Jökull - 01.12.1984, Page 149

epicenters were north or northwest of Gríms-
vötn. The first two earthquakes nray be regarded
as foreshocks of the M = 4.3 earthquake in
Bárðarbunga on February 21st, suggesting that
the January earthquakes were unrelated to
Grímsvötn and the jökulhlaup.
1965
Jökulhlaup began in the end of August and
culminated on September the 7th ( Rist 1973).
Four Vatnajökull earthquakes (M > 3.0) were
recorded in Reykjavík that year, in May and
November. (Fig. 2). Apart from these quakes
many smaller ones were recorded on the AKU
and SID seismometers. Four smaller earthquakes
with epicenters possibly in Grímsvötn coincide
with the culmination of the jökulhlaup on the 8th
of September i.e. at 1856 (M=2.4), 1857 (M=
2.4), 1857 (M=2.2) and 1858 (M=2.5). Accord-
ing to the Seismological Bulletin (1965) they all
originate in the Grímsvötn area and the bulletin
comments, „some more tremor during this time”.
This tremor was recorded on the SID seismo-
meter and does not resemble any known volcanic
tremor but the wind tremor often recorded at
SID.
1972
A jökulhlaup culminated on March 23th ( Rist
1973). During February and March three Vatna-
jökull earthquakes were recorded but none of
them seems to originate at Grímsvötn (Fig. 2).
An unusual tremor was recorded at Skammadals-
hóll (SK) on March 13 1900-March 14 0500 and
on March 20th at 1800-2400. The seismogram
from SID on March 13 is not readable but on the
20th of March wind disturbance was recorded
during the evening, especially from 1900—2000
hours, and culminating around 1200 hours on the
21 st. Helgi Björnsson and Magnús Hallgrímsson
were stationed in Grímsvötn from March 28th
until April the 4th. They felt earthquakes twice
during the night between March 28 and 29
{Björnsson and Hallgrímsson 1976), but nothing
was recorded on the seismometers at SID and
SK. Helgi and Magnús operated a seismometer
during their expedition in Grímsvötn. The
seismometer recorded many small ice shocks
(Björnsson pers. comm. 1984). According to
analysis of the sediment discharge in Skeidará
during the jökulhlaups in 1965, 1972 and 1976,
the ratio of dark glass/light glass in the suspended
sediment was exceptionally high in 1972. The
coarse silt fraction became almost 90% glass with
the same refractive index as the Grímsvötn volca-
nic ash (Tómasson 1974, Tómasson et al. 1980).
Tómasson (1974) explained this exceptional
increase in the glass ratio as a result of a small
subglacial eruption that lasted less than a day and
produced big quantities of sand and silt that was
carried away with the jökulhlaup water. Stein-
thórsson and Oskarsson (1983) concluded that
their geochemical studies of jökulhlaup water in
1972 supported the idea of a volcanic event.
Taking into account all available seismic data
collected before and after 1972 nothing supports
the idea of a small volcanic event in 1972 or in
later years until May 1983 when a small eruption
was discovered from seismic analyses ( Einarsson
and Brandsdóttir 1984).
1976
Jökulhlaup culminating on Sept. 22nd ( Rist
1976). Helgi Björnsson and Magnús Hallgríms-
son were stationed in Grímsvötn from Sept. 22nd
until October the 6th. They did not sense earth-
quakes when the jökulhlaup was at its peak
(Björnsson and Hallgrímsson 1976). No Vatna-
jökull earthquakes were recorded during the four
months following a big Bárðarbunga earthquake
in July 1976 (Fig. 2).
1982
An unusually small jökulhlaup occurred in
February (Björnsson 1983). No earthquakes (Fig.
2).
CONCLUDING REMARKS
Available data from the jökulhlaups and erup-
tions described in this chapter is summarized in
Table 2, along with all available data on the
earthquake activity. Note that no reliable reports
are of earthquakes felt during these events and
only twice have they been accompanied by earth-
quakes (M > 3.5).
EARTHQUAKESAND
SKAFTÁRHLAUPS
According to farmers in Skaftártunga and
Meðalland jökulhlaups in Skaftá and outlet rivers
were frequently observed in the first half of this
century ( H. Björnsson 1977). During the Skaft-
árhlaup in 1955 an ice cauldron was formed about
15 km NW of Grímsfjall ( Thórarinsson and Rist
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