Jökull - 01.12.1979, Page 39

5 Earthquakes in Iceland
PÁLL EINARSSON and SVEINBJÖRN BJÖRNSSON
Science Institute, University of Iceland, Reykjavík
Most of the seismicity of Iceland is related to the
mid-Atlantic plate boundary that crosses the
country. The boundary approaches Iceland from
the southwest along the crest of the Reykjanes
Ridge (Fig. 1) and from the north along the Kol-
beinsey Ridge. In Iceland the plate boundary is
displaced to the east by two major fracture zones,
the South Iceland seismic zone in the south and the
Tjömes Fracture Zone in the north. Because of the
lack of a clear topographic expression, both zones
are defined primarily by their high seismicity,
earthquake focal mechanisms and configuration
with respect to the spreading axes. The largest
earthquakes in Iceland occur within these zones
and may exceed magnitude 7. Earthquakes also
occur along the volcanic rift zones between the
fracture zones, but they only rarely exceed magni-
tude 5. A large part of this seismicity appears to be
related to central volcanoes.
Epicenters of earthquakes of the period 1962—77
large enough to be located by seismograph net-
works outside of Iceland are shown in Fig. 1. The
map includes events down to magnitude 4, but is
complete for magnitude 4.5 and larger events.
Single event focal mechanism solutions available so
far are also shown. This map shows many of the
characteristics of the Icelandic seismicity, even
though some of the locations are in error by as
much as 40 km. Concentration of activity is seen in
the Tjömes Fracture Zone near the coast of N-
Iceland, and in SW-Iceland on the Reykjanes
Peninsula and in the South Iceland seismic zone.
The focal mechanisms indicate strike-slip faulting.
If the easterly striking nodal planes are taken as the
fault planes, the sense of motion is right-lateral in
N-Iceland and left-lateral in SW-Iceland, which is
consistent with a transform fault interpretation of
these zones. Outside of the fracture zones clusters of
activity are seen in the Borgarfjördur area in W-
Iceland, in the volcanic zone in Central Iceland,
and near the volcanoes Katla in S-Iceland and
Krafla in N-Iceland. Each of these zones will be
considered separately.
Reykjanes Peninsula
The Reykjanes Peninsula is an area of high seis-
micity and recent volcanism that forms a transition
between the Reykjanes Ridge to the west, and the
western volcanic zone and the South Iceland seis-
mic zone to the east (Fig. 2). The mid-Atlantic
plate boundary as defined by the seismicity enters
Iceland near the tip of Reykjanes and then runs
along the peninsula in an easterly direction (Fig. 2).
Detailed studies show that the seismic zone is less
than 2 km wide in most places. The earthquakes are
mostly at the depth of 1—5 km and are not located
on a single fault. The seismicity seems to be caused
by deformation of the brittle crust above a deeper
seated and aseismic deformation zone. Small scale
structures can be resolved in the seismicity within
the zone. Several seismic lineations or faults can be
identified, striking obliquely or even transversely to
the main zone.
Focal mechanisms have been determined for a
large number of small earthquakes, using data
from dense, local networks, and for two earth-
quakes larger than magnitude 5 using teleseismic
data. The minimum compressive stress is consis-
tently oriented in a horizontal, NW direction. The
maximum compressive stress rotates between the
vertical direction, causing normal faulting on NE-
striking faults, and the horizontal NE direction,
causing strike-slip faulting on N or E striking faults.
Thus the stress regime is characterized by the
NW-trending minimum stress. The other principal
stresses are probably nearly equal and may change
directions according to local, or time depe'ndent
conditions. Dykes open up against the minimum
stress and strike NE, in accordance with the erup-
tive fissures observed on the surface.
The mode of strain release changes systemati-
cally along the peninsula. Near the tip of Reykja-
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