Jökull - 01.01.2019, Blaðsíða 46

Historical accounts of pre-eruption seismicity in Iceland
Table 2: Precursor times of historical Hekla eruptions with documented seismicity. – Tafla 2. Forboðatímar
gosa í Heklu á sögulegum tíma þar sem jarðskjálfta er getið.
Eruption Day Duration Precursor time
1300 July 11 or 12 about 12 months ?
1510 July 25 unknown very short, few minutes
1554 May-early June six weeks uncertain, possibly two weeks
1597 January 3 six months very short, few minutes
1693 February 13 > seven months very short, few minutes
1725 April 2 unknown several hours
1766 April 5 23 months a few hours
1845 September 2 seven months less than one hour
1878 February 27 about two months 3-4 hours
1913 April 25 24 days three hours
1947 March 29 13 months very short, minutes
1970 May 5 two months 25 minutes
1980 August 17 three days 23 minutes
1981 April 9 about one week (4 hours)
1991 January 17 51 days 30 minutes
2000 February 26 10 days 79 minutes
earthquakes with very short precursor times, whereas
the eruptions in the surrounding areas were associated
with strong earthquake activity with precursor times
longer than 3 hours.
Öræfajökull
This off-rift volcano (Figure 1) has attracted consid-
erable attention in the last few years because of clear
signs of inflation that began in late 2016 (Geirsson et
al., 2018). The volcano has erupted twice in historic
times, a rather catastrophic eruption in 1362 and a
smaller eruption in 1727. Both were apparently pre-
ceded by felt earthquakes.
1362: Reports of this eruption are rather scant, al-
most like legends. The eruption was very large and
explosive, and left a thick ash deposit in SE-Iceland,
mainly east of the volcano, but has been detected also
overseas. The volume of ash is estimated to be 10
km3 (Thorarinsson, 1958). The inhabited area around
the volcano was totally devastated and was apparently
not habitable for several decades following the erup-
tion (Gudmundsson, 1998). According to a legend,
three quakes were heard shortly (minutes to hours)
before the eruption broke out. Recent archaeological
excavation of a ruined farm indicates that the walls
of the farmhouse had fallen before the ruin was cov-
ered by an ash layer, indicating strong pre-eruption
earthquake activity (B.F. Einarsson, 2008). The lack
of archaeological artifacts found in the ruins may sug-
gest long precursory activity. The people may have
had sufficient warning to move away with their be-
longings.
1727: Weak earthquakes were felt at the nearby
church at Sandfell in the morning of August 7, strong
enough to frighten people, according to contemporary
account by the pastor Jón Þorláksson (Thoroddsen,
1925). The earthquakes became more frequent and
stronger the following morning. Things were shaken
down from shelves indoors, but houses did not col-
lapse. The shaking was accompanied by noises,
strong as thunder. An eruption was detected around
9h that morning, accompanied by jökulhlaups and ash
fallout. Fire fountains were seen in 5–6 places high
on the west flank of the volcano. Hot floods rushed
down the gullies, one of them killing three people on
August 11. The eruption continued until April 1728.
JÖKULL No. 69, 2019 45