Jökull - 01.12.1984, Page 146

1922
In late September 1922 a large jökulhlaup com-
menced. This Skeiðarárhlaup was accompanied
by an eruption in Grímsvötn, first noticed on the
29th of September. The jökulhlaup culminated
on the 4th of October on the same day as the
eruption became more vigorous. Earthquakes
were felt (Thórarinsson 1974). The reports of felt
earthquake(s) during this eruption came from
Vestfirðir (Hesteyri, Vatnsfjörður and Barða-
strönd) on the 4th of October. Reports on the
eruption from other parts of Iceland did not
mention earthquakes although heavy rumble
(dynkir) could be heard there as well as at Vest-
firðir (Thorkelsson 1923b). It is difficult to
believe that the reported earthquake(s) felt at
Vestfirðir (320—350 km from Grímsvötn) are
related to the Vatnajökull eruption. If earth-
quakes originating in Vatnajökull were felt at
Vestfirðir their consequences must have been
catastrophic in the inhabitated areas around Vatna-
jökull. Either the quake(s) were local in Vest-
firðir (very unusual) or what is more likely, they
were detonations that originated in Grímsvötn.
Such detonations have been observed before e.g.
in the Hekla eruption in 1947 (Thórarinsson
1950). The first seismometer in Reykjavík was
managed in cooperation with the International
Association of Seismologists stationed in Strass-
burg. During the war this cooperation was cut off
and limited finances along with unsuitable hous-
ing prevented seismic recordings in 1918—1925
(Thorkelsson 1923a).
1927
Sigurður Björnsson (1977) suggests, that prob-
ably an eruption took place in Vatnajökull in
September 1927. A large jökulhlaup came in
Jökulsá á Breiðamerkursandi in the beginning of
September 1927. The jökulhlaup was not big
compared to Skeiðarárhlaups. Strong sulphuric
stench was felt on Breiðamerkursandur that
autumn and once northerly winds carried ash
down Breiðamerkurjökull. No reported or instru-
mentally recorded earthquakes.
1933
From November 30th to December the lOth
1933 numerous observers from North and South
Iceland report fires and red glows that judged
from directions given seem to have originated in
southwest Vatnajökull. No tephra fall was
reported (Áskelsson 1936, p.15-16). No earth-
quakes originating in Vatnajökull were recorded
in Reykjavík during this time.
1934
A jökulhlaup in Skeiðará began on March 22.
First reports of an eruption came at 2030 hours
on the 30th at the same time as the jökulhlaup
culminated. The eruption column reached max-
imum height on April the 3rd, viewed from Núps-
staður 50 km SSW of Grímsvötn. The eruption
was not visible from inhabited areas after 7th of
April (Thórarinsson 1974). Áskelsson (1934) was
at the eruption site on the 13th of April and
observed two craters at the SE caldera wall in
Grímsvötn. The only report of earthquakes felt
during this eruption came from Nielsen (1937,
p.56), who felt a small earthquake on the 30th of
April when on an expedition in Grímsvötn. The
expedition discovered that three craters had
formed in Grímsvötn. The seismometer in
Reykjavík recorded five earthquakes with
epicentral distance approximately 200 km on the
30th of March at local hours: 2004 (M=4.5), 2057
(M=3.75), 2102 (M=3.75), 2106 (M=3.5) and
2134 (M=3.75). The largest quake was also
recorded on stations in Greenland and the
United Kingdom indicating that the epicenter
was east of Reykjavík. From this information
Tryggvason (1960, 1978a) concluded that the
earthquakes originated in Vatnajökull and were
related to the eruption. Apart from these five
earthquakes traces of smaller quakes with similar
appearance can be seen on the seismogram. This
earthquake sequence resembles the 1983 erup-
tion earthquakes as relates to magnitude distribu-
tion and maximum magnitude (Einarsson and
Brandsdóttir 1984). Tryggvason (1960) stated
that the simultaneous occurrence of earthquakes
and the visible eruption indicated, that the erup-
tion had no subglacial (not visible) stage but
commenced at the time of the first recorded
earthquake at 2004. Hence this sequence would
support, the theory that the jökulhlaup triggered
the eruption. Thórarinsson (1974) stated that the
earthquakes, especially the first one, were so
large that they must partly be caused by the
eruption, although the subsidence of the ice
cover of the Grímsvötn lake also triggered the
earthquakes (Tltórarinsson 1974 p. 151). How-
ever, no earthquakes have been found to accom-
pany later jökulhlaups. It is therefore concluded
144 JÖKULL34. ÁR