Jökull - 01.12.1984, Síða 144

Fig. 1. A map of Vatnajökull, showing calderas
and inferred subglacial calderas (Sœmundsson,
1982, Jóhannesson et al. , 1982), also the ice
cauldrons associated with jökulhlaups in the river
Skaftá (H. Björnsson 1977) and the cauldron
formed in 1938 during a jökulhlaup in Skeiðará
(Thórarinsson 1974).
Mynd 1. Kort af Vatnajökli er sýnir öskjur,
teiknaðar samkvœmt grein Kristjáns Sæmunds-
sonar (1982), korti Hauks Jóhannessonar o. fl.
(1982) og gervitunglamyndum. Einnig eru sýndir
sigkatlar tengdir Skaftárhlaupum Helgi Björnsson
1977) og ketill sem myndaðist norðan Grímsvatna
í Skeiðarárhlaupinu 1938 Sigurður Þórarinsson
1974).
May—June 1983 authors have started reevaluat-
ing previous data on the volcanic history of
Grímsvötn (Björnsson 1983, Jóhannesson 1983,
Einarsson and Brandsdóttir 1984, Björnsson and
Kristmannsdóttir 1984 and Grönvold and
Jóhannesson 1984). The aim of this study is to
contribute to the discussion of previous volcanic
activity in western Vatnajökull by introducing the
compiled seismic history of the Vatnajökull area
during this century.
SEISMOMETERS IN ICELAND
AND SEISMIC DETECTION FROM
VATNAJÖKULL
The first seismometer in Iceland was installed
in Reykjavík (REY) 1909. This seismometer was
a horizontal Mainka (N), with a pendulum that
had a mass of 135 kgs, and the earth's movement
was magnified by a factor of 80. Another Mainka
(E) seismometer was installed in Reykjavík 1913.
The seismometers were not in operation from
1914-1925. It was not until 1951 that a vertical
Sprengnether short period seismograph, magnify-
ing the earth’s movement 4000 times, was instal-
led in Reykjavík. In 1952 two horizontal Spreng-
nether seismometers were installed in Reykjavík.
The Mainka seismometers were transferred to
Akureyri (AKU) in 1954 and Vík (VIK) 1955. A
vertical Willmore (Z) seismometer was installed
at Kirkjubæjarklaustur (SID) 1957 and in 1964 a
world wide network station was set up at
Akureyri. From 1971, the number of seismo-
meters operating in Iceland has steadily increased
and today there are close to 40 seismographs
around the country.
Because of low magnification the Mainka
seismometers in Reykjavík could not detect
earthquakes with epicentral distance 200—250
km, if their magnitudes were less than 3.5—4.0.
Seismic detection and location accuracy of earth-
quakes from the Vatnajökull area was improved
when the Sprengnether seismometers started
operating in Reykjavík and the Mainka seismo-
meters were moved to Akureyri and Vík. Earth-
quakes with M < 2.5 originating in Vatnajökull
became recordable when the seismometer at
Kirkjubæjarklaustur started operating in 1957.
Because of its favourable position with respect
to Bárðarbunga, Grímsvötn and other seismic
areas in the western part of Vatnajökull, the
seismometer at Kirkjubæjarklaustur made it
possible to distinguish with much better accuracy
between epicenters in these areas, especially if
the earthquakes were big enough to be recorded
at Akureyri as well. In 1973—1977 many new
vertical, short-period stations were installed in
North and South Iceland lowering the detection
threshold to M = 2.0 and improving the location
accuracy of Vatnajökull earthquakes signifi-
cantly. Epicenters, with standard location error
usually Iess than 3 km, have been computed for
most Vatnajökull earthquakes of M 2= 2.5,
recorded since 1975.
The frequency of Vatnajökull earthquakes
increased greatly in 1952—1961 compared to ear-
lier years (Table 1). During these years 104 earth-
quakes (M 2= 3.5) were recorded compared to
less than 20 quakes during the years 1925—1951.
A plot of all Vatnajökull earthquakes (M 2= 3.0)
in 1952—1982 (Fig.2), shows a marked increase in
earthquake magnitude in 1974—1980, during
which 7 earthquakes with M ^ 5.0 were
142 JÖKULL 34. ÁR