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

Abstract

In 1993 and 1998 a 38‐point GPS network was surveyed on the Reykjanes Peninsula, SW Iceland. According to the NUVEL‐1A plate motion model the spreading rate of the North American and Eurasian plates in SW Iceland is 18.9±0.5 mm/yr toward N102.7°±1.1°E, highly oblique to the plate boundary. Instead of oblique spreading, the measurements indicate left‐lateral shear strain accumulation parallel to the Reykjanes Peninsula seismic zone (∼N76°E) at the rate of about εyx ≈ −0.2 μstrain/yr (tensor shear strain). Subsidence is generally 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 Hengill triple junction. This is probably a result of magma accumulation beneath! mount Hrómundartindur, as previous seismic and geodetic measurements have 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 zone, we estimate locking depth of ∼6.5 km and deep slip rate of ∼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 observed value of 11.9±0.5 mm/yr. If the Hengill area is excluded, little extension is observed across the peninsula. The discrepancy between the NUVEL‐1A oblique spreading and the observed transcurrent motion is thought to be caused by lack of magma intrusion into the crust during this time period.