Jökull - 01.12.1979, Page 43

been very low in recent years, and the fault remains
to be confirmed by geological mapping. Its
existence should therefore be considered specu-
lative.
One can conclude that a large part of the seis-
micity of the Tjörnes Fracture Zone can be
attributed to the activity along two and possibly
three WNW-striking faults. The existence of
further seismic zones can certainly not be excluded.
The transform motion is thus taken up by at least
two parallel faults within a broad deformation
zone.
Volcanic zones
Earthquakes in the volcanic zones are generally
smaller than in the fracture zones. Volcanic erup-
tions are usually accompanied by earthquakes, but
between eruptions most parts of the volcanic zones
are seismically quiet. A few areas of persistent seis-
mic activity are found, the most prominent ones in
Central Iceland and near the subglacial volcano
Katla in South Iceland (Fig. 1).
The seismic area in Central Iceland is largely
covered by the ice cap Vatnajökull, and the tec-
tonic structure is poorly known. Recent studies of
ERTS images of this area seem to indicate that the
structure is dominated by a group of central volca-
noes and it is tempting to relate the earthquakes to
volcanic processes. The seismic activity of this area
has been unusually high in recent years. Six earth-
quakes of magnitude 5 and larger occurred in the
period 1974—79, but before 1974 no such large
events were known.
The Katla volcano is located near the southern
end of the eastern volcanic zone, south of its junc-
tion with the South Iceland seismic zone. The
structure of this part of the zone is characterized by
several central volcanoes, rifting structures are less
significant. Historic eruptions of Katla have been
preceeded by felt earthquakes, and because of the
potential danger of future eruptions the seismicity
at Katla is monitored by a relatively dense seismo-
graph network. The epicenters located so far
delineate two active areas. One is under the SE-
part of the Mýrdalsjökull ice cap and coincides
with the eruption sites in the latest Katla eruptions.
The other area is under the SW-part of Mýrdals-
jökull, about 15 km W of the first one. The depths
of hypocenters in both areas are in the range 0—30
km. The hypocenters thus delineate two chimney-
like features that penetrate the crust and extend
well into the anomalous upper mantle.
The seismic activity in the Mýrdalsjökull area
shows a pronounced annual cycle. The probability
of an earthquake occuring within a given time in-
terval is several times higher in the second half of
the year than in the first half. This annual cycle was
first noted by E. Tryggvason (1973) for the years
1952—58 and has been confirmed by later data.
The Heimaey eruption in 1973 was preceeded by
an intensive swarm of small earthquakes that
started 30 hours before the eruption. Earthquakes
also accompanied the eruption, but the seismicity
declined as the lava production diminished. No
shock reached magnitude 4. The earthquakes dur-
ing the eruption occurred at the depth of 15 — 25
km and occupied a spherical Volume centered un-
der Heimaey. It seems likely that the erupted
magma either was stored or formed within this
volume.
The depth of the Heimaey and Katla earth-
quakes is much larger than observed elsewhere in
Iceland. In these areas the upper boundary of the
anomalous mantle underlying Iceland is at the
depth of 12—15 km. Earthquakes at the depth of
20—30 km may be taken to imply brittle failure in
the mantle where creep or ductile behaviour is
normally assumed. In these volcanic regions it is
possible, however, that high strain rates associated
with magmatic processes may cause brittle failure
in material that would be ductile at lower strain
rates.
A major rifting episode has been in progress since
1975 in the volcanic rift zone in NE-Iceland. The
activity has been confined to the Krafla central
volcano and its associated fissure swarm (Fig. 3),
and provides a demonstration of a process that
seems to play an important role in Icelandic tec-
tonics. The activity is characterized by repeated
cycles of relatively slow inflation and rapid
deflation of the volcano. Magma apparently
accumulates at a constant rate under the volcano
during the inflation periods and during the
deflation events the magma escapes from the
reservoir area. Each cycle of activity is
accompanied by a characteristic pattern of seismic
activity. Continuous volcanic tremor starts in the
caldera region at about the same time as the
deflation. Small earthquakes also occur in the cal-
dera, but the epicentral area is soon extended along
the Krafla fissure swarm to the north or to the
south. The rate of propagation of the seismic
activity is highest during the first few hours, typi-
cally 0.5 m/sec., but the speed decreases as the
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