Jökull - 01.12.1979, Page 42

Fig. 3. Map of the main tectonic features of North Iceland. The fissure swarms in the volcanic zone
are taken from a map by Saemundsson in Björnsson et al. (1977). The trough structures are based on
McMasteret al. (1977), and the seismic lineaments are based on Einarsson (1976) and later data.
are not accurate enough for detailed tectonic in-
terpretation. Detailed studies have shown that a
considerable part of the seismicity is associated
with a WNW trending line that runs slightly north
of the island Grímsey and joins the Krafla fissure
swarm in the Axarfjördur Bay (Fig. 3). The sense of
motion along this seismic line is right-lateral
strike-slip as evidenced by two focal mechanism
solutions. The Grímsey seismic line has no clear
expression in the topography. Instead, the surface
structure in this area is characterized by northerly
trending troughs and ridges that are arranged en
echelon with respect to the fracture zone and the
seismic line. In some respects this structural
relationship resembles that in SW-Iceland, where
the epicentral belts also lack a surface expression.
Even though the Grxmsey seismic line has been
responsible for the majority of earthquakes in this
area for the last decade, it is not the only seismically
active line. The Húsavík faults form a distinctive
fault swarm exposed on the Tjörnes Peninsula. The
faults can be traced from the shore near the town
Húsavik east-south-eastwards into the volcanic rift
zone (Fig. 3). Off shore the fault can be traced as a
topographic offset of the Grímsey shoal and a
strong, negative anomaly in the free air gravity
field. Earthquake locations in recent years have
shown that significant earthquake activity occurs
on the Húsavlk faults, and large, historic earth-
quakes have been accompanied by surface faulting
near Húsavík.
In addition to the Grímsey and the Húsavík
faults a third major WNW-striking earthquake
fault near the town Dalvík has been suggested (Fig.
3). The main evidence for the existence of this fault
is the earthquake (M = 6'A) that caused extensive
damage in Dalvík in 1934, the topography on the
east side of Eyjafjördur and the alignment of low
temperature geothermal springs between Eyja-
fjördur and Skagafjördur. The 1963 earthquake
(M = 7) in the mouth of Skagafjördur possibly
occurred on this fault. The fault plane solution of
that earthquake would imply right-lateral strike-
slip along the fault. The seismicity of this region has
40 JÖKULL 29. ÁR