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in this paper (Figure 4). According to Saemundsson (5) the troughs
have successively assumed the roles of spreading centers, the east-
ernmost trough (the Axarfjördur trough) presently heing the most
significant one. Within such a framework the transform motion
between the active spreading centers may shift with time between
the different strike-slip faults. If the Axarfjördur trough is pre-
sently the most active spreading center, the Grimsey fault or some
other fault farther to the north should be the most active trans-
form fault. Significant seismic activity south of the Grímsey fault
suggests that other spreading centers are presently active to the
west of the Axarfjördur trough.
The linear magnetic anomaly pattem associated with the spread-
ing Kolheinsey Ridge extends as far south as 67°N (16). The cen-
tral anomaly appears to bifurcate near the island Kolbeinsey. South
of Kolbeinsey the position of the spreading plate boundary is un-
certain, but the N-S trending trough west of Grímsey (Figure 1)
was suggested by Tryggvason (9) and Saemundsson (13) as a
possible spreading center. This trough, the Eyjafjördur trough, is
bounded at its south end by the extension of the Húsavík faults
(5). This geometric configuration suggests that the Húsavík faults
may terminate as a transform fault at the Eyjafjördur trough. The
large earthquake of 1963 was located about 40 km west of the
Eyjafjördur trough. It would therefore take a mislocation of 40
km to move the epicenter to the Húsavík faults whereas a mis-
location of 15 km would move the epicenter to the Dalvík fault.
For this reason the 1963 earthquake is more likely to have occurred
on the Dalvík fault, although the evidence is admittedly rather
poor.
We are still left with the problem of what happens at the west-
em end of the proposed Dalvík fault. If the Dalvík fault is to be
interpreted as a transform fault it has to connect to a spreading
plate boundary at its westem end, which would require an addi-
tional spreading center or a zone of spreading west of the Eyja-
fjördur trough. The earthquake swarm of October 1973 (Table 1)
may have occurred on such a spreading zone, but more data are
needed to verify this suggestion.
Many questions on seismological aspects of the Tjörnes Fracture
Zone remain unanswered. One interesting observation is, for ex-