seismi c zone (similar to N76 degreesE) at the rate of about (epsilon) over dot (y x) approximate to -0.2 mu strain/yr (tensor shear strain). Subsidence is ge nerally observed toward the seismic zone. A local maximum subsidence of 60 mm was measured in the Svartsengi geothermal area. Subsidence in this area has previously been detected with geodetic measurements and is considered a result of geothermal usage. Expansion was observed in the area of the Heng ill triple junction. This is probably a result of magma accumulation beneat h! mount Hromundartindur, as previous seismic and geodetic measurements hav e indicated. Using a simple screw dislocation model, we fit the majority of the data. Assuming a left-lateral shear zone at depth along the seismic zo ne, we estimate locking depth of similar to 6.5 km and deep slip rate of si milar to 16.5 mm/yr. The maximum left-lateral displacement predicted by the screw dislocation model, 11.85 +/- 0.06 mm/yr, is consistent with the obse rved value of 11.9 +/- 0.5 mm/yr. If the Hengill area is excluded, little e xtension is observed across the peninsula. The discrepancy between the NUVE L-1A oblique spreading and the observed transcurrent motion is thought to b e caused by lack of magma intrusion into the crust during this time period.