Changes and improvements in PRENLAB-2

In the first phase of PRENLAB-2, the models developed in PRENLAB-1 were improved:

A

At the western end of the SISZ, segments with aseismic oblique slip (mainly normal faulting with a smaller component of left-lateral strike-slip) were introduced, to better fit the Reykjanes ridge (RR) between the SW tip of the Reykjanes peninsula to Hengill triple junction (Figure 28).

B

The test-point density was increased from 56x44 (5 km distance) to 280x220 (1 km distance) to get more details of the stress field and to reduce interpolation errors.

C

A layered model, including an inelastic asthenosphere below a brittle seismogenic upper layer, is in preparation.

D

To investigate the model resolution a set of different models is produced: Besides the main model, several extreme cases are assumed and the variation of the main results under these assumptions is observed.

E

The stress field at 1912, the end of the series of strong events with M$\ge$6, is extrapolated to April 1999.

Concerning item A, the RR on Iceland is treated now completely as a zone of aseismic rifting, consisting of 2 sections with changing rifting and strike-slip components to model the bend of the ridge from SW towards the Hengill triple junction, as displayed in Figure 28. Doing so, not only the geometry of the rift is better fitted, but also stress build-up by plate motion is concentrated in the west near the Hengill triple junction instead of farther west in the RR. Now, seismic slip is confined to the SISZ ((125, -5) -- (250, 0)), where the series of strong earthquakes simulated here, took place.

This model was calculated with a test-point spacing of only 1 km (item B). It will be named "improved model" below.

  

Figure 28: Map of Iceland and surrounding area. Thick red lines indicate mid-Atlantic ridge segments, as used in the PRENLAB-TWO modelling. The E-W segment, continuation of RR, was not assumed in the PRENLAB-1 model (see text). The smaller box shows the region of the model on the South Iceland seismic zone. The SISZ extends approximately between (338.6$^\circ$E, 63.95$^\circ$N) to (341.2$^\circ$E, 64$^\circ$N). The large box gives the region for the Iceland rift model.

A selection of the results obtained by this model is given in Figures 29, 30, and 31. Figure 29 gives the initial stress field again with the dark red areas subject to the highest shear stress, now at the rift tip east of Hengill.

  

Figure 29: Shear stress field in the South Iceland seismic zone and its surroundings as assumed in 1706 ("improved model").( The same values of the isolines apply as in Figure 32.)

  

Figure 30: The stress field after the last strong event May 6, 1912, M=7.0 earthquake occurred at (187, -11) in the "improved model". Here, only the central region of the modelled area is displayed, so that the details inside the SISZ are clearly visible.

The pre-seismic stress level is expected to be smoother than before with the wider spacing of the test-points. As - at the same change, however - the high stress tip of the SISZ was shifted to Hengill, the westernmost events (1706, 1784b, 1896c and e) got into a higher stress region. This produces a larger scatter of the pre-event stress level, cf. Figure 31. However, the stress level for the main events remains in a similar range as before (between 2.0 and 2.8, now between 1.7 and 2.9 MPa).

  

Figure 31: Cross plot of the pre-seismic shear stress level at the site of the impending earthquakes vs. occurrence time. Here in the "improved model", the stress values at 10 to 70 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.