To be able to analyze the Tjörnes Fracture Zone, first it has to be realized that it extends from Skagafjörður to Skjálfandi, and in the East and West coincides with two different volcanic zones. The western volcanic zone of the North, Skagafjörður Volcanic Belt, is actually neglected by most geologists, because it is volcanically extinct. Still it exists seismically, as shown here.
Recently, many earthquakes occurred at Reykjanes, and now within the Tjörnes Fracture Zone. Here, the convection rolls division line of Reykjanes is pointed out, to show how it divides the TFZ into two areas, east and west of the line. The earthquakes start on the line, and then start spreading along the Kolbeinsey Ridge.
Here I point out: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000TC900022