Task 2: Pin-pointing of stress concentrations in space and time

At present, stresses are concentrated around 19.9$^\circ$W and at 20.8-21.2$^\circ$W. As there have not been earthquakes since 1912 and 1896, respectively, this is to be expected and was reproduced by the model.

Once more, we would like to point to the fact that, in the model, stresses are steadily released at those areas (marked with dashed lines) where the SISZ approaches the ridge segments, as noted under item "C" of the list of improvements above. It are sections of the SISZ between the high stress areas just mentioned and the rift tips.

Is this a weakness of the models or is it the real situation? Could these high stress areas be excluded from the model? The answer is essentially "no". The origin of the stress concentrations at the end of the SISZ, i.e. at the tips of the adjoining ridges, is the fact that the ridges do not extend to infinite depth, but are assumed to reach only 7 km depth and enter than an inelastic, hot region not capable of supporting stresses for time periods of years. Deeper penetration of the brittle layer there would homogenize the stress field between rift tips at some average value. Although this is not very likely, a similar effect would be produced by drag at the base of the adjacent plates. Nevertheless, such a redistribution of stress would not lead to low stresses at the ends of the SISZ.

Possibly, there might have been stronger events in 1546 and 1632 at the western end (about 21.3$^\circ$W) and one event in 1311 (about 18.9$^\circ$W) (cf. Halldórsson 1991). But all other evidence only shows small and medium sized events there. The main argument against high seismogenic stress release there is, that there was no large event (M$\ge$6) since 1706. There are several indications that the stress release indeed mainly takes place in small and medium events at the ends of the SISZ. In 1987, there was a medium sized earthquake (M_S=5.8) at 63.91$^\circ$N 19.78$^\circ$W (198, -9) near Vatnafjöll (see Figure 50) at the east end of the SISZ. And in 1998, there were 2 similar events at the Hengill-Ölfus triple junction at the western end of the SISZ: June 4 (M=5.1) and November 13 (M=5) both accompanied by a lot of smaller events.

Therefore, we believe that the tips of the SISZ might only be the place where medium sized events take place and the stress concentrations given around 19.9$^\circ$W and at 20.8-21.2$^\circ$W represent those for future strong earthquakes.

As the stress build-up by plate motion is very low in these models (compare the stress field in 2000 in Figure 56 with that of 1912 in Figure 55), the uncertainty in time is very large in the occurrence time, see also the discussion below.