Jökull - 01.12.1984, Side 145

period M>3.5 M>3.0
1925-1931 (0) 9 •
1932-1941 (6) ?
1942-1951 (1-10) ? #
1952-1961 46 88
1962-1971 20 61
1972-1981 38 97
Table 1. Frequency of Vatnajökull earthquakes
in the magnitude range 3.5 > M ^ 3.0 and M ^
3.5. The seismicity can be divided into two peri-
ods. A low seismicity period 1925—1953 and
higher seismicity period since 1954. The higher
seismicity period coincides with increased geo-
thermal activity in the cauldron area northwest of
Grímsvötn. (Fig. 1).
Tafla 1. Taflan sýnir fjölda Vatnajökulsskjálfta á
stœrðarbilinu 3.5 > M > 3.0 og M g- 3.5 á
árunum 1925—1981. Skjálftavirknin eykst til
muna 1954, um leið og vart verður aukningar í
jarðhita á sigkatlasvœðinu norðvestan Grímsvatna
(Mynd 1).
recorded. All these large earthquakes originated
in the Bárðarbunga area ( Páll Einarsson , pers.
comm.).
SKEIÐARÁRHLAUPS AND
ERUPTIONS IN WESTERN
VATNAJÖKULL THIS CENTURY.
1903
On the 25th of May 1903 local farmers at
Skaftafell noticed that a jökulhlaup was begin-
ning in Skeiðará. During the following days the
flow rate of Skeiðará steadily increased to reach a
maximum on the 28 and 29 of May. Uncertain
reports from people in Öræfi tell of earthquakes
felt on two nights (no dates) during the jökul-
hlaup along with heavy rumble and crashing noises
from the jökulhlaup which in a western breeze
could be heard in Hornafjörður 100 km east of
Skeiðará. The quakes and noises reached their
maximum in the afternoon of May 28th and the
following night. A great steam and ash coluntn
indicating eruption was first noticed Thursday
afternoon the 28th of May coinciding within few
hours of the jökulhlaup maximum. The eruption
originated in the vicinity of Thórðarhyrna 20 km
SSW of Grímsvötn (Thórarinsson 1974). On the
other hand, (Haukur Jóhannesson 1983) has sug-
gested that unusual water bursts in glacier rivers
north of Vatnajökull in December 1902 and May
1903 were caused by an eruption in the Gríms-
vötn area that continued in 1903, at the same
time as the Thórðarhyrna eruption. He considers
the Grímsvötn eruption to be the largest during
this century. The tephra fall during this eruption
was greater than in most Grímsvötn eruptions but
the composition of the tephra was the same as
that of Grímsvötn tephra (Thórarinsson 1974),
suggesting a relationship of the Thórðarhyrna
eruption with the Grímsvötn volcano. The erup-
tion was visible on and off until January 1904. No
reports are of felt earthquakes during the erup-
tion. In 1903 seismometers had not been installed
in Iceland.
1910
An eruption believed to originate in western
Vatnajökull, close to Hamarinn, was first noticed
on the 18th of June 1910. Tephra fall from this
eruption was frequent in August and lasted until
October same year ( Tltórarinsson 1974). There
are no reports of felt earthquakes or jökulhlaups
related to this eruption. No earthquakes were
recorded on the Reykjavík seismometer, indicat-
ing that if there were earthquakes associated with
this eruption, they were all smalier than magni-
tude 3.5—4.0.
1913
A large jökulhlaup discharged to Skeiðará in
April 1913 but no visible eruption or tephra fall
reported (Thórarinsson 1974). Askelsson (1936,
p. 12) claimed that „in all probability” there was
an eruption in Grímsvötn accompanying the
jökulhlaup. Jóhannesson (1983) pointed out, that
an eruption in Hekla coinciding with the jökul-
hlaup might have drawn attention from a possible
Grímsvötn eruption at the same time. According
to a diary of Ari Hálfdánarson froni Kvísker an
earthquake was felt on the 25th of April 1913, but
he does not comment on the intensity of the
quake (Thórarinsson 1975). No earthquakes that
could possibly originate in Vatnajökull were
recorded in Reykjavík during that time. There-
fore the earthquake on April 25th was probably a
local quake in Öræfi.
JÖKULL 34. ÁR 143