Jökull - 01.12.1992, Side 30

large springs. No such springs are found in the most
probable area, which excludes this path for the sub-
surface drainage of lake Höskuldsvatn.
Drainage to the northwest, along the Húsavík fault,
would carry the water towards the lake Botnsvatn,
where water flows from several rather large springs.
However, the location of the springs does not favor
drainage from the Höskuldsvatn area towards lake
Botnsvatn.
This leads to the conclusion that the probable
path of the subsurface drainage of lake Höskulds-
vatn is towards east, through Holocene lavas and the
Þeistareykir fissure swarm, towards Lón in Keldu-
hverfi. This path lies in an area where considerable
ground deformation did occur during the period of the
Krafla fires, 1975—1984, although limited observa-
tions exist in this area.
The flanks of the Krafla fissure swarm are known
to have been compressed in E-W direction simulta-
neously with the widening of the fissures (Tryggvason,
1984). This compression occurred in the area east of
lake Höskuldsvatn and may have changed the flow
characteristics of the aquifers. Another characteristic
of the ground deformation is the uplift of the flanks of
the fissure swarm. This caused tilt up in easterly direc-
tion in the area of lake Höskuldsvatn, as measurements
of 1976 and 1980 show when related to earlier mea-
surements (Tryggvason, 1981). The ground tilt from
1972to 1980nearlake Höskuldsvatnwas about 12mi-
croradians, up towards northeast. In the Þeistareykir
fissure swarm, this tilt was about 15 microradians up
towards east, and at a location due east of Höskulds-
vatn and 7 km west of the central axis of the Krafla
fissure swarm, a tilt of 75 microradians was observed
from 1972 to 1980. This tilt represents uplift of the
region through which the subsurface drainage occurs,
relative to lake Höskuldsvatn.
Thus, both horizontal compression and ground tilt
is likely to restrict somewhat subsurface drainage from
lake Höskuldsvatn towards east.
Measurements of one short line of precision level-
ing in October 1991 indicate that the ground tilt in this
area has continued after 1980, which is not surpris-
ing in light of the continued volcanic activity in 1980
through 1984.
It cannot be estimated at this time how much effect
these tectonic processes have had on the subsurface
drainage of lake Höskuldsvatn. They may be insignif-
icant.
MEASUREMENTS 1991
In the fall of 1991 the elevated surface of lake
Höskuldsvatn caused some concern among the local
population. A farmer found fissures in the ground
in an area 12 to 15 km south of lake Höskuldsvatn,
which he judged as recent. These fissures were found
in soil. Their direction was irregular and the width
was frequently about 5 cm.
An expedition was arranged on October 21 to in-
vestigate these fissures. At that time, some snow had
covered the ground, making search for small fissures
difficult. However, some irregular fissures were found
at a location 3.5 km southeast of the farm Heiðarbót
at about 350 m elevation. These fissures were in thick
soil, clearly caused by frost action, and thus not re-
lated to any tectonic processes, nor to the unusually
high water level of lake Höskuldsvatn. It has not been
verified if the fissures found beneath the snow cover
were the same as seen by the farmer in snow free
ground one week earlier, but his description indicates
the same type of fissures.
It was decided to relevel a line of precision lev-
eling, immediately southeast of the south end of lake
Höskuldsvatn, to determine if any significant ground
deformation had occurred since last leveling of this
profile in July 1980. This leveling was conducted on
October 22-23 under less than ideal conditions. The
ground was frozen and partly covered by snow at the
beginning of this leveling, but the weather was warm,
causing snow and frozen ground to thaw. However,
an acceptable result was obtained by covering the line
three or four times.
This leveling indicated a ground tilt of 10.8 micro-
radians up towards an azimuth of 115 degrees, or 25
degrees south of east since 12 July 1980. This com-
pares with observed tilt of 10.6 microradians, up to-
wards an azimuth of 64 degrees, or 26 degrees north
of east, between 30 June 1976 and 12 July 1980.
Thus the up-slope tilt vector points towards the
region where greatest tectonic and volcanic activity
28 JÖKULL, No. 42, 1992