Jökull - 01.01.2019, Side 42

Historical accounts of pre-eruption seismicity in Iceland
houses might collapse. In the evening of the same day
at sunset frequent lightning was seen, that continued
throughout the night together with tremors and move-
ments of the ground. . . .
. . . In the night following October 17 and the day
a flood and jökulhlaup originated in the so called
Kötlugjá.)
This description indicates that strong earthquakes
preceded this large eruption of Katla. The electrical
activity probably marks the beginning of the eruption,
indicating that the precursory time may have been
about 6 hours. The jökulhlaup appears to have taken a
few additional hours to develop and reach the inhab-
ited areas.
1823: The eruption of Katla in 1823 had a rather un-
usual prelude, in that the neighboring volcano Eyja-
fjallajökull had been in a state of eruption since De-
cember 1821 and the two volcanoes were active si-
multaneously for a while in 1823 (e.g., Einarsson and
Hjartardóttir, 2015). The 1823 Katla eruption was ob-
served and described by two keen observers, rev. Jón
Austmann in Mýrar (Figure 2, 30 km SE of Katla) and
Sveinn Pálsson, med. doctor in Vík (Figure 3, 20 km
S of Katla), see Jónsson (2018, p. 222). The eruption
began on June 26.
„Hér um bil klukkan 6 eftir middag fannst hér í
Vík (. . . ) fyrst jarðskjálfti, nokkrir harðir og stuttir
kippir, þó ekki meiri en að vart hrikti í húsum og þar á
eftir hægari en viðvarandi hræringu er að síðustu létu
á milli með hörðum kippum, viðlíkt og fyrst þangað til
klukkan 9, þá allt varð kyrrt svo sem 1/4 tíma, sást þá
fyrst mökkur koma upp með ógnarlegri ferð . . . “
P. 232: „. . . Nálægt miðri nótt . . . byrjaði vatns-
hlaup nokkurt með smá íshroða og jökulleirsbleytu,
austan með Víkurfjalli . . . “
(About 6 pm an earthquake was felt here in Vík
(. . . ), a few sharp and short shocks, but not hard
enough to rock houses significantly, followed by
weaker but persistent movements, later mixed with
harder shocks similar to the first one, until 9h PM,
then all became quiet for a quarter of an hour. Then
an eruption column ascended at high speed . . .
. . . Near midnight . . . a water flood began, mixed
with ice slush and glacier clay, east along the Víkur-
fjall mountain . . . )
This eruption appears to have been preceded by
felt, but weak earthquakes. They are not mentioned
by Jón Austmann in Mýrar, who otherwise describes
the course of events very thoroughly. The precursor
time is about three hours. The first jökulhlaup arrives
about three hours after the beginning of the eruption.
1860: The most detailed account of the beginning of
this rather small eruption was written by Jón Jónsson
in Höfðabrekka (Figure 2), 20 km S of Katla, (Jóns-
son, 2018, p. 267):
„Þriðjudaginn hinn 8. maí klukkan 6 til 7 um
morguninn komu jarðskjálftar svo miklir að hús
kipptust við og brakaði í þeim. Klukkan 51/2 um
kvöldið braust vatn fram Múlakvíslaraura með mikilli
ferð. . . .“
And for May 9: „. . . Þá um morguninn fyrst sást
dimmsvartur mökkur er lagði langt upp á loft úr jökl-
inum . . . “
(On Tuesday May 8 at 6 to 7h AM earthquakes oc-
curred large enough to shake houses and make crack-
ing noises. At 5 1/2 h PM a jökulhlaup came down the
Múlakvísl river (Figure 3) with great speed . . .
And for May 9 the diary reads: Then, in the morn-
ing, a dark, black eruption column was first seen
standing high above the glacier . . . )
The 1860 eruption appears to have been only
moderately large. It was, for example, considered
“the least damaging eruption of all historical Katla
eruptions” by rev. Magnús Hákonarson who lived in
Vík during the eruption (Jónsson, 2018, p. 277). It
was clearly preceded by an earthquake sequence but
the precursor duration is uncertain because the onset
time of the eruption cannot be determined from the
available reports. The eruption column was not seen
until in the morning of May 9. This is the only Katla
eruption of recent centuries where the jökulhlaup is
detected before the eruption column is seen. Assum-
ing, however, the eruption began 1–3 hours before the
detection of the jökulhlaup the precursor time is about
9 hours. This is probably a lower limit of the estimate
for the precursor time.
1918: Gísli Sveinsson (1919) writes: „Rúmlega einni
stundu eftir hádegi fundust mjög snögglega jarð-
skjálftakippir allmargir og linti ekki hræringunum.
Hélst það áfram, þó meira dræmt en í fyrstu.“
JÖKULL No. 69, 2019 41