The Bláfjöll area is here used as a name for a 35 km EW elongated area, between the seismically very active Hengill area in the east and the high activity Krísivík area in the west. Let us introduce here the term Bláfjöll seismic zone (BSZ). It can be consideredas a westward prolongation of the SISZ. As SISZ it is characterized by very low present day seismicity. Historical evidence does not reveal any earthquakes in BSZ which in size are comparable with the largest SISZ earthquakes. The largest earthquake known since 1700 occurred in 1929, in the western part of the zone with measured magnitude of 6.3. This earthquake would only release 60 years of strain build up on a 14 km long fault, i.e. the strain build up since 1868. In 1868 an earthquake occurred in SW Iceland that may have been in this area, but magnitude was below 6. An earthquake of magnitude around 6 which occurred in 1724 has been related to the BSZ area . In 1968 an earthquake occurred in the western part of the zone of magnitude 5.5.

In the existing hazard assessment for BSZ it is assumed that the largest earthquake in the area may reach magnitude 6.5. This is on basis of the faint historical evidence and the assumtion that the tectonics of this part of the Reykjanes peninsula is similar to the more western parts, i.e. a mixture of rifting and strike slip tectonics. On basis of these assumptions it is not justified to claim that larger earthquakes can occur in this area.

It can however to be of enormous significance to make a better constrained hazard assessment for the area which is only at 10 km distance from the Reykjavík area. In the present hazard assessment 0.2 g accelleration is expected in Reykjavík. This would increase to 0.4 g in the center of Reykjavík if magnitude 7 earthquakes would be expected in the area instead of 6.5 and still higher accelleration in the parts of Reykjavík closest to the zone. A better hazard assessment must be based on a better understanding of the present day tectonics of the area, a better model.

It is possible that shear strain energy, to be released in a large earthquakes, has been building up in this area for 200-300 years. The period after 1700 is a period of relatively good historical accounts, so a magnitude 7 earthquake during the last 300 years should have been in reports. About 200 years ago events happened near this area which may have considerably changed the stress conditions in the area. In 1789 a large rifting epizode occurred in the NE-SW Hengill fissure swarm, a part of the western volcanic zone, which was observed on a 50 km long segment of the zone which intersects the area between SISZ and BSZ. The central part of the rift was probably sinking a couple of meters at least, as observed at two locations during the epizode. This epizode may have partly relaxed the shearing stresses already then built up in the BSZ and partly increased the compression in the surrounding of the fissure swarm, i.e. would close it and make earthquakes and intrusive activity less likely in the BSZ.

The Reykjanes peninsula has two tectonic modes to allow for the EW transform notion of the SISZ. One is the mode of rifting and volcanism on NE-SW trending fissure swarms. This mode was active with volcanism from year 900 - 1400, and at least the most eastern part of it was active, without volcanism, in 1789.

The present tectonic mode of BSZ is dominated by large compressive stresses and horizontal shearing that will probably be released in earthquakes similarly to the SISZ. N-S earthquake faults are found in the BSZ, as far as it has been studied, side by side similar to what is seen in the SISZ. These faults are in lavas which are prehistorical, i.e. a few thousands years old possibly created by earthquakes in former seismotectonic modes of similar type as the present one. We cannot conclude that earthquakes can reach magnitude 7 during these tectonic modes just as in the SISZ, but we cannot exclude it either with present knowledge. To reach an understanding of this is one of the major challenges for PRENLAB.

It is then also significant to consider if the release of compressive stresses in large earthquakes in this area would start a new mode of tectonics, i.e. a mode that would be more characterized by rifting and volcanism.