Jökull - 01.12.1977, Page 63

gravity variations in connection with the Leir-
hnjúkur event, and facilitate the evaluation of
future higli precision gravity measurements in
northern Iceland.
PREVIOUS INVESTIGATIONS
OF GRAVITY VARIATIONS WITH TIME
IN NORTHERN ICELAND
Monitoring of secular and long-periodic
gravity variations with time in northern Iceland
started in 1938, when Schleusener observed
gravity in 40 stations situated mainly along the
100 krn long line Akureyri—Grímsstadir á Fjöll-
um (Schleusener 1943). From 1964 to 1970,
about 1000 new gravity stations have been
established in the young volcanic zone and the
adjoining Tertiary plateau basalts in the region
between 65.5°—66° N and 18°—15° W, giving
a more detailed picture of the regional gravity
field in this area (Schleusener et al. 1976). The
gravity stations of 1938 have been reconstructed
and reobserved in 1965, the number of points
by additional stations increasing to about 100.
Further repetitions of this “main profile”,
which was extended to the village of Hof in
1971, took place in 1970/71 (Schleusener et al.
1974) and 1975. The profile (</> ~ 65°40') covers
now a total length of about 150 km and con-
tains (1975) 176 gravity stations, the average
station distance being about 1 km. This profile
coincides more or less with the seismic profile
measured by Pálmason (1971).
Since 1965, the profile stations are monu-
mented by concrete disks and centrical iron
bars, or in rocky areas by iron bolts or engraved
crosses. The observations have been performed
since 1964 with La-Coste-Romberg gravity met-
ers, resulting in an accuracy of ± 0.01 mgal at
the main profile and ± 0.02 . . . ± 0.03 mgal at
the regional field stations. Heiglit control lias
been carried out by technical levelling (± 0.05
m) at the main profile and by barometric level-
ling (±3 m ... ±5 nt) at the regional fielcl.
The comparison 1970—1965 indicated gravity
variations of 5 . .. 10 ^ugal/a along the main
profile, in the sense of increasing gravity in the
young volcanic zone compared to gravity in the
western plateau basalts (Schleusener and Torge
1971). From the comparison 1975—1970/71, a
similar result has been found (Torge and
Drewes 1977). Assuming that the variations are
completely due to surface displacements, these
results correspond to a subsidence of 0.03 . . 0.05
m/a in the young volcanic zone, if a Bouguer-
type relationship is valid. The maximum varia-
tion gradient occurs at Námaskard. From level-
ling results, Tryggvason (1974) found a subsid-
ence of 0.01 m/a in the central part of the
North Iceland rift zone.
THE 1976 GRAVITY SURVEY
IN NORTHERN ICELAND
The 1976 field survey took place from 1.8.
to 21.8. 1976. In addition to the authors, the
following persons participated at the survey:
cand. geod. W. Adolf, Ing. (grad.) H. Lehrke,
Dipl.-Ing. G. Terwey, Miss cand. geod. A. Weise.
In the monumented main profile, 44 gravity
stations between Másvatn and Jökulsá á Fjöll-
um and other 4 base stations in Akureyri, Laug-
ar and Grímsstadir have been reoccupied (Fig.
1), using LaCoste-Romberg gravity meters no.
G 79, G 85, D 14. Furtherntore 4 stations of the
Icelandic gravity survey in the Námafjall region
have been observed. Overlapping gravity con-
nections between six especially permanent stat-
ions form a base network (Fig. 2), which yields
a Iiigh stability of the system. The gravity dif-
ferences in the base network have been observed
at least once with three gravity meters. The
misclosures (—20... +25 pigal) give an idea of
the precision achieved. The other profile stat-
ions liave been occupied at least once witli two
instruments and connected with the base stat-
ions by loop measurements. Network design was
optimized according to investigations for the
network planning 1975. Gravity values have
been calculated from the 185 observed gravity
differences by a free net adjustment (Table 2).
The gravity level is derived from the gravity
of the station Akureyri, Menntaskóli entrance,
first step, no. 60932
gAkureyri = 982,348.39 ± 0.05 mgal
(Schleusener et al. 1974),
and refers to the Potsdam gravity system, with
the conventional Bad Harzburg (station no.
21510 A, IAG-SSGð-catalogue) gravity value
JÖKULL 27. ÁR 61