Jökull - 01.12.1959, Blaðsíða 16

að loka augunum fyrir þeirri staðreynd, að land
okkar er land náttúruhamfara, eldgosa, jarð-
skjálfta og ferlegra vatnsflóða. Okkur tjóar ekki
að smeygja okkur undan ábyrgð með því að
láta sem við vitum ekki, hverju við getum átt
von á í sambandi við næsta Ivötlugos. Fyrir því
hafa séð þeir vakandi og samvizkusömu klerk-
ar, klausturhaldarar og sýslumenn, sem settust
niður og skráðu við skímu frá skjá eða glætu
frá tólgarkerti sannsögulegar og greinargóðar
frásagnir af þeim náttúruhamförum, er yfir
dundu í þeirra tíð, svo að síðari kynslóðir
mættu þar af einhvern lærdóm draga.
SUMMARY:
ON THE POSSIBILITIES OF PREDIC-
TING THE NEXT ERUPTION
OF KATLA
By S. Thorarinsson
Hidden beneath the ice cover of the intake
area of Kötlujökull (also called Höfdabrekku-
jökull), a SE outlet of Mýrdalsjökúll, is one of
Iceland’s ?nost active volcanoes, Katla. The
eruptions of this volcano have often caused
great damage, both because of heavy tephra
falls and because of the tremendous jökulhlaups
(glacier bursts) which accompany the eruptions.
From 1580 onwards we have preserved written
contemporary records of all Katla eruptions but
before 1580 the records are less reliable. Tephro-
chronological studies have proved that there
was a big eruption in Katla about 1490 A. D.
and the tephra was carried ivestwards.
Since 1580 the average length of interval be-
tween Katla’s eruptions has been 42 years and
the largest deviation from that interval is 26
years. Curiously enough short and long inter-
vals liave occurred alternatively so that eruptions
have found place either at the end of the
second or the sixth decade of each century, with
a maximum deviation of only five years from
this rule. Katla erupted in 1625, 1721, 1823 and
1918 and on the other hand in 1660, 1755 og
1860, and it is thus no wonder that people are
beginning to ask: Is not the next Katla erup-
tion bound to start soon?
One of the problems the Iceland Glaciological
Society has tried to tackle is how to predict
subglacial erúptions and the jökulhlaups accom-
panying them. The Society’s resources, however,
are limited, a:id it is not much it has been able
to do hitherto with regard to Katla. The
ablation area of Kötlújökull ivas mapped in
1949 by an exploration party from Durham
University, England, led by Hal Lister. Seismic
soundings of the ice tlúckness within the intake
area ivere carried out in June 1955 by a French
geophysicist, Jean Martin, ivho took part in an
Icelandic expedition to this area. The average
thichness of the ice cover in the area ivas found
to be about 300 m.
The small jökulhlaup — nearly on experi-
mental scale — from the Katla area on June 25,
1955 proved that some subglacial melting is
going on in this area and it also gave a clue to
the understanding of the very high discharge
maximum of the jökulhlaups from Katla, com-
pared with tlie total amount of water dischar-
gecl.
Kötlujökull and its intake area were photo-
graphed from air in the summer of 1955 and
reconnoitring flights have since then been macle
once or txvice a year in order to study eventual
changes within the area.
Efforts have also been made to try to predict
the next Katla eruption with the lielp of seis-
mographs vhich have been installed both at
Vík (1955), 23 km S of Katla and at Kirkju-
bœjarklaustur (1958), 53 km ENE of tlie volcano.
Studies of the ivritten records of Katla erup-
tions since 1625 lead t.o the conclusion that earth-
quakes strong enough to be felt by most in-
habitants of the village Vik will nearly certainly
occur at least one hour before the jökulhlaup
cuts off the motor-road over Mýrdalssandur.
And as Vík lies outside the main seismic areas
in lceland and earthquakes so strong as to be
felt by most people hardly ever occur there,
except at the beginning of the Kat.la eruptions,
it seems justified to proclaim that all traffic
over Mýrdalssandur should be stopped imme-
diately and the districts Alftaver and Medal-
larid, which have most to fear from the next
jökulhlaup, should. be warned as soon as such
earthquakes are felt in Vík. The traffic over
the sandur should not be allowed again until
it has been established that the focus of the
earthquake is not xvithin the Katla area.
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