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

The Tjornes 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 Husavik-Flatey Fault, a major 7-9 Myr old right-lateral fault, In addition to this fault, there are two major seismic lineaments as sociated with the Tjornes Fracture Zone; the Grimsey lineament and the Dalv ik lineament. These lineaments are marked by concentrations of seismicity w ith the largest earthquakes reaching magnitude 7. The maximum depth of eart hquakes is 10-12 km and increases with distance from the spreading axis. We determined accurate relative locations and focal mechanisms of more than 8 00 earthquakes in 62 clusters on the principal seismic lineaments. The esti mated relative location uncertainty for most of the relocated earthquakes i s 2-20 m. The best fitting plane through each cluster is assumed to coincid e with the fault plane of the group of earthquakes. For clusters near the H usavic-Flatey Fault the fault planes are right-lateral and strike N122 degr ees E-N140 degrees E, similar to the overall strike of the Husavik-Flatey F ault. This agrees with the right-lateral displacement on the fault as well as with field observations of numerous transform-parallel right-lateral fau lts associated with the main fault. By contrast, earthquake clusters on the lineaments of Grimsey and Dalvik define (mostly) left-lateral planes strik ing roughly north-south, i.e., at 40 degrees-90 degrees to the overall tren d of these lineaments. Field observations show that left-lateral, north-sou th trending fault planes are also common in the on-land parts of the Dalvik lineament. The different style of faulting probably represents transform f aults at different stages of development.