Jökull - 01.12.1979, Page 44

deflation rate decreases and the epicentral zone is
extended. The earthquake activity culminates after
the maximum in tremor and deflation rate is
reached. The largest earthquakes are located
within a well defined, but each time different sec-
tion of the fissure swarm. The magnitude only
rarely exceeds 4.5. The depth of hypocenters is in
the range 0—6 km. Extensive fault movements,
both normal faulting and fissuring, occur in the
area of maximum earthquake activity. The propa-
gating seismic activity suggests that the magma
escaping from the Krafla reservoir is injected
laterally into the fissure swarm to form a dyke. The
dykes may be as long as 40—60 km.
The first and the most violent deflation event
started on Dec. 20, 1975. The deflation of the cal-
dera exceeded 2 m and the accompanying earth-
quake swarm lasted about 8 weeks. Most of the
epicenters that appear in the northern part of the
volcanic zone in Fig. 1 belong to this swarm. The
largest earthquakes were confined to two separate
areas. One area was within the caldera and the
earthquakes were apparently associated with
faulting above the magma reservoir. Depth of most
hypocenters was 0—4 km. The largest earthquakes
reached magnitude 5. The other epicentral area
was near the junction between the Krafla fissure
swarm and the Grímsey fault (Fig. 3). The largest
earthquake was of magnitude 6 and the focal
mechanism shows right-lateral strike-slip along the
Grímsey fault. This earthquake sequence demon-
strates well the relationship between rifting along
the diverging plate boundary and transform fault-
ing in the fracture zone. The present Krafla events
are assumed to be the result of interaction between
magma pressure under the Krafla volcano and
rifting of the plate boundary. The rifting is trig-
gered by increasing magma pressure in the reser-
voir and a fluid filled extensional crack propagates
horizontally along the Krafla fault swarm. The
driving force of this process is the tectonic stress at
the plate boundary, but the mode of strain release is
modified by the presence of fluid.
Intraplate earthquakes
Earthquakes are rare outside of the volcanic
zones and the seismic zones in South and North
Iceland. Intraplate earthquakes are known in the
Iceland region, however, for example near the insul-
ar shelf margin east of Iceland. A very significant
sequence of intraplate earthquakes occurred in
Borgarfjördur in West Iceland in 1974. The sequence
lasted more than two months and culminated with a
shock of mb = 5.5. The epicenters were located in
two intersecting linear zones. The main epicentral
zone was about 25 km long and had an E-W trend.
The second zone had a NE-SW trend and inter-
sected the first one in the middle. The depth of the
hypocenters was 0—8 km. Focal mechanism was
determined for the main shock and several small
shocks in the western part of the epicentral zone.
All the obtained solutions show normal faulting,
consistent with observed surface faults. The Borg-
arfjördur area seems to be undergoing horizontal
extension. The direction of the least compressive
stress rotates from the WNW in the center of the
epicentral zone to N-S or even NNE in the western
part. Here the epicentral zone is spatially related to
the Snaefellsnes volcanic zone, where the tectonic
structure is characterized by block faulting on
WNW striking faults and volcanism on WNW
trending lines. This structure also implies horizon-
tal extension in the NNE-SSW direction. The
crustal extension in West Iceland may be caused by
subcrustal flow radially away from a mantle plume
under Central Iceland, but could also be the result
of gravitational stresses induced by the regional
topographic high of Iceland.
SELECTED REFERENCES
Björnsson, A., K. Saemundsson, P. Einarsson, E.
Tryggvason and K. Grönvold, 1977: Current
rifting episode in North Iceland. Nature 266:
318—323.
Björnsson, A., G. Johnsen, S. Sigurdsson, G. Thor-
bergsson and E. Tryggvason, 1979: Rifting of
the plate boundary in North Iceland. J.
Geophys. Res, 84: 3029—3038.
Björnsson, S, 1975: Jardskjálftar á íslandi (Earth-
quakes in Iceland), Náttúrufraedingurinn, 45:
110—133.
Björnsson, S. and P. Einarsson, 1974: Seismicity of
Iceland. In Kristjánsson, L. (Ed.): Geodynamics
of Iceland and the North Atlantic Area. D.
Reidel, Dordrecht, Holland, 225—239.
Brandsdóttir, B. and P. Einarsson, 1979: Seismic
activity associated with the September 1977
deflation of the Krafla central volcano in NE-
Iceland. J. Volcanol. Geothermal Res. (in
press).
Einarsson, P, 1976: Relative location of earth-
quakes within the Tjörnes Fracture Zone. Soc.
Sci. Islandica, Greinar V: 45—60.
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