Jökull - 01.01.2019, Side 45

Einarsson
1766: This longest eruption of Hekla in Historic times
began in the morning of April 5 and lasted for 23
months. It was preceded by earthquakes. According
to a report by bishop Hannes Finnsson the earthquakes
were felt all through the night before the eruption be-
gan, around 03:30. He also mentions a long-term
precursory phenomenon, that all the streams in the
Hekla area got smaller during the previous two years.
Withering vegetation was also mentioned (Thorarins-
son, 1967).
1845: An eruption began on September 2 and became
visible around 10h. Weak trembling of the ground
was noticed less than one hour befor the visible erup-
tion. The detailed report of this eruption was written
by Oddur Erlendsson at Þúfa (Figure 4) in Landssveit
(Thorarinsson, 1967).
„. . . Að ofanverðum dagmálum fór í einu vetfangi
að heyrast dunur og ofsa niður í austri með þvílíkum
undirgángji, að jörð iðaði við undir fótum manna,
var niður þessi því líkastur, sem menn stæði undir
ógurlegustu árgljúfrum, eða stríðasta öldufalls hljóð,
fundu þeir sem voru í húsum inni hæga jarðskjálpta 3
eða 4 þar er næst var fjöllum, en ekki nema einn, þar
er fjær var, en þeir er úti voru að verkum staddir, urðu
þess ei varir. Sló þá upp á austur landnorður loptið
blásvörtum myrkva æði miklum . . . “
The timing of this description is not very precise.
The wording „ofanverð dagmál“ may be take to mean
„well after 9 AM“. In another report it is mentioned
that the eruption column was seen at 10 AM. The pre-
cursor time is therefore likely to be shorter than one
hour, probably considerably shorter. It is stated that
the earthquakes were not felt by people at work out-
side. This would suggest a value of III on the Mercalli
Intensity Scale. The nearest farms were at distance of
15 km, so the magnitude of the earthquakes was not
much more than 3.
1878: The eruption began on February 27 accompa-
nied by strong earthquake activity. Continuous activ-
ity was felt in the neighbouring areas from 15h until
5h the following morning (Thoroddsen, 1925; Peder-
sen et al., 2018). Earthquakes were felt at least as far
as 75 km from the eruption site. The eruption site was
at Krakatindur (Figure 4), about 9 km east of the sum-
mit of Hekla and 23 km from the nearest inhabited
area. Fire was seen at 18–19h and appears to have in-
creased strongly around 20h following a strong earth-
quake. After that the seismicity gradually decreased
but the visible fire slowly increased.
1913: An eruption began on a 5 km long fissure at
Mundafell (Figure 4), some 6 km east of Hekla on
April 25 at about 6h, preceded by vigorous earthquake
activity for three hours (Bárðarson, 1930; Pedersen
et al., 2018 ). The earthquakes were widely felt, the
strongest ones also in Reykjavík (Figure 1) at the dis-
tance of 120 km. About 10 hours later another erup-
tive fissure, about 5 km long, opened up at Lambafit
(Figure 4), 12 km NE of the first one. Both fissures
produced basaltic lava with only minor ash. The erup-
tion on the first fissure apparently only lasted about 24
hours, but the second one remained active until May
19 (Bárðarson, 1930).
1947: The latest (so far) of the large eruptions of
Hekla began on March 29 without a significant warn-
ing. The estimated time of the beginning is 06:41, but
nine minutes later an earthquake occurred that was felt
over a large part of SW-Iceland (Thórarinsson, 1967).
A summary of Hekla eruptions with documented
seismicity is in Table 2. Data on the five latest erup-
tions are based on instrumental observations and in-
cluded for comparison (Einarsson, 2018). The gen-
erally low seismic activity associated with histori-
cal Hekla eruptions is remarkable and is consistent
with the findings of Soosalu and Einarsson (2005)
for recent eruptions of Hekla. Exceptions are erup-
tions outside the main edifice of Hekla, such as 1554,
1725, 1878, and 1913. The last two, at least, were
lava eruptions from eruptive fissures. These eruptions
may have a strong tectonic component reflecting the
unique tectonic position of Hekla at the intersection
between the Eastern Volcanic Zone and the South Ice-
land Seismic Zone. Two of the bookshelf faults of
the SISZ transform were identified beneath the Hekla
edifice by Soosalu and Einarsson (2005) extending
into adjacent areas. These are potential source faults
of larger earthquakes. The difference in seismic be-
haviour between eruptions originating from the main
edifice and those occurring in the surrounding areas
was noted by e.g., Thorarinsson (1970). The „proper“
Hekla eruptions were generally preceded by weak
44 JÖKULL No. 69, 2019