Jökull - 01.01.2014, Side 65

Earthquake Sequence 1973–1996 in Bárðarbunga volcano
ity was shallow (<10 km) (Björnsson and Einarsson,
1990). Einarsson (1991) noted that earthquakes in the
volcanic zone in the Vatnajökull region do not delin-
eate major plate boundaries, but cluster on the central
volcanoes, i.e. on Bárðarbunga, Grímsfjall, Hamar-
inn and Kverkfjöll, as well as the east-west trending
volcanic Loki Ridge (Lokahryggur in Icelandic) (Fig-
ure 1). Of these, Kverkfjöll have been the least seis-
mically active since the 1970s. The seismicity also
shows that the stratovolcano Öræfajökull, which lies
south of the volcanic zone, has been even less seis-
mically active than Kverkfjöll. The seismic cluster-
ing around the Vatnajökull volcanoes has been ex-
plained partly by stress changes associated with a de-
flating magma chamber in the case of Bárðarbunga
(Einarsson, 1991) and an inflating magma chamber
in the case of Grímsvötn (Einarsson and Brands-
dóttir, 1984). Although the seismicity does not de-
lineate the major plate boundary faults, they may
align on smaller faults. The seismicity around Bárð-
arbunga does e.g. delineate an arch shaped structure
that approximates the caldera rim fault (Björnsson and
Einarsson, 1990).
Eruptions and seismicity, short term correlation?
In the period 1934–1996 there have been 4 con-
firmed eruptions in Vatnajökull and one small erup-
tion (0.1 km3) in the Askja volcano, 25 km north
of Vatnajökull (Einarsson, 1962; Jóhannesson, 1983;
Björnsson and Einarsson, 1990) (Figure 2). Seis-
mic tremor signals associated with jökulhlaup from
sub-glacial geothermal areas in the Loki Ridge area
were interpreted as small subglacial eruptions (Þor-
bjarnardóttir et al., 1997; Einarsson et al., 1997), but
their certainty has not been confirmed by other meth-
ods. Three additional eruptions suggested in Gríms-
vötn in the years 1933, 1945 and 1954, respectively
(Jóhannesson, 1983, 1984), have been disputed (Gud-
mundsson and Björnsson, 1991).
An intense earthquake swarm was recorded on
the Reykjavík seismometers on March 18–19th 1945.
Based on S–P time, the location of this event was
traced to the Vatnajökull region (Tryggvason,1978b).
The largest of these earthquakes was 41/4 ML. It
was followed by a single earthquake on March 20th
(ML=41/4) in similar location. An experienced Vatna-
jökull traveller visited Grímsvötn in the summer of
1944 and again in July 1945. In July 1945 he ob-
served at the southern rim of the subglacial Grímsvötn
caldera a new opening with turbulent boiling water in
the ice cover that floats on the subglacial lake (Þórar-
insson, 1974). No signs of such activity were seen the
year before. Two months after his visit in July 1945,
a jökulhlaup burst out from Grímsvötn. Björnsson
and Guðmundsson (1993) have estimated the thermal
output of the Grímsvötn caldera between 1922–1991.
During this period the largest heatflux in Grímsvötn
is associated with the 1934 Grímsvötn eruption. The
second largest heatflux pulse in Grímsvötn, accord-
ing to this report, occurred in the years 1945–1948.
From the observations described, it is proposed, that
the earthquake swarm of March 18–19th in 1945 sig-
nifies the onset of an eruption in Grímsvötn, which
was never directly observed.
All confirmed eruptions in Vatnajökull in 1934–
1996 were accompanied by earthquakes of magnitude
4 or more within 2 months of the beginning of erup-
tions (Figure 2). The last 3 eruptions in Grímsvötn
(1998, 2004 and 2011, respectively) were all accom-
panied by events of magnitude 4.0 or larger (ML or
mb) on the first day of eruption. During the eruption
of Grímsvötn in 1983, earthquakes of this magnitude
did, however, not happen (ISC, 2012). The recipro-
cal relationship does not hold, i.e. not all earthquakes
of magnitude 4 and greater are associated with erup-
tions within the Vatnajökull region. As many of the
eruptions in the Vatnajökull area are accompanied by
earthquakes smaller than Mc (see definition above),
the disputed eruptions of Grímsvötn in 1933, 1945
and 1954, cannot be rejected on the ground of the seis-
mic bulletins available.
Subsurface pressure connection between volca-
noes in Iceland and elsewhere has been postulated
(Einarsson, 1991; Gonnermann et al., 2012). There-
fore, it is tempting to ask if volcanic events occur-
ring close to Vatnajökull may have stimulating seis-
mic effects on the Vatnajökull region. The neighbour-
ing Askja eruption in 1961 does not seem to have in-
duced seismicity at the level > 4.0 in Vatnajökull. On
the contrary Vatnajökull remained seismically quiet at
the time (Figure 2).
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