Reconstruct the current stress fields associated with the test areas applying inversion of large sets of focal mechanisms of earthquakes.

We inversed of large population of earthquake focal mechanisms to derive the regional seismotectonic field. We studied a population of 48669 double-couple focal mechanisms of earthquakes, from 67717 events as evaluated by the Icelandic Meteorological Office during the years 1995 to 1997. Magnitudes range between -1.8 and 4.8. The propose of this study was to determine whether such a large mass of data has the potential to indicate the general tectonic field. Many reasons suggested that this may be not the case: uncertainties of determinations, perturbations in tectonic regimes, and so on.

We considered two zones surrounding the major transform-rift zones north and south of Iceland: the Tjörnes quadrangle (66$^\circ$-67$^\circ$N, 16.5$^\circ$-19.75$^\circ$W) and the South Iceland seismic zone (SISZ) quadrangle (63.7$^\circ$-64.25$^\circ$N, 19.8$^\circ$-21.1$^\circ$W). Inversion was carried out based on a new direct method established by one of us during the PRENLAB-1 project (J. Angelier), using 10547 and 4413 double-couple focal mechanisms in these two quadrangles respectively. In both cases, the reconstructed s2 axes plunge 78$^\circ$ or steeper, indicating dominating strike-slip mode. The average direction of extension (s3 trend) is N66$^\circ$E for the Tjörnes quadrangle and N143$^\circ$E for the SISZ one. Selecting only the data fit significant quality requirements results in no or little change, with N66$^\circ$E and N146$^\circ$E trend respectively: 9831 and 1916 mechanisms are thus retained (respectively). This stability of the inversion indicates that the resuls are significant. The ratio F between principal stress differences average 0.6-0.7, indicating that in terms of magnitudes, s2 is closer to s1 than to s3. This is consistent with the association of normal and strike-slip faulting modes. Considering the general trend of plate separation and related extension in Iceland, that is, N104$^\circ$E, these results in the main zones where transform faulting occurs north and south of the Icelandic rift (right-lateral and left-lateral respectively) are of particular interest. To the north, the trend of average extension is deviated counterclockwise of 38$^\circ$. To the south, it is deviated clockwise of 42$^\circ$. These deviations are in perfect agreement with the pattern of transform motions between the segments of the North-Atlantic oceanic spreading axis.