Jökull - 01.01.2019, Blaðsíða 40

Historical accounts of pre-eruption seismicity in Iceland
„. . . Anno Christi 1625 þann annan dag sept-
embermánaðar um morguninn snemma, um fyrstu
birting, þá fundust og heyrðust hér í Veri nokkrir
jarðskjálftar, smáir en ekki stórir. Þar skammt eftir
heyrðist dunur og gnýr svo mikill og þungur að jörðin
öll rigaði sér undir mönnum. Því jökullinn ásamt eldi
og vatni gjörði þá hið fyrsta (eftir skjálftann) úr stað
að hrærast og upp að springa. En um það bil sem úti
voru mjaltir þá tók strax vatnsflóðið að koma í þá á
sem fellur næst við staðinn. . . . “
(. . . Anno Christi 1625 on the second day of Sep-
tember early in the morning, around first light, a
few earthquakes were felt here in Ver, small and not
large. Shortly thereafter thundering noises and din
were heard so strong that the Earth moved under our
feet. Because the glacier first now (after the earth-
quake), together with fire and water, moved and ex-
ploded. But about the time milking was finished the
river nearest to the farm began flooding. . . . )
A few firm conclusions can be drawn from this
description. There were felt earthquakes prior to the
outbreak of the eruption and the initiation of the jökul-
hlaup, but they were small. The beginning of the erup-
tion was probably accompanied by a low-frequency
earthquake, judged from the verb „riga“ used in the
icelandic text. The timescale is uncertain. It appears
that there was only short time between the felt earth-
quakes and the beginning of the eruption, possibly
only minutes or tens of minutes. First light may be
assumed to be about 5h in the morning on September
2. Later in the text it is mentioned that milking was
around „dagmál“, which is about 9h.
1660: A contemporary account of the beginning of the
eruption is by Jón Salómonsson in Höfðabrekka (H
in Figure 2), about 20 km south of the Katla caldera
(Jónsson, 2018, p. 83):
„Anno 1660 þann 3. nóvember um kvöldið móti
dagsetri sást fyrst eldur upp koma til norðurs
tilsýndar frá Höfðabrekku er stóð við Mýrdal í
Skaftafellssýslu, með langvaranlegum landskjálfta,
hartnær sem menn meintu heila stund, þó stundum
yrði nokkur kyrrð á, áður en eldurinn uppgaus. Þar
eftir um kvöldið þess sama dags, þá úti var vana-
legur vökutími og menn vildu hvílast, kom fram
að Höfðabrekku jöklagangur með ofurmáta miklum
vatnaþunga, hver sitt útfall hafði í Kerlingardalsá sem
flóar fyrir austan Fagradal. . . . “
(Anno 1660 on November 3, in the evening
towards sunset, fire was seen erupting north of
Höfðabrekka near Mýrdalur in Skaftafellssýsla dis-
trict, with persistent earthquake for about an hour
before the fire erupted, though with interruptions.
Later that night, at the end of the working day, when
people wanted to rest, a glacier outburst arrived at
Höfðabrekka with tremendous and heavy water cur-
rent, following the course of the Kerlingardalsá river
(Figure 3) east of Fagridalur. . . . )
The interpretation here is straight foreward. Earth-
quakes were felt for an hour before the eruption be-
gan. The jökulhlaup arrived at Höfðabrekka 3–4 hours
after the earthquakes were felt. If compared with de-
scriptions of earthquakes prior to eruptions in the
following century one may conclude that the events
in 1660 were rather weak.
1721: A contemporary account is available by Þórð-
ur Þorleifsson in Kirkjubæjarklaustur, 50 km east of
Katla, and Erlendur Gunnarsson, Þykkvabæjarklaust-
ur, 35 km SE of Katla (Jónsson, 2018, p. 89–90):
„Anno 1721 þann 11. maímánaðar, kl. 9 fyrir
middag, kom svo mikill jarðskjálfti að á sumum
bæjum í Mýrdal voru menn ei óhræddir í húsum vera.
Við hvern jarðskjálfta einnin var vart á Síðu, undir
Eyjafjöllum og í Fljótshlíð; hverjir næstu viku þar
eftir jafnlega voru en viðhéldust öðru hverju til mið-
sumars.“
„Á sama degi, klukkan 1 eftir middag, heyrðust
miklir dynkir með stórbrestum, hverjum að fylgdi ógn-
arlegur eldgangur með mökk og svælu, sem uppkom
í jöklinum norður af Höfðabrekku, nefnt Kötlugjá,
. . . Klukkan 2 kom krapavatnshlaup, hastarlega fram-
rennandi með nokkrum jökum . . . Klukkan 3 kom
sjálft jökulhlaupið með hraðri ferð, . . . “
(Anno 1721 on May 11, at 11 h AM an earthquake
occurred, so strong that people in Mýrdalur were
afraid of staying inside their houses. This earth-
quake was also felt in Síða, in Eyjafjöll and Fljótshlíð.
Earthquakes were felt frequently during the following
week and off and on after that, until the middle of the
summer.
JÖKULL No. 69, 2019 39