Seismotectonic analysis of the Tjörnes Fracture Zone, an active transform fault in north Iceland

The Tjörnes Fracture Zone is a transform fault connecting the rift zone of the Kolbeinsey Ridge with that of north Iceland. The main transform motion takes place on the Húsavík‐Flatey Fault, a major 7–9 Myr old right‐lateral fault. In addition to this fault, there are two major seismic lineaments associated with the Tjörnes Fracture Zone; the Grímsey lineament and the Dalvík lineament. These lineaments are marked by concentrations of seismicity with the largest earthquakes reaching magnitude 7. The maximum depth of earthquakes is 10–12 km and increases with distance from the spreading axis. We determined accurate relative locations and focal mechanisms of more than 800 earthquakes in 62 clusters on the principal seismic lineaments. The estimated relative location uncertainty for most of the relocated earthquakes is 2–20 m. The best fitting plane through each cluster is assumed to coincide with the fault plane of the group of earthquakes. For clusters near the Húsavík‐Flatey Fault the fault planes are right‐lateral and strike N122°E–N140°E, similar to the overall strike of the Húsavík‐Flatey Fault. This agrees with the right‐lateral displacement on the fault as well as with field observations of numerous transform‐parallel right‐lateral faults associated with the main fault. By contrast, earthquake clusters on the lineaments of Grímsey and Dalvík define (mostly) left‐lateral planes striking roughly north‐south, i.e., at 40°–90° to the overall trend of these lineaments. Field observations show that left‐lateral, north‐south trending fault planes are also common in the on‐land parts of the Dalvík lineament. The different style of faulting probably represents transform faults at different stages of development.