As a simple assumption, one might expect, that earthquakes in a certain fault zone usually occur at about the same critical shear stress level. Here, we try to find out, if such an expectation matches the known facts about the earthquakes and the stress field in the SISZ.
In all models, the pre-seismic stress level for most main shocks is high and fairly stable. This indicates that the rather simple model can already explain the main features of the behaviour of the SISZ. This is especially astonishing, when the fact is kept in mind, that most (all but one) events used are not instrumentally recorded. Even though the earthquake rupture planes strike N-S, the stress changes calculated here affect the whole area of the SISZ.
The tendency with time towards slightly lower values, is an indication that the stress increase due to rifting might have been assumed too low, i.e. the spreading rate between 1706 and 1912 might be higher than 2 cm/year. Moreover, the initial unknown stress field of 1706 could be reduced in the eastern part and the central part, where the first events did not occur before 1732 and 1734, respectively.
A closer look, yields that for the "improved model" the stress level before the earthquakes is between 1.9 and 2.9 MPa, if only the main shocks and no aftershocks are considered (Figure 31). However, in this model, there are 4 events with pre-seismic stress level not much above the background stress, which would not be expected. In the "short rupture model" (same seismic moment as before, but half of the rupture lengths and twice of the co-seismic displacements) the stress level is higher and the pre-event stress level varies between 6.5 and 7.4 MPa for the main shocks (cf. Figure 34). It is more stable than the level in the previous models, if relative values are compared, and 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). So, the stress field analysis gives some indication that the strong change in rupture lengths used, means to tune the model into the right direction. Nevertheless, the strong variation in model parameters does not lead to totally different results, i.e. the model is rather stable in this respect. To further check the sensitivity of the model results to changes, a model with stronger influence of the permanent stress build-up by plate motion will be calculated next.
In general, the models go beyond the standard earthquake moment release and hazard analysis as they include the spatial location and extension of the events and provide an extrapolation to the present stress situation.