Jökull - 01.12.1984, Side 143

Seismic Activity in Vatnajökull in 1900—1982
with Special Reference to Skeiðarárhlaups,
Skaftárhlaups and Vatnajökull Eruptions
BRYNDÍS BRANDSDÓTTIR
Science Institute, University of Iceland.
Dunhagi 3, 107 Reykjavík.
ABSTRACT
Seismometers have been operated in Iceland
since 1909. The seismic detection threshold of
Vatnajökull earthquakes was M — 3.5 until 1951
when a new seismometer with higher sensitivity,
lowered the detection threshold to M > 2.5. The
location accuracy of Vatnajökull earthquakes
improved significiantly when a seismometer was
installed at Kirkjubœjarklaustur in 1957, and
again when the world wide network station was
installed at Akureyri in 1964. Since 1964 the num-
ber of seismic stations has increased from 4 to 38
lowering the detection threshold of Vatnajökull
earthquakes to M — 2.0. Epicenters, with standard
location error less than 3.0 km, have been com-
puted for earthquakes with M 3= 2.5, recorded in
the last 9 years.
Since 1903 sixteen jökulhlaups have occurred in
Skeiðará. Twice have they definitely been accom-
panied by eruptions in Grímsvötn, in 1922 and
1934. In 1922 the seismometer in Reykjavík was
not in operation but in 1934 five earthquakes of M
55 3.5 were recorded. In 1910, an eruption in
vicinity of Hamarinn did not produce earthquakes
big enougli to register on the Reykjavík seismo-
meter, i.e. with M 3=3.5. Eruptions in Vatnajökull
before 1957, such as the one in 1910, could occur
undetected by seismometers. After the seismo-
meter was installed at Kirkjubœjarklaustur and the
Akureyri WWSSN station started operating, an
eruption in Vatnajökull would have been seis-
mically detected. No earthquakes have been found
to accompany jökulhlaups.
The Vatnajökull seismicity since 1909 can be
divided into two periods. A low seismicity period
from 1909—1914 and 1925—1953, during which
less than 15 earthquakes (possibly only 5) with M
3= 3.5 were recorded and a higher seismicity
period from 1954, with more than 100 earthquakes
of M 3= 3.5. The higher'seismicity period coincides
with increased geothermal activity in the cauldron
area northwest of Grímsvötn.
INTRODUCTION
During the last 15 years the number of seismic
stations operating in Iceland increased from 6 to
38, greatly increasing the data available for seis-
mic studies. The seismic stations around Vatna-
jökull have, since installed in 1971-1977, pro-
duced data that not only yielded new information
on the Vatnajökull seismicity, but also made
possible reevaluation of previously reported and
instrumentally recorded data.
The Vatnajökull ice sheet covers a group of
central volcanoes. A few of them have developed
calderas (Fig. 1). The Grímsvötn volcano with its
6—8 km wide caldera has been one of the most
active volcanoes in Iceland during historical
times. A subglacial, ice dammed lake is situated
in the Grímsvötn caldera, fed by a powerful
geothermal area, that constantly melts the glacier
ice from below. The melt water accumulates in
the lake until a critical water level is reached, at
which the lake is partly drained, in a sudden burst
lasting from several days to a few weeks. The
glacier bursts (jökulhlaups in Icelandic) run
beneath the glacier to the glacier rivers that drain
Skeiöarárjökull (Skeiðarárhlaup).
At least two geothermal areas are situated in
Vatnajökull NW of Grímsvötn. There melt water
is also accumulated and released into the glacier
river Skaftá (Skaftárhlaup).
The volcanic history of Grímsvötn has been
thoroughly described, (see e.g. Thorkelsson
1923b, Áskelsson 1936, Nielsen 1937 and Thór-
arinsson 1974). In their conclusions, these investi-
gators have disagreed on the number of eruptions
in Grímsvötn this century. After the eruption in
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