The tectonic setting

The SISZ is situated between two sections of the mid-Atlantic ridge, the Reykjanes ridge (RR) and the eastern volcanic zone (EVZ). Even though the angle between the SISZ and the neighbouring ridges is far from 90$^\circ$, it is considered as a transform fault. Following the transform fault hypothesis, left-lateral shear stress is expected along the E-W striking zone. This is equivalent to right-lateral shear stress with N-S orientation. In fact, earthquakes seem to occur on N-S trending en-echelon faults [20,30] and further references there). They are located side by side between the Hengill triple junction, where the RR meets the low activity western volcanic zone (WVZ) and Hekla volcano, a part of the EVZ [20] and Figure 27). As we further know from Subprojects 4 and 5, the orientation of the larger horizontal principal stress is NE-SW, i.e. fits to an active N-S or E-W trending fault, which is not a weak fault like the San Andreas fault, and the stress orientation seems to be constant since Pliocene time.

  

Figure 27: The South Iceland seismic zone showing mapped surface breaks and regions in which over half of the buildings were destroyed in historic seismic events [20]. The N-S dashed line near Vatnafjöll indicates the estimated location of the fault on which the May 25, 1987, earthquake occurred [7]. The structural features and the coastline are after Einarsson and Sæmundsson (1987).

In detail, the questions to be solved are:

• Do these events, placed on parallel faults, release all the energy stored in the 3-D volume of the SISZ?
• Do the earthquakes always take place in areas of high stress?
• What is the critical stress level? How large is its variability?
• Where are the highest stresses nowadays?

The area investigated extends from 18 to 24$^\circ$W and from 63 to 65$^\circ$N. The origin is set to 24$^\circ$W, 64$^\circ$N (cf. Figure 28) it includes the SISZ, $\pm 1^\circ$north and south of 64$^\circ$N, the SW edge of the EVZ, and the north eastern most part of RR.