Jökull - 01.12.1992, Blaðsíða 5

Table 1. Calculated magnitude at places where earthquakes were felt during the Askja-Sveinagjá rifting events.
A variation in intensity by one degree (IV or VI) changes the calculated magnitude by approximately 0.8 units.
- Reiknuð stœrð jarðskjálfta í mismunandi fjarlœgð frá upptökum. Eitt stig ofar eða neðar á Mercalli kvarða
jafngildir um 0.8 stiga breytingu á reiknaðri stærð.
Region Intensity Epicentral distance (km) and Magnitude
Víti in Askja S-Sveinagjá C-Sveinagjá N-Sveinagjá
Svartárkot, Bárðardalur IV 41 4.7 35 4.6 42 4.7 52 4.9
Gautlönd, Mývatn district V 60 5.9 34 5.3 32 5.3 36 5.4
Grímsstaðir á Fjöllum VI 71 6.9 32 6.1 19 5.7 11 (5.3)
Möðrudalur VI 54 6.6 29 6.0 30 6.0 37 6.2
Brú, Jökuldalur IV 56 5.0 55 5.0 62 5.1 71 5.3
earthquakes were felt during the Askja-Sveinagjá rift-
ingevents is4.9 (R= 19km) toó.l (R = 71 km),foran
intensity V earthquake, (Table 1). If we assume that re-
ports from Möðrudalur are exaggerated or that some of
the earthquakes originated closer to Möðrudalur,i.e. in
Veggjastykki, then earthquakes in the northern part of
Sveinagjá may have reached magnitude 5.4-5.7. This
mganitude is higher than expected as rifting earth-
quakes during the recent Krafla rifting episode seldom
exceeded magnitude 4 on the Richter scale, in spite of
extensive surface faulting (Einarsson, 1991a).
An eruption was discovered in Askja on January
3, 1875, but increased steam activity had been ob-
served in the Askja region, some days prior to the
eruption. One eruption column, substantially wider
close to the horizon, was visible from the Mývatn
district at the same time (Anonymous, 1875a; Anony-
mous, 1876a), indicating fissure eruption or many ac-
tive craters close together. Two eruption columns were
seen from eastern Iceland and one of them originated
without doubt from an eruption site within Askja. Ac-
cording to Gunnarsson (1875) the distance between the
two columns was somewhat greater than the distance
between the northem column and the tablemountain
Herðubreið, 20 km northeast of Askja. An eruption
column originating south of Askja cannot be recon-
ciled with Gunnarsson’s description unless the erup-
tion site was at least 20 km south of Askja but in
that case people in the Mývatn district would have
detected two separate columns as well. In order to
fit the above descriptions, one has to assume that the
second column originated somewhere in the fissure
swarm north-northeast of the Öskjuvatn caldera. This
eruption site remains to be discovered.
Poor visibility prevented further observation of the
eruption the following days but earthquake activity
decreased markedly after the outbreak (Anonymous,
1876a). Still, relatively strong earthquakes were felt
at Grímsstaðir during the next weeks (Guðmundsson,
1932). Frequent earthquakes were also felt in the be-
ginning of February 1875, in Bárðardalur and in the
Mývatn district. Farmers there observed occasionally
one or two separate eruption columns in the direction
of Askja (Anonymous, 1875c).
These descriptions are analogous to observations
made during the recent Krafla eruptions. Intense
earthquake activity accompanied lateral magma intru-
sions within the Krafla volcanic system but decreased
abruptly once an intrusion reached the surface in an
eruption. It is therefore reasonable to assume that con-
siderable intrusive activity took place within the Askja
fissure swarm during the autumn of 1874, prior to the
January 2 eruption, during which intrusive activity di-
minished markedly or even came to a halt but resumed
in early February.
Four farmers from the Mývatn district visited
Askja on February 16 1875, and reported fierce geo-
thermal activity and new craters in a depression in the
southeast comer of the Askja caldera. Their report
does not mention volcanic activity within the caldera
at that time but the farmers felt earthquakes frequently
in the vicinity of the depression (Anonymous, 1875d).
JÖKULL, No. 42, 1992 3