Greinar (Vísindafélag Íslendinga) - 01.01.1976, Qupperneq 48

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INTRODU CTION
The Tjömes Fracture Zone is associated with a zone of seismic
activity off the northern coast of Iceland. The purpose of this paper
is to try to relate the earthquake activity to the structure and tec-
tonic setting of the Tjörnes Fracture Zone.
Epicenters determined by Sykes (1) for the period 1955—1963
and by the World Wide Network of Standardized Seismographs
(taken from the Preliminary Determinations of Epicenters) for
the period 1963-1974 are hsted in Table 1 and plotted in Figure 1.
The seismic zone so delineated is approximately 150 km long and
80 km wide, elongated in the E-W direction. The eastem end of
the zone is connected to the axial rift zone in northem Iceland and
the westem end joins with the submarine Kolbeinsey Ridge. The
Kolbeinsey Ridge is a typical segment of the Mid-Atlantic Ridge,
spreading at a rate of 0.8 cm/year (2). The geometric configura-
tion of presumed spreading centers and seismicity together with
a focal mechanism solution for one earthquake led to the inter-
pretation of the Tjömes zone as a transform fault (3). This inter-
pretation was supported by Ward (4), who noted that the trans-
form motion probably took place over a broad zone, the southwest-
em edge of which was exposed on land on Tjömes. The name of
the Tjörnes Fracture Zone comes from this interpretation and is
therefore based more on the seismicity than topographic features.
Saemundsson (5) described the stmcture of the Tjörnes zone as
consisting of several N-S trending ridges and troughs arranged en
echelon withi n a WNW trending zone. Saemundsson furthermore
demonstrated right-lateral displacement along the Húsavík faults
which, according to him, form the southem boundary of the Tjör-
nes Fracture Zone.
The Húsavik faults form a distinctive fault swarm exposed on
the Tjömes peninsula. The fault swarm can be traced from the
shore east-south-eastwards into the axial rift zone of Iceland. Off
shore the faults can be traced as a topographic offset of the Grímsey
shoal (5) and a strong, negative anomaly in the free air gravity
field (6).
The Tjörnes Fracture Zone appears to play an important geo-
chemical role. The smooth changes in the chemical composition of