Jökull - 01.12.1960, Blaðsíða 21

TABLE I
Earthquakes recorded in Reykjavík at times of jökulhlauþs from Icelandic glaciers.
(M = magnitude of the earthquakes).
Year Date Origin time M
GMT
1934 March 30 21 04 13 4i/,
March 30 21 57 22 33/4
March 30 22 02 22 33/
March 30 22 06 07 314
March 30 22 34 27 33/
1954 Juiy 17 08 22 13 3.3
Juiy 17 16 04 47 2.9
Juiy 21 05 00 18 3.1
1955 June 25 14 32 13 2.4
June 25 17 20 40 2.5
June 25 17 31 27 2.9
June 25 17 41 50 2.8
June 25 18 01 10 2.9
June 25 18 03 54 3.2
June 25 18 22 31 3.1
June 25 19 00 37 3.6
1960 Jan. 21 20 21 20 3.0
Jan. 22 06 26 16 3.1
Jan. 24 01 41 44 2.8
of water is the primary cause of the earth-
quakes.
3. Same as 2, except that the pressure decrease
releases a volcanic eruption, and the earth-
quakes are caused by the eruption.
4. The earthquakes are caused by fracturing
in the glacier ice, as the glacier roof of a
water container, beeing emptied by the
jökulhlaup, collapses.
5. The earthquakes are the primary cause of
the jökulhlaup, by weakening an ice dam,
which has kept the water in a reservoir.
These hypotheses regarding the physical rela-
tion between earthquakes and jökulhlaups in
the cases included in Table I will now be
discussed briefly.
Grimsvötn, March 1934:
The recorded earthquakes occurred simulta-
neously with the commencement of the visible
Remarks
Jökulhlaup from Grimsvötn had been increas-
ing for several days, to reach a maximum at
the time of the recorded earthquakes (Rist
1955). A visible eruption commenced at the
same time.
Jökulhlaup from Grímsvötn had been increas-
ing for several days, to reach a maximum on
Julyl7—18. No visible eruption (Thorarinsson
1954 a).
Jökulhlaup from Mýrdalsjökull commenced at
about 20h GMT, lasted only few liours. No
visible eruption (Thorarinsson 1955).
Jökulhlaup from Grimsvötn reached its max-
imum on Jan. 24, after having been increasing
for more than two weeks.
volvanic eruption, and some 8 days after the
beginning of the jökulhlaup. The simultaneous
occurrence of earthquakes and the visible erup-
tion supports the opinion, that the eruption
had no subglacial (not visible) stage but com-
rnenced at the time of the first recorded earth-
quake, at 20h 04m icelandic tirne. This indicates,
that the explanation no. 3 is valid for this
case.
Grimsvötn, January 1960:
Here exactly the same arguments can be
stated as regarding the jökulhlaup of July 1954
and the same conclusion will be obtained. Tlie
Grímsvötn region is seismically active, not only
at times of jökulhlaups, so there is a possibility
of there being no physical relation between
the recorded earthquakes and the jökulhlaups
of 1954 and 1960, but the similarity in the
sequences is a strong support for a physical
relation.
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