Perturbation of stress and oceanic rift extension across transform faults shown by earthquake focal mechanisms in Iceland

Major sources of stress perturbation are expected in the brittle crust, and documented by local studies in Iceland. Whether or not a significant average stress state may emerge from regional-scale inversion of very large sets of focal mechanisms is thus subject to doubt. We carried out stress inversion of double couple focal mechanisms recorded by the Icelandic seismological network within a set of 126 588 shallow earthquakes from July 1991 to July 1999. We performed mass inversion of two main data sets of 12 191 and 71 889 focal mechanisms in the transform zone areas of North and South Iceland respectively. The inversion reveals surprisingly high levels of consistency within these data sets, with regard to the expected dispersion. Adopting a threshold value of +40% in an individual fit scale from -100% (total misfit) to +100% (perfect fit), the proportion of acceptable data is as high as 78% for 65 571 focal mechanisms of the major regime in these two areas (53% for 18 519 mechanisms of the minor regime). The major stress regimes thus calculated show nearly vertical intermediate principal axes and the azimuths of extension (minimum stress σ₃) are 065° in the Tjörnes Fracture Zone and 140° in the South Iceland Seismic Zone. With respect to the 105° azimuth of plate separation, these directions show nearly symmetrical angular deviations: 40° anticlockwise in the right-lateral transform zone area of North Iceland and 35° clockwise in the left-lateral one of South Iceland. Such large deviations reveal first-order stress perturbation in the areas where major rift offset resulted from Late Cainozoic rift jumps related to plate boundary migration with respect to the Iceland Mantle Plume.