Jökull - 01.12.1984, Blaðsíða 147

that the earthquakes recorded in March 1934
were purely of volcanic origin. Whether the
jökulhlaup triggered the eruption or not is
impossible to judge from the data available.
1934
On December 23rd 1934 many reports came
from N—Iceland that flames and red glow were
visible in the direction towards Vatnajökull.
Eruption was also reported on December 21st
and 26th. Possibly a minor eruption took place in
Vatnajökull, (in Grímsvötn ? ) (Askelsson 1936).
No recorded earthquakes can be related to the
eruption.
1938
Jökulhlaup began on the 23rd of May and
peaked on the 26th of May. This jökulhlaup was
unusual. It came earlier than expected, rose to a
maximum in three days and receded slowly in two
weeks. Aerial observations of Grímsvötn on the
28th of May revealed a subsidence not only of the
caldera floor but also in an area north of it
('Thórarinsson 1974). Thórarinsson was of the
opinion that a small subglacial eruption may have
caused the subsidence north of Grímsvötn.
Björnsson (1983) has shown that the heat flux of
the Grímsvötn geothermal area increased from
5000 MW to 15000 MW in 1938, indicating erup-
tive activity. Earthquakes were not mentioned
except a questionable report from Rangárvellir
(Tryggvason 1978a). There people felt earth-
quakes (no dates) which they thought were
associated with the jökulhlaup. They must, how-
ever, have been small local quakes as they were
not felt in Öræfi and no earthquakes were
recorded in Reykjavík. If a magma migration at
shallow depths or a small subglacial eruption
occurred in this event, it did not manage to
produce recordable earthquakes (i.e. M > 3.5).
1939
A small jökulhlaup in Skeiðará culminated
on the 15th of June. Thórarinsson (1974)
related this burst to the emptying of ice dammed
lakes. Björnsson (1983) pointed out, that the rate
of accumulation of water in Grímsvötn was rather
high following the events in 1938 and that the
1939 jökulhlaup had a sulphurous odour which
disproves the former hypothesis that the flood
originated from draining of a marginal lake. No
earthquakes.
1941
A small, exceptionally slowly increasing
jökulhlaup culminated on the 16th of May (Thór-
arinsson 1974). No earthquakes.
1945
People visiting Grímsvötn on the 26th and 27th
of July 1945 discovered increased geothermal
activity at the south caldera rim. A jökulhlaup
commenced on the 16th of September and
culminated eleven days later (Thórarinsson
1974). Áskelsson (1959) was at Grímsvötn on the
5th of October. He discovered a small crater in
the northwestern caldera wall in Grímsvötn and
judging from the small amount of tephra and
sand that was erupted claims that a small érup-
tion produced the pit, probably on the night
before the 26th of September. He was also of the
opinion, that there was a relationship between
the initial eruption and the jökulhlaup such, „that
the flow rate of the jökulhlaup begins to drop
rapidly when the eruption surfaces”. No earth-
quakes were recorded or felt during this jökul-
hlaup.
1948
Jökulhlaup in the beginning of February,
culminating on the 23rd. No eruption in Gríms-
vötn (Thórarinsson 1974). According to
Björnsson (1983) the rate of melting at Gríms-
vötn was about 50% higher in 1948 than in an
average year. No earthquakes with possible
epicenters in Vatnajökull were recorded on the
seismometers from April 1945 to February 1952
(Tryggvason 1978b, 1979).
1954
Jökulhlaup in July, culminating on the 18th
(Rist 1955). An explosion pit was formed in the
northern part of the Grímsvötn caldera and
another at the southern caldera rim; according to
Thórarinsson's (1974) interpretation, formed by
steam explosions. The new seismometer in
Reykjavík recorded three Vatnajökull earth-
quakes in July, July 17th at 0822 (M=3.3), July
17th at 1604 (M=2.9) and July 21 at 0500 hours
(M=3.1). A total of 26 Vatnajökull earthquakes
(M ^ 2.5) were recorded during that year.
According to the seismometer at Akureyri the
epicentral distance of the earthquakes is 140 km
wich could place them in Grímsvötn (Tryggva-
son 1960, 1973, 1979). The seismic recordings
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