Jökull - 01.12.1977, Page 70

TABLE4 - TAFLA 4
Gravity and height for Icelandic control points between Mývatn and Námaskard.
Þyngd og hœð islenskra fastmerkja milli Mývatns og Ndmaskarðs.
Station No. g (mgal) H (m) Remarks
5276 982 297.109 284.16 Station of main profile.
Kóngsp. 298.128 279.03
5604 294.568 294.66
6412 289.260 321.02 St. no. 171 of main profilc.
5603 289.169 320.23 Identical no. 5689.
115 282.134 358.89
this region. At the earthquake area of Axar-
fjördur, we fincl a gravity decrease (up to 0.1
mgal) at Vestursandur, and a gravity increase
(up to 0.2 mgal) in the southern adjoining
fissure zone of Lyngás. From earthquake activi-
ty, vertical displacements of up to 1 m and
fissure formation have been observed here. The
gravity increase (up to 0.1 mgal) at the north-
eastern coast of Tjörnes might not be real, since
point idendfication was difficult there due to
terrain inclination and movements by road con-
struction.
In the Gjástykki fissure zone, a general gravi-
ty decrease of about 0.1 mgal occured. In the
southern part of Gjástykki, a monumentecl pro-
file lias been established 1938 by Niemczyk and
Emschermann (1943) and controlled in position
1965, 1967, 1971 by Gerke (1974) and in height
1965 ancl 1967 by Spickernagel (1966, 1968).
Spickernagel found a subsidence of the central
part of the graben of the order of 1 cm/a
(1965-1938) and 4... 7 mm/a (1967-1965).
From the gravity measurements we derive a
gravity increase of 0.2 mgal for the central part
relative to the flanks (Fig. 5), for the period
1976—1965. Assuming the gravity-height factor
— 0.2 mgal/m, this gravity increase would cor-
respond to a subsidence of 1 m, a process wltich
according to the result of Spickernagel probably
did not happen continuously during 1965—1976
but by a discontinuous change.
CONCLUSION
The 1976 gravity and levclling survey in
northern Iceland indicated, that gravity- and
correlated heiglit-variations in connection with
g1976-g1965 lmGol)
Fig. 5.
Gravity variation 1976—
1965 along the Gjástykki
profile.
Mynd 5.
Þyngdarbreyting 1976—
1965 eftir mœlilinu
i Gjdstykki.
68 JÖKULL 27. ÁR