Jökull - 01.12.1977, Page 69

h1976 ' h1S75 ícr[l'
ip =65ö40'
Fig. 3. Height and gravity variations 1976—
1975 along the main gravity profile.
Mynd 3. Hœðar- og þyngdarbreytingar eftir
aðalmœlilinu 1976—1975.
variations, an accumulation of systematic level-
ling errors is to be assumed.
The gravity increase and height decrease at
Námaskard is the effect ot the subsidence at
the Krafla area, being very strongly reduced
already at a 8 km distance. Frorn the gravity
and height change at Námaskard we obtain an
average gravity-height factor of —0.1 ... —0.2
mgal/m. This could correspond to the combin-
ed effect of the removal of a Bouguer plate and
a superposed gravity decrease.
7’he gravity variations 1976—1975/70 found
in the regional gravity field between Mývatn
and Axarfjördur, are listed in Table 3, figure 4
shows the regional distribution of the varia-
tions. These stations are not monumented like
those in the main profile and had to be re-
stored by point sketches. So the accuracy of the
variations is influenced not only by the r.m.s.e.
of the gravity measurements at the first (± 0.02
. .. ± 0.03 mgal) and the second (± 0.02 mgal)
epoch, but also by remaining identification
errors. Taking also local mass displacements in-
to account, we estimate the r.m.s.e. of the deter-
mined variations at ± 0.05 mgal. At some stat-
ions, especially along the road from Námafjall
to Krafla, the uncertainty might be greater, of
the order of 0.1 mgal or more. Only the ten-
dency of remarkable gravity increase should be
recognized here. At the monumented Gjástykki-
NW-SE-profile, accuracy is higher (± 0.03 . ..
0.04 mgal).
From Fig. 4 we see, that gravity variations
have a rather local character and occur only in
á few areas. There is a strong gravity increase
(up to a few tenths of a mgal) from Námafjall
to Krafla, confirming the large subsidence of
Fig. 4. Gravity stations between Mývatn and
Axarfjördur and gravity variations 1976—1965/
70 (t/ioo mgal).
Mynd 4. Þyngdarmœlistöðvar milli Mývatns og
Axarfjarðar og þyngdarbreytingar 1976—1965/70
/Vioo mgal).
JÖKULL 27. ÁR 67