Jökull - 01.12.1992, Page 4

1961. An E-W trending fissure opened in the north-
eastem part of the Askja caldera and the lava flowed
through Öskjuop and fanned out across the eastem
plain (Einarsson, 1962; Þórarinsson, 1963).
REPORTS OF EARTHQUAKE ACTIVITY
ASSOCIATED WITH THE ASKJA RIFT-
ING EPISODE, 1874—1876.
Nothing is known about the seismic activity of
Askja prior to the rifting episode in 1874-1876. The
first records of renewed activity in Askja date back to
February 1874, when unusually dense steamclouds
were observed over Askja from afar (Thoroddsen,
1905). New fissures were discovered in the autumn of
1874, in the northem part of the Askja fissure swarm.
These fissures extended and widened during the next
months and this process was accompanied by intense
earthquake activity which was felt in Möðrudalur and
at Grímsstaðir á Fjöllum, situated 54 km and 71 km
NNE of the Askja caldera and approximately 30 km
from the southem end of Sveinagjá (Figure 1; Table 1)
(Anonymous, 1875a; 1875b; Guðmundsson, 1932).
During the last two weeks of December, strong and
frequent earthquakes were felt in northern and east-
ern Iceland (Anonymous, 1875a; Gunnarsson, 1875).
The earthquake activity reached a maximum on Jan-
uary 2, 1875, when continuous seismic activity was
felt at farms in the Mývatn district (Anonymous,
1875a; 1876a). Earthquake intensity estimates indi-
cate that the biggest earthquakes reached intensity IV
on the modified Mercalli scale in Jökuldalur in north-
eastem Iceland and intensity V in the Mývatn district.
The earthquakes most likely reached intensity VI at
Grímsstaðir and in Möðrudalur where some of the
houses came close to collapse (Anonymous, 1875b;
Guðmundsson, 1932). The above descriptions indi-
cate that the earthquakes originated in the northem
part of Sveinagjá, within 20 km from Grímsstaði. It
is, however, strange that the earthquakes seem to have
been much stronger in Möðrudalur than at Gautlönd,
which are situated at approximately the same distance
from Askja and Sveinagjá (Table 1). This could indi-
cate that some of the earthquakes originated closer to
Möðrudalur, possibly in Veggjastykki (Figure 1).
Figure 1. Index map of northeastern Iceland. Faults
and eruptive fissures are drawn according to Jóhan-
nesson and Sæmundsson, 1989. - Kort af gosbeltinu
á norðausturlandi sem sýnir sprungusveima Kröflu og
Oskju.
Halldórsson et al., (1984) derived the following
equation for the intensity-distance relation for Ice-
land by calculating the attenuation of intensities from
isoseismal maps from 8 earthquakes with magnitude
range from 5.2 to 7.0;
I-I„ = 0.8767 -0.0123R- 1.5691 logR (1)
I is the intensity for a given earthquake at an epicentral
distance, R and I0 is the calculated epicentral intensity.
Equation (1) is valid for R > 20 km. The relationship
between I0 and magnitude, M was found to be;
lo = 0.33+ 1.24M (2)
The intensity-magnitude relationship is obtained by
combining the above equations.
1 = 1.2067+ 1.24M-0.0123R- 1.5691 logR (3)
According to equation (3) an intensity V earth-
quake at a distance of 30 km has a calculated magni-
tude of 5.2. The calculated magnitude at places where
2 JÖKULL, No. 42, 1992