Jökull - 01.12.1984, Side 150

1955). Another cauldron approximately 4 km
west of the 1955 cauldron was first noticed in
1971. H. Björnsson (1977) was of the opinion that
the jökulhlaups in Skaftá are caused by a draining
of a water cupola formed by geothermal meltwa-
ter trapped beneath a depression in the glacier
surface. Since 1954 16 Skaftárhlaups have occur-
red, 12 draining the eastern (1955) cauldron and
4 draining the western cauldron (Björnsson
1983). None of these jökulhlaups have been
accompanied by increased seismic activity (Fig.
2), and nothing in the seismic data indicates that
the Skaftárhlaups could be related to volcanic
activity in the cauldron area, at the time of each
jökulhlaup. It is noteworthy, that the increase in
the Vatnajökull seismicity since 1954 is followed
by a volume increase in Skeiðarárhlaups since
1955.
INTERPRETATION AND
CONCLUSIONS
The Vatnajökull seismicity during this century
can be divided into two periods, a low seismicity
period from 1909—1914 and 1925—1953 and a
higher seismicity period from 1954 (Table 1,
Fig.2). None of the earthquakes recorded since
seismic observations began in 1909 were felt in
inhabitated areas, not even the largest ones, such
as the Bárðarbunga earthquakes in 1974—1980
with M > 5.0. The higher seismicity period from
1954 coincides with increased geothermal activity
northwest of Grímsvötn, most likely resulting
from increased magmatic activity at shallow
levels.
Jökulhlaups are generally not found to be
accompanied by earthquakes. Two eruptions
in western Vatnajökull this century have pro-
duced earthquakes with M 3= 3.5, i.e. the erup-
tions in Grímsvötn in 1934 and 1983 (see Table
2). The Reykjavík seismometers were not in
operation during the eruption in 1922. The erup-
tion of 1910, and the probable eruptions of 1938,
1945 and 1954 were not accompanied by signifi-
cant earthquakes.
Before 1957 eruptions in Vatnajökull could
occur undetected by seismometers.
ACKNOWLEDGEMENTS
Páll Einarsson provided unpublished data and
suggested several improvements on the manu-
script.. Ragnar Stefánsson at the Meteorological
Office permitted access to old seismograms.
Beg Month Year Seismometers in operatioa uj^^2n:i/)cokh>h-.cqli_ cc<>lolooi/)cc5é:5:<o Earthq. felt rec. Eruption Grímsv. other SkeiSarár- hlaup km3
May 1903 ? ? X 5
June 1910 | ? x No
April 1913 I ? 6
Sept. 1922 X 6
Sept. 1927 ? No
Nov. 1933 ? ? No
Mar. 1934 X x 6-7
Dec. 1934 9 9 No
May 1938 ? 6-7
June 1939 ? -1
May 1941 ? 1-2
Sept. 1945 ? 2-3
Feb. 1948 ? 2-3
July 1954 ? 3.5
Jan. 1960 3.0
Aug. 1965 3.0
Mar. 1972 3.2
Sept. 1976 24
Feb. 1982 1.3
May 1983 x X No
Dec. 1983 I I I 0.55
Table 2. The table lists the beginning month and
year of all Skeiðarárhlaups, possible eruptions in
Grímsvötn and in other volcanic areas in
SW-Vatnajökull, during this century. Principal
seismometers in operation during each event and
available data on the eartquake activity is also
shown. (Reykjavík (REY), Akureyri (AKU),
Vík (VIK), Kirkjubæjarklaustur (SID), Skamma-
dalshóll (SK), Grímsstaðir (GS), Snæbýli (SB),
Reynihlíð (RI), Kvísker (KV), Miðfell (MI), Að-
alból (AB) and Grímsfjall (GF)). Note that no
reliable reports are of earthquakes felt during
these events and only twice have the been accom-
panied by earthquakes M & 3.5.
Tafla 2. Taflan sýnir öll Skeiðarárhlaup og eldgos
sem vitað er um á þessari öld, einnig alla tiltæka
vitneskju um jarðskjálfta. Athyglisvert er, að eng-
ar áreiðanlegar heimildir eru um að skjálftar hafi
fundist í þessum umbrotum og einungis tvisvar
hafa skráðst skjálftar M 3= 3.S. í bœði skiptin
fylgdu skjálftarnir eldgosi í Grímsvötnum.
REFERENCES
Áskelsson, Jóhannes 1934: Á Vatnajökli. Ferða-
sögubrot eftir Jóhannes Áskelsson. (An
excursion at Vatnajökull, in Icelandic). Les-
bók Morgunblaðsins 9. árg. 24. og 25. tbl.
185-187 og 193-196.
Askelsson, Jóhannes 1936: On the Last Eruptions
in Vatnajökull. Soc. Sci. Islandica 18: 55p.
Áskelsson Jóhannes 1938: Rannsókn eldstöðv-
anna í Vatnajökli, ferðasaga. (The Vatna-
jökull expedition May—June 1938, in Ice-
landic). Morgunblaðið 9 júní 1938.
Björnsson, H. and M. Hallgrímsson 1976:
Mælingar í Grímsvötnum við Skeiðarárhlaup
1972 og 1976. (Measurements in Grímsvötn
148 JÖKULL 34. ÁR