Jökull - 01.12.1992, Side 6

A basaltic fissure eruption started in Sveinagjá on
February 18, 1875. Sveinagjá is situated in the east-
ern part of the Askja fissure swarm, around 40 km
north of the Askja caldera and about 25 km from the
Grímsstaðir farm. Another outbreak, slightly north of
the first eruption, began on March 10 (Anonymous,
1876a). Three farmers from the Mývatn district vis-
ited the eruption site in Sveinagjá, ön March 12. One
of the farmers wrote a report on the state of the erup-
tion during the three hours they stayed at the eruption
site but does not mention earthquakes (Hálfdánarson,
1875). The third eruption occurred on March 18-
19, on the southemmost part of Sveinagjá. During
March the main eruption site seems to have frequently
shifted along the whole eruptive fissure of Sveinagjá.
This eruption was accompanied by earthquakes big
enough to wake up people in the Mývatn district on
March 18 and 23. On March 23 the main eruption was
at the northem end of the Sveinagjá fissure, about 25
km from Grímsstaðir. Forty active craters were vis-
ible from Grímsstaðir that day (Anonymous, 1875e).
Earthquakes large enough to wake up people at a dis-
tance of 35 km are IV-V on the Mercalli scale and their
calculated magnitude is 4.5-5.3 (Table 1).
A big explosive eruption started in the Askja
caldera in the early morning on March 29. This plinian
outbreak was accompanied by heavy tephrafall in the
Jökuldalur district east of Askja (Gunnarsson, 1875;
Thoroddsen, 1905; Sigurðsson and Sparks, 1978; Sig-
valdason, 1979). Many farms in Jökuldalur had to be
evacuated following the tephrafall on March 29. The
next eruption started on April 4, on the southern part of
Sveinagjá and two new outbreaks took place on April
10-11 and April 20-24 at about the same site as the
February 18 and March 10 eruptions, on the northern-
most part of Sveinagjá (Anonymous, 1875e; 1875f).
Reports from Mývatn indicate that the eruption on
April 4, was noisier than the two earlier eruptions
(Sigurðsson, 1875b) but no reports exist of felt earth-
quakes. An eruption cloud in Askja was observed from
the Mývatn district on May 19 (Anonymous, 1875g).
Three additional outbreaks were observed during
1875, on July 2, near Fjallagjá on the northem part of
Sveinagjá (Anonymous, 1875h), on August 15 (Watts,
1875) and on October 17th, just north of the erup-
tion site on August 15 (Anonymous, 1875i; 1875j;
Jónsson, 1945). The beginning of the August 15
eruption was accompanied by felt earthquake activ-
ity (Anonymous, 1875a; Jónsson, 1945). There exist
vague reports of eruptive activity in the Askja cen-
tral volcano in late 1875 and early 1876 (Anonymous,
1876b). Eruptions in the Askja central volcano were
also reported to have occurred in October and late De-
cember 1876. Earthquakes were felt in Bárðardalur at
that time (Anonymous, 1877; 1878). Eleven fissure
eruptions in Sveinagjá were reported during 1875 and
the eruptive activity continued, on a smaller scale, in
1876. The Öskjuvatncaldera was formed in the course
of these events.
EARTHQUAKE ACTIVITY DURING THE
1921-1933 AND 1961 ERUPTIONS IN
ASKJA
Volcanic activity in the Askja region was next ob-
served in 1921. During this episode a fissure eruption
took place south of the central volcano, an island was
formed within the Öskjuvatn lake and several erup-
tions broke out along the caldera rim of Öskjuvatn.
No reports of felt earthquakes during the rifting event
in 1921-1933 are known and the one-component seis-
mic station in Reykjavík, reinstalled in the beginning
of 1925, did not detect any earthquakes (Seismolo-
gical Bulletin, 1925-1933). Jónsson (1945), Einars-
son (1962) and Þórarinsson (1963) found the total lava
production during these eruptions to be 0.2 km3 and it
is 0.33 km3 according to Sigvaldason et al. (1992).
Between October 6 and 26, 1961, seven earth-
quakes were recorded with epicenters in the Askja
region. These earthquakes ranged in magnitude from
2.8 to 4.0 (Figure 2a) but were not felt (Seismolo-
gical Bulletin, 1961). The seismic stations operating
in 1961 were located 90, 160, and 270 km away from
Askja and the most sensitive station, at Kirkjubæjar-
klaustur, 160km southwest of Askja, could only detect
earthquakes bigger than M 2.4 originating in the Askja
region. The increased earthquake activity in early
October coincided with increased geothermal activity
north of Öskjuvatn. The biggest earthquake, at 11:56,
October 26, M 4.0, marked the outbreak of an eruption
4 JÖKULL, No. 42, 1992