MAIN FEATURES OF THE SEISMOTECTONICS OF THE SISZ

The SISZ in a narrow sense is an E-W transverse seismic zone where earthquakes up to magnitude 7 occur intermittently on N-S faults which cross the zone. Continuously occurring small earthquakes and surface observable faults of large earthquakes indicate that the SISZ in a narrow sense is less than 10 km wide and almost exactly E-W oriented.  Stefansson/etal:1993,Einarsson:1991 The direction of the relative plate motion in this area is N103$^{\circ}$E at velocity 1.86 cm/year according to the NUVEL-1A plate model  DeMets/etal:1994. If all the plate motion is taken up by the SISZ area this means that the relative motion along the plate boundary is 18 mm/year and a spreading motion or opening component across the plate boundary could be 4 mm/year (Figure 1).

  

Figure 1: The figure indicates the main rift zones of SW Iceland, the western volcanic zone (WVZ) and the eastern volcanic zone (EVZ). The SISZ in the sense of a 10 km wide zone and its prolongation in the Reykjanes peninsula are shown by rectangular boxes. The direction of the relative plate motion according to NUVEL-1A plate model is shown by large arrows. The transversal plate motion and the extension across the SISZ is shown by lighter arrows.

This would, however, be a maximum as it is possible that a minor part of the order of 1/10 of the motion would be taken up by the western volcanic zone. This rifting is complemented or pushed apart by build-up of pore pressure, which is significant because of intrusion of basaltic fluids, mostly from below the base of the seismogenic crust. The high pore pressures in the seismic zone create N-S compression perpendicular to the plate boundary. Thus one should treat the stresses active near the SISZ as composed of two factors. Firstly the stresses which stem from the general opening of Iceland because of rifting of the Eastern Volcanic Zone (EVZ) and of the Reykjanes ridge and the resulting E-W transversal and shearing motion, and secondly the stresses which stem from the expansion of the SISZ. Further to the west, in the Reykjanes peninsula, the angle between the plate motion and plate boundary is 27$^{\circ}$, so the opening component there is around 8 mm/year. As a consequence of the faster opening, the Reykjanes peninsula has more volcanism and not as large earthquakes as the SISZ.