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Two problems were addressed next

To see how sensitive the results depend on the model parameters, it was begun to check extreme cases and their outcome.
The average stress level before some events is only slightly above and even below the background stress of about 2.65 MPa (see the main shocks 1706 and 1784a, and 1896a and 1912, respectively). This is, among other reasons, due to the fact that the rupture planes, used until now, extend rather far to the north and south of the SISZ.

The damage areas from historical records are not gathered by scientists and are usually biased by uneven population density. So the magnitudes and locations are not very accurate, as stated earlier. As mentioned in the footnotes of Table 5, there are doubts on the correct rupture size from global relations between magnitude and rupture length.

From both reasons, given here, a model was calculated that uses the same seismic moment of the events, but cuts the fault length to 50% while doubling the co-seismic displacement. It will be termed "short rupture model". One side-effect of this change is an increase of the stress level by a factor 2.5, as the moment release is concentrated to a smaller area. The background stress field amplitude was increased accordingly, because - as described above - this field is adjusted to the average stress change of the strongest event. It is important to note that the increase in stress level does not change the stress pattern of the initial stress field; as we are not looking for specific stress amplitudes but for stress concentrations, the change in level is not important.

The resulting pre-seismic stress level is expected to be smoother than before due to the concentration of stress release to high stress areas.

For comparison with the models above, some results obtained in the "short rupture model" are given in Figures 32 through 34.

Figure 32: Shear stress field in the South Iceland seismic zone and its surroundings as assumed in 1706 - the starting field for the model calculations in the "short rupture model". The background stress is about 6.5 MPa. It is increased between the rift tips which are at (125, -5) and (250, 0).

Figure 33: The stress field after the last strong event May 6, 1912, M=7.0 earthquake occurred at (187, -6) ("short rupture model"). The values of the isolines as in Figure 32 apply.

The pre-event stress level now varies between 6.5 and 7.4 MPa for the main shocks (for more details cf. Figure 34). It is more stable than the level in the previous models, if relative values are compared. For most events, the initial stress level is considerably higher than the background. Only for two main shocks it is near the background (1706 and 1896a) and only for two strong aftershocks it is below (1896b and d). The differences to the previous model are not very large, but a further improvement of the "improved model" could be achieved in using shorter rupture planes. Concerning the extension to a layered crustal structure with an inelastic substratum to include post-seismic relaxation processes, these models will be addressed as soon as the elastic ones are finished. A new code has been prepared for this, much faster, more accurate, and capable of including even more layers than the existing code. The extension of the computer programme for the superposition of stress fields with the new code has already begun. The results will not only be compared to those from the purely elastic models, but also to continuous GPS crustal deformation data, as soon as these are available.

Figure 34: Cross plot of the pre-seismic shear stress level at the site of the impending earthquakes vs. occurrence time. Here in the "short rupture model", the stress values at 5 to 35 test-points near the surface trace of the rupture plane were averaged. -- Letters "a" through "e" denote the events in one year in temporal sequence.

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Next: Extrapolation of the stress Up: Changes and improvements in Previous: Changes and improvements in
Margret Asgeirsdottir