Seismotectonics of the central part of the South Iceland Seismic Zone

The largest earthquakes in Iceland are associated with strike-slip faults i n the South Iceland Seismic Zone (SISZ). Major destructive earthquake seque nces occur in this zone at an average interval of 80 years, the last two se quences being in 1784 and 1896. The last major single earthquake in the zon e was an earthquake of magnitude 7 (M) in 1912. The seismicity is mostly as sociated with a conjugate system of NNE-trending (mainly) right-lateral fau lts and ENE-trending (mainly) left-lateral faults, but in addition there ar e many NW-trending faults in the SISZ, Particular attention is paid to the comparison between present-day fault activity (earthquake mechanisms) and Q uaternary faulting (fault-slip data). This comparison reveals a general sim ilarity despite the temporal difference between these subsets independently collected. The fault populations are well exposed in the hyaloclastite mou ntain Vordufell, near the central part of the South Iceland Seismic Zone, w here considerable fault slip occurred during the earthquake sequences of 17 84 and 1896. The cumulative displacement on most of these faults is 1-6 m. In addition to the large-scale faults, we measured 79 minor faults (displac ements of the order of centimetres) in the Vordufell Mountain. Most of the minor faults are left-lateral. Analysis of these minor faults recording pal aeostresses in rocks ranging in age from 3.1 to 0.7 Ma, as well as 50 focal mechanisms from a swarm of present-day microearthquakes occurring beneath the Vordufell Mountain, gave consistent results in terms of seismotectonic stress (orientations of stress axes and ratios of principal stress differen ces). This shows that the tectonic regime remained unchanged through recent times. The results indicate, however, that there are two contrasting stres s regimes in this part of the SISZ. The primary subset indicates NW-SE tens ion and NE-SW compression, whereas the secondary subset indicates NW-SE com pression and NE-SW tension. We propose that the primary subset is consisten t with the dominating time-averaged regional stress field in the SISZ. This dominating stress field can be largely explained as a consequence of simpl e plate pull parallel with the spreading vector in South Iceland. The secon dary subset and the associated stress field, however, may be partly related to stress release and rebound and partly to dike injections in the nearby segments of the rift zone in South Iceland. (C) 1998 Elsevier Science B.V. All rights reserved.