Jacques Angelier (1), Françoise Bergerat (1), Sébastian Garcia (1), Catherine Homberg (1) and Sigurdur Rögnvaldsson+(2)
(1) ESA 7072, Université P. et M. Curie, Paris, France
(2) Icelandic Meteorological Office, Reykjavik, Iceland

The SIL network monitored by the Icelandic Meteorological Office allowed determination of earthquake focal mechanisms even for small magnitudes. Because of this sensitivity of the seismological network, large numbers of double couple mechanisms were available in the Tjörnes Fracture Zone, northern Iceland, where right-lateral transform motion occurs along a N120°E trend between the rift of Northern Iceland and the Kolbeinsey Ridge. The determination of seismotectonic regimes is made by using an inverse method that aims at calculating for each set of mechanisms a single common stress tensor by minimising the misfit between observed and calculated vectors of shear stress. Two particular problems occur. On one hand, the seismogenic states of stress are inhomogeneous and involve a variety of sub-regimes that markedly differ in nature and orientation, as a result of stress perturbations and permutations that take place even in relatively small regions ; it is thus necessary to determine several tensors corresponding to mechanically homogeneous subsets. On the other hand, most minimisation criteria require distinction between the two nodal planes for each focal mechanism, which result in increasing complexity of the inversion process as compared with fault slip data inversion. This problem is solved by using a new method of direct inversion which does not require this distinction to be made. The application to 670 focal mechanisms with magnitudes larger than 1 allowed determination of nine seismotectonic stress regimes. Four are strike-slip in type, three are normal and the remaining two are reverse. The two most important regimes are strike-slip in type, and reveal NE-SW and E-W directions of extension. Similar trends of extension are revealed by two normal-type regimes. The behaviour of the transform fault zone indicated by this variety of seismotectonic regimes reflects a combination of (1) stress permutations (switches) related to elastic rebound and block accommodation phenomena, and (2) variations in mechanical coupling across the transform zone that induce abrupt changes in extensional trends. This behaviour is consistent with that independently indicated by geological surveys of brittle tectonics in Flateyjarskagi.