Task 2.1: Inversion of large sets of focal mechanisms of earthquakes

Using a new inversion technique suitable for focal mechanisms of earthquakes (in that it does not require the choice of a nodal plane as the actual fault plane) was particularly worthwhile in order to process the very large numbers of focal mechanisms of earthquakes determined within the SIL network set-up and monitored by IMOR.DG. This was because of the mathematical aspects which imply analytical resolution so that at the core of the inversion process itself the runtime does not depend on the quantity of data (Angelier 1998).

The inversion was carried out with a total dataset of about 50000 double-couple focal mechanisms of earthquakes (Angelier et al. 2000a). It revealed regional deviations as large as $40^\circ$ in the direction of extension near the major transform zones (clockwise for the South Iceland seismic zone, counterclockwise for the Tjörnes fracture zone), relative to the N$104^\circ$E trend of plate separation across the mid-Atlantic ridge. However, the mean axis of minimum stress obtained from these inversions for the whole of Iceland is quite consistent with the trend of plate separation independently reconstructed by DeMets et al. (1990).