ALBERT

Description: The Tjörnes Fracture Zone (TFZ), North Iceland, is a 120 km transform offset of the Mid-Atlantic-Ridge that accommodates 18 mm yr –1 plate motion on two parallel transform structures and connects the offshore Kolbeinsey Ridge in the north to the on-shore Northern Volcanic Zone (NVZ) in the south. This transform zone is offshore except for a part of the right-lateral strike-slip Húsavík-Flatey fault (HFF) system that lies close to the coastal town of Húsavík, inducing a significant seismic risk to its inhabitants. In our previous work we constrained the locking depth and slip-rate of the HFF using 4 yr of continuous GPS measurements and found that the accumulated slip-deficit on the fault is equivalent to a M w 6.8 ± 0.1 earthquake, assuming a complete stress release in the last major earthquakes in 1872 and a steady accumulation since then. In this paper we improve our previous analysis by adding 44 campaign GPS (EGPS) data points, which have been regularly observed since 1997. We extract the steady-state interseismic velocities within the TFZ by correcting the GPS data for volcanic inflation of Theistareykir—the westernmost volcano of the NVZ—using a model with a magma volume increase of 25  x 10 6  m 3 , constrained by InSAR time-series analysis results. The improved velocity field based on 58 GPS stations confirms the robustness of our previous model and allows to better constrain the free model parameters. For the HFF we find a slightly shallower locking depth of ~6.2 km and a slightly higher slip-rate of ~6.8 mm yr –1 that again result in the same seismic potential equivalent to a M w 6.8 earthquake. The much larger number of GPS velocities improves the statistically estimated model parameter uncertainties by a factor of two, when compared to our previous study, a result that we validate using Bayesian estimation.