Tjörnes Fracture Zone is found both on land and under sea at the north coast of Iceland.
It is similar to the SISZ, but differs as the weakness of the tectonic plate underneath is manifested in different pattern of epicenters.
The description found in an article by Bryndís Brandsdóttir and others, here linked, is interesting, as it says that actually Tjörnes Fracture Zone is aligned EW. The three main lines of epicenters show how the polygons brake up in a regular pattern.
Sometimes manipulating with reality can make things clearer. To compare the earthquake zones with the main underlying factors of mantle convection and its polygon pattern and other implications, let us draw this map with Icelands main three axis:
The NS-axis through the craters of Eyjafjallajökull, Hekla, Hveravellir area and Drangey of the Skagafjörður Volcanic Zone, the EW-axis from Snæfellsjökull to Snæfell with Hveravellir at the center, clarify the locations of SISZ and TFZ.
Therefore, focusing on the mantle convection cell pattern gives a very clear picture. Tectonic drift is of course the reason for the dynamics of these earthquake zones, but this ‘reason for the reason’ is quite important.
The latitude of 64.75°N plays a major role, and the shift from NE trend to NS coincides with the division between large polygons in the southern half of Iceland and the smaller and more irregular polygons of the northern half.