Jökull - 01.01.2019, Blaðsíða 43

Einarsson
Table 1: Precursor times of historical Katla eruptions. – Tafla 1. Forboðar eldgosa í Kötlu á sögulegum tíma.
Eruption Day Duration Precursor time Jökulhlaup time
1311 January 25 ? 15 days? ?
1580 August 11 ? ? ?
1625 September 2 12 days short 3 hours
1660 November 3 9 days 1 hour 3–4 hours
1721 May 11 ∼5 months 4 hours 5 hours
1755 October 17 >21 days >6 hours >12 h
1823 June 26 4 weeks 3 hours 6 hours
1860 May 8 19 days 9 hours? 11 hours
1918 October 12 24 days 2 hours 2+ hours
Guðgeir Jóhannsson (1919) writes: „Um kl. 1 e.h.
verður snarpur jarðskjálftakippur; hriktir í húsum og
glamra saman lausir smámunir, er þétt standa í hillum
og borðum. Eru síðan sífelldir smákippir og titringur
í 1/2 kl.st. Um kl. 3 e.h. sést héðan einkennilegt þykkni
í hánorðri, sem nær til norðausturs og ber hátt á
loft yfir Höttu og Hrafnatinda. . . . . . Um sama leyti
og hér varð vart við mökkinn, eða litlu síðar, sést
hvar jökulflóð brunar eftir farvegi Múlakvíslar alla
leið fram í sjó. . . . “
The eruption began on October 12. An earthquake
was felt around 13h followed by a sequence of smaller
events for about half an hour. An eruption column was
seen at 15h and at the same time, or a little later, a
jökulhlaup was seen on the Mýrdalssandur plain (Fig-
ure 2), a flood wave of meltwater mixed with ice,
ash and debris (Jóhannesson, 1919; Sveinsson, 1919;
Tómasson, 1996).
Out of 18 large, historic eruptions of Katla (Thór-
arinsson, 1975) eight were accompanied by felt earth-
quakes according to written documents (Table 1).
This does not mean that the other nine were not ac-
companied by earthquakes also, but they are not men-
tioned. In the eruptions of 1625, 1721, 1823, 1860,
and 1918 it is specifically stated that the felt earth-
quakes occurred well before the eruption was seen,
possibly as long as 9 hours. The jökulhlaup times, i.e.
the time from the first felt earthquake until the flood
reaches the inhabited area, varies between two and 11
hours, in one case possibly exceeds 12 hours.
Hekla
The eruptive behavior of Hekla in Historical times is
relatively well documented due to its proximity to,
and visibility from Skálholt (Figure 4), the bishop’s
seat and cultural center of Iceland for centuries. The
structure of the volcanic system is unusual, consider-
ing that it is located at a rift-transform intersection.
The Hekla edifice has the form of an elliptic cone or a
ridge. The associated fissure swarm is relatively short
and lacks a characteristic graben structure.
Eruption frequency has been rather constant dur-
ing the last millennium, with about two eruptions per
century. The last century deviates notably from the
long-term frequency with five eruptions between 1947
and 2000. Earthquakes are mentioned in historical
documents in association with onset of the following
eruptions of the Hekla system (Thoroddsen, 1925;
Thorarinsson, 1967):
1157 or 1158: Eruption began on January 19. Earth-
quakes caused some casualties. Relation to eruption
is uncertain.
1157: Eldsuppkoma í Heklu 19. janúar og land-
skjálfti sá er manndauði varð af. (Ísl. ann. 1847, p.
64–66, Biskupasögur I, p. 85)
1300: An eruption of Hekla, beginning on July 11 or
12 and lasting about 12 months, was accompanied by
earthquakes in late December that destroyed several
farms (Thorarinsson, 1967). Relation to eruption is
uncertain.
42 JÖKULL No. 69, 2019