Seismotectonics of the central part of the South Iceland Seismic Zone

Abstract

The largest earthquakes in Iceland are associated with strike-slip faults in the South Iceland Seismic Zone (SISZ). Major destructive earthquake sequences occur in this zone at an average interval of 80 years, the last two sequences being in 1784 and 1896. The last major single earthquake in the zone was an earthquake of magnitude 7 ( M) in 1912. The seismicity is mostly associated with a conjugate system of NNE-trending (mainly) right-lateral faults and ENE-trending (mainly) left-lateral faults, but in addition there are many NW-trending faults in the SISZ. Particular attention is paid to the comparison between present-day fault activity (earthquake mechanisms) and Quaternary faulting (fault-slip data). This comparison reveals a general similarity despite the temporal difference between these subsets independently collected. The fault populations are well exposed in the hyaloclastite mountain Vördufell, near the central part of the South Iceland Seismic Zone, where considerable fault slip occurred during the earthquake sequences of 1784 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 (displacements of the order of centimetres) in the Vördufell Mountain. Most of the minor faults are left-lateral. Analysis of these minor faults recording palaeostresses 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 Vördufell Mountain, gave consistent results in terms of seismotectonic stress (orientations of stress axes and ratios of principal stress differences). This shows that the tectonic regime remained unchanged through recent times. The results indicate, however, that there are two contrasting stress regimes in this part of the SISZ. The primary subset indicates NW–SE tension and NE–SW compression, whereas the secondary subset indicates NW–SE compression and NE–SW tension. We propose that the primary subset is consistent with the dominating time-averaged regional stress field in the SISZ. This dominating stress field can be largely explained as a consequence of simple plate pull parallel with the spreading vector in South Iceland. The secondary 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.