Jökull - 01.12.1984, Blaðsíða 148

from Akureyri are not sufficient to distinguish
whether the earthquakes were in Grímsvötn or
any other seismic area in western Vatnajökull.
Thórarinsson (1974) was of the opinion that it is
„nearly certain” that these earthquakes were trig-
gered by the subsidence in Grímsvötn. Much
better data collected during later years reveal
that earthquake activity generally does not
accompany jökulhlaups. There is no indication
that the July earthquakes were in any way dir-
ectly connected to the jökulhlaup from Gríms-
vötn. They were a part of the seismicity in west-
ern Vatnajökull that increased greatly in 1954
(Table 1).
1960
A jökulhlaup commenced in the beginning of
January and culminated in the evening of the
24th. No explosion pits formed in the Grímsvötn
valley (Thórarinsson 1974). Earthquakes
originating in western Vatnajökull (possibly in
Grímsvötn) were recorded on the REY, AKU,
VIK and SID seismometers on January 21st at
2021 (GMT, M=3.0), January 22nd at 0626 (M=
3.1) and January 24 at 0141 hours (M=2.8)
Tryggvason (1960) believed that although these
earthquakes might not be physically related to
the jökulhlaup the similarity of this sequence and
the 1954 sequence would be a strong support for
such a relationship. Fig. 2 shows a plot of all
earthquakes in 1952-1983 with epicentral dist-
ance 200—250 km from Reykjavík i.e. in Vatna-
jökull. The seismicity of the Vatnajökull region
increased significantly in 1954. There is no clear
seismic pattern visible except that the seismicity
often seems to diminish markedly or even stop
for a period of time after big earthquakes. An
example of this was in February 1960 following
the M=4.3 earthquake in Bárðarbunga. Accor-
ding to the SID seismometer the January earth-
quakes had different epicentral distances 100-
110 km (2021), approx. 100 km (0626) and
approx. 90 km (0141), indicating that they did not
originate in the same area and most likely their
1962 1963 1964 1965 1966 1967 1968 1969 1970 1971
Fig. 2. The time history of earthquakes (M 3= 3.0) in western Vatnajökull 1952-1982, magnitude
versus time. Jökulhlaups in Skeiðará (G) and Skaftá (S) are shown. Note the absence of a relationship
between jökulhlaups and changes in earthquake activity.
Mynd 2. Myndin sýnir alla jarðskjálfta (M 3= 3.0) með upptök í vestanverðum Vatnajökli á árunum
1952—1982. Stœrð hvers skjálfta er teiknuð á móti tíma. Einnig eru merkt inn Skeiðarárhlaup (G) og
Skaftárhlaup (S). Engin fylgni er á milli jökulhlaupa og breytinga á skjálftavirkni.
146 JÖKULL 34. ÁR